prescribed by national curriculum and textbook board as a

Prescribed by National Curriculum and Textbook Board as a Textbookfor class VII from the academic year 1997

General ScienceFor class VII

Written by Dr. A.K.M Shamsudduha

Dr. Md. Golam Rasul Miah

Prof. Dr. Md. Abdul Wahab

Zohurul Islam Khan

Mohammad Hamiduzzaman Khan Chowdhury

Translated ByS. M. Haider

Rukhsana Shaheen,

A.S. M. Lukman

M. Rafiqual Islam

Belayet Hossain

National Curriculum and Textbook Board, Dhaka.

Published byNational curriculum & Textbook Board

69-70, Motijheel commercial Area, Dhaka

[All rights reserved by the Publisher]

First Print : 1997

Revised Edition : 2007

Reprint : 2010

Reprint : 2011

Computer Compose Perform Color Graphics (Pvt.) Ltd

Cover

Selim Ahmed

Illustrator

Kazi Saifuddin Abbas

Arif Reza Khan

Nazrul Islam Dulal

Design

NCTB, Dhaka.

For free distribution from academic year 2010 by the Government of Bangladesh

Printed by :

Preface

Education is the key to development. A progressively improved education system largely determines the pace and the quality of national development. To reflect the hopes and aspirations of the people and the socio-economic and cultural reality in the context of the post independent Bangladesh, new textbooks were introduced in the beginning of the 1980s folIowing the recommendations of the National Curriculum and Textbook Committee.

In 1994, in accordance with the need for change and development, the textbooks of lower secondary, secondary and higher secondary were revised and modified. The textbooks from classes VI to IX were written in 1995. In 2000, almost all the textbooks were rationally evaluated and necessary revision were made. In 2008, the Ministry of Education formed a Task Force for Education. According to the advice and guidance of the Task Force, the cover, spelling and information in the textbooks were updated and corrected.

The aims & objectives of the study of General Science is to make the learners curious and observant about nature and its environment- science teaches a person to have clear and logical thinking ability. It also makes a person able to do things on his/her own. The General Science book has been prepared according to the general objectives and learning outcomes of the curriculum. Topics of physics, chemistry, botany, zoology, geography and population study have been included in this book. It is hoped that learners will be able to gain both theoretical and practical skills on these topics from study of this book.

We know that curriculum development is a continuous process on which textbooks are written. Any logical and formative suggestions for improvement will be considered with care. On the event of the golden jubilee of the Independence of Bangladesh in 2021, we want to be a part of the ceaseless effort to build a prosperous Bangladesh.

In order to develop learners interest in science some modifications have been made in this book. This book of General Science for class VII is the English Version of the original textbook entitled 'Sadharan Biggan' written in Bangla.

In spite of sincere efforts in translation, editing and printing some inadvertent errors and omissions may be found in the book. However our efforts to make it more refined and impeccable will continue. Any constructive suggestion towards its further improvement will be gratefully considered.

I thank those who have assisted us with their intellect and efforts in the writing, editing and rational evaluation of this book. We hope that the book will be useful for the students for whom it is written.

Prof. Md. Mostafa Kamaluddin

ChairmanNational Curriculum and Textbook Board

Dhaka.

Contents

Chapter Topics Pages

1 Heat 1

2 Pressure of a liquid 14

3 Atmosphere 24

4 Oxygen 36

5 Hydrogen 48

6 Carbon dioxide 55

7 Solution 63

8 Morphology of plants : leaf, stem, flower and fruit 75

9 Microbial world 91

10 Animal kingdom 99

11 Interdependency of Plants and Animals 110

12 The Vertebrates 121

13 Light 127

14 Magnet 138

15 Electricity 147

16 Earth, Crust and Rock 155

17 Sea and Ocean 160

18 Weather and Climate 168

19 Common laboratory Processes 175

20 Production of Essential Goods from 182

Discarded Raw Material

21 Tissue 188

22 Human Body: Digestive and Excretory Systems 195

23 Population Growth and Environmental Pollution 203

24 Flood, River Erosion and Draught in Bangladesh 214

25 AIDS 222

Chapter - 1

Heat

We cannot see heat. Heat has no weight, colour or odour. But we can feel its

existence. If you go out and stand in the sun, you will feel hot. Touch a piece of ice,

you will feel cold. The external cause for which the sensation of hotness or coldness is

felt is called heat.

Again we know that heat is one kind or energy that gives us the sensation of hotness

or coldness. An object becomes hot when it absorbs heat and becomes cold when it

releases heat.

Nature of heat: Caloric and kinetic theories

About 200 years ago heat was also considered to be an invisible, weightless and

indestructible fluid or gaseous substance. Scientists named this strange matter caloric.

It was held that all objects contained caloric in them. When the caloric enters into any

object, its caloric increases, the object becomes hot. On the other hand, if caloric is

released or its caloric decreases, the object becomes cold. This theory is known as the

caloric theory of heat.

Later the caloric theory failed to exist due to the research works of the scientists called

Count Ramford (1753-1814), James Joules (1818-1889) and others. It is also natural

that caloric should have weight, if it was a matter, since all matters have weight.

Weightless matters may be imagined, but they do not exist in reality. Different objects

are found not to differ in weight, when they are measured in both hot and cold

condition.

Kinetic theory of heat

The prevailing idea about the nature of heat is known as the kinetic theory of heat.

You know, every object is made of innumerous atoms. These atoms are always in

motion. If motion of atoms increases, kinetic energy also increases. This kinetic

energy is transformed into heatenergy. As a result, heat of the object increases in the

object becomes hot. On the other hand, if motion of the atoms decreases, kinetic

energy decreases resulting in the decrease of heat of the object i.e. it becomes cold. In

short, heat is another form of kinetic energy. The total kinetic energy created in an

object due to the motion of atoms is transformed into heat energy. This is kinetic

theory of heat.

Sources and production of heat

We know that energy transforms from one form to another. Heat is one kind of energy.

So, heat can be transferred into different energy and different energy may be

transformed into heat energy. Rub the palms of your hands for a number of times, they

will be warm. This means that rubbing of hands produces heat. Here, mechanical

energy has transformed into heat energy. Again we need heat for our survival too. We

get this heat from our food. Chemical energy in food transforms into heat energy and

supplies us strength to work. We need heat to cook. We get this heat by burning wood,

coal, gas or oil. Here, heat originates from the chemical energy stored in wood, gas or

oil (petrol, kerosene, diesel). This heat energy is transformed into mechanical energy

to drive trains, motor, launches and steamers. Moreover, it is transformed into

electrical energy to supply electricity to the mills, factories and houses. Heat may also

be produced from electricity. Electric heaters, irons, cookers etc. are its examples.

Heat is required on earth to sustain lives of men and animals and to grow plants and

trees. We get this heat from the sun. It is the heat of the sun that keeps the earth hot.

Animals and plants gather energy from this heat. Again, when plants and animals die

and remain buried under the soil for millions of years, they transform into coal, gas or

oil. It means that the sun is the main source of heat of coal, gas and oil. Hence, the sun

is the main source of heat although coal, gas and oil are seen as its direct sources.

Hence we can say that

Sun is the main source of heat and other sources of heat are wood, coal, gas and oil

(kerosene, petrol, diesel). Heat may be produced from other forms of energy, such as

mechanical energy, chemical energy and electrical energy.

Influence of Heat

The properties of an object change when heat is applied to it such as:

1. Change of temperature: When heat is applied to an object, its temperature

increases and when it is removed, its temperature decreases.

2. Change of state: When heat is applied, solids generally turn into liquids and

liquids turn into vapours. But when heat is removed, the situation is reversed, i.e.

vapours turn into liquids and liquids turn into solids. However, temperature does not

change as long as the change of state continues to take place. Exceptions- some solids

such as camphor, iodine, naphthalene, ammonium chloride etc. turn into vapour

directly when heat is applied. This incident is called distillation.

3. Expansion of matter: When heat is applied, expansion takes place in length, area

and volume of an object. All the three kinds of expansion may take place in solids but

in liquids and gases expansion takes place in volume only.

2 General Science

4. Change of physical properties: Application of heat destroys the magnetic

properties of a magnet, decreases conductivity of electricity in metals, changes

solubility of liquids and increases pressure in gases.

5. Chemical change: Sometimes chemical reaction takes place on application of heat.

For example, when carbon is heated it reacts with oxygen of air to form carbon

dioxide.

6. Combustion: Heat helps combustion of combustible substances. Kerosene, petrol,

paper, wood, etc. burn on heating.

7. Production of light: Application of excessive heat leads to incandescence of a

substance i.e. light is produced. The filament of an electric bulb emits light when it is

heated.

Temperature

Suppose, you are given a glass of water and a cup of tea. Touch the glass of water, you

will feel cold. Touch the cup of tea, you will feel warm. We say that the temperature

of a cup of tea is higher than that of a glass of water. The temperature of a hot object

is high and that of a cold object is low. It means temperature of an object is a measure

of how much hot or cold the object is. In other words, temperature is the thermal

condition of an object.

Now drop out some water from the glass and pour tea from the cup into the glass.

Now touch the glass. You will feel it warmer than before. Temperature of tea was

higher than that of water. But when tea was mixed with water, heat was transferred

from tea to water and the temperature of water increased. In the same way, if a piece

of hot iron is immersed into water, iron turns cold and water turns hot. This happens

when heat flows from hot iron to cold water, i.e. from iron of higher temperature to

water of lower temperature. Iron becomes cold by releasing heat and water becomes

hot by absorption of heat. Generally, when two objects are brought in contact with

each other, it depends on the difference of temperature of the two as to which one will

loose temperature and which one will gain it. Heat flows from objects of higher

temperature to objects of lower temperature. Therefore, it can be said that temperature

is the thermal condition of a body. It determines which body will release heat or

receive the same from the other when the two are kept in contact.

Temperature of an object increases when it receives heat and decreases when it gives

out heat. That is, heat is the cause and temperature is the result.

Take a cup of water from a bucket of hot water. Temperature of the water of the bucket

and that of the cup is the same. But the amount of heat in the bucket with much water

is greater than the amount of heat in the cup with less water. So, the two objects with

Heat 3

the same temperature may differ in the amount of heat in them. Conversely, the two

objects even with the same amount of heat may differ in temperature.

Difference between heat and temperature

Heat

1. Heat is a form of energy.

2. Temperature of an object increases

when it receives heat, but decreases

when it gives out heat. It means heat is

the cause of temperature (Exception:

Heat does not increase during melting

and boiling).

3. Flow of heat from one object to

another does not depend on the amount

of heat in them.

4. The amount of heat of two objects

may be the same but their temperature

may be different.

5. The name of the instrument used to

measure heat is calorimeter.

6. Unit of heat is known as calorie in C.

G. S unit and Joule in S.I. unit.

Measurement of temperature

Experiment: You can practice this experiment in your home. Take three bowls or

such other containers. Pour cold water in one; slightly warm water in another and hot

water that your hands can endure in the third bowl. Now immerse one hand (say right

hand) in hot water and the other in cold water. Count up to sixty in imagination. Now

raise both the hands and immerse them in warm water immediately. How do you feel?

You will feel that the same water gives you the sensation of cold in your right hand

and hot in your left hand.

Therefore, temperature of an object cannot be determined correctly by means of

touch. For correct measurement, an instrument is necessary. The name of the

instrument used to measure temperature is thermometer.

4 General Science

Temperature

1. Temperature is the thermal condition

of an object.

2. Temperature is the result of heat.

3. Direction of the flow of heat depends

on temperature.

4. Two objects may be of the same

temperature but the amount of heat in

them may be different.

5. The name of the instrument used to

measure temperature is thermometer.

6. Unit of temperature is degree Celsius

in C.G.S unit and Kelvin in S.I. unit.

Hot Room temperture Ice-cold

Fig. 1.1: Interpretation of temperature on touching may be wrong.

We cannot see temperature. So, it cannot be measured directly. The volume of a liquid

changes with the change of temperature. This change can be observed and measured.

Temperature is measured indirectly by using this property of liquid. Liquids such as

alcohol, mercury etc. are used for this purpose.

Mercury Thermometer

The type of thermometer that uses mercury as an indicator of temperature is called the

mercury thermometer. It consists of a thick-walled glass tube with a narrow and

uniform bore. At one end, there is a thin walled bulb. The other end is kept open at

first. The bulb is filled with pure and dry mercury through the open end. The bulb is

heated and cooled again and again to fill the bulb with mercury. After filling the bulb,

a little portion of the tube is also filled with mercury. Now the entry end of mercury is

heated so that mercury reaches the neck of the tube. At this stage, the open end with

mercury is heated and closed by melting glass. Next the tube is cooled. As a result,

mercury comes down. The rest of the tube does not contain anything except a small

amount of mercury vapour. The tube is then graduated according to a fixed scale for

measuring temperature. The bore in the tube of the thermometer is extremely narrow.

So, mercury goes upward to a great extent if the temperature of the bulb slightly

increases. The walls of the bulb are thin so that heat can pass through it quickly. But

the tube is thick to make it strong.

Scales of temperatureYou know that in order to measure anything the first step is to fix a unit of the thing as

a standard for measurement. In measuring temperature, two fixed temperatures are

Heat 5

considered as standards. These are

known as the fixed points one is the

lower fixed point and the other, the upper

fixed point.

The lower fixed-point: The temperature

at which pure ice melts under normal

atmospheric pressure is called the lower

fixed point.

The upper fixed-point: The temperature

at which pure water boils to vapour

under normal atmospheric pressure is

called the upper fixed point.

The interval between the upper fixed

point and the lower fixed point is called

the fundamental or primary. This interval

is divided into a suitable number of

equal divisions. Each division is said to

be one degree (1̊ ). There are different scales of temperature depending on the

different number of divisions in the fundamental interval. Presently, two scales of

temperature are followed-they are the Celsius scale and the Fahrenheit scale.

1. Celsius scale: The lower fixed point in this scale is marked 0˚ and the upper fixed

point is marked 100˚. The interval between the two points is divided into 100 equal

divisions. Each division is called one degree Celsius (1˚C)

Celsius, a Swedish scientist invented this scale in the year of 1742. The scale is named

the Celsius scale after his name. This scale is used in all types of scientific work.

Many of the countries including ours use this scale. As the interval between the two

points is divided into 100 equal divisions, it is also called centigrade ('centi' means

100 and 'grade' means division).

2. Fahrenheit scale: In this scale, the lower fixed point is marked 32˚ and the upper

fixed point is marked 212˚. The interval between the two points is divided into 180

equal divisions. Each division is called one degree Fahrenheit (1˚F). A German

scientist named Fahrenheit invented this scale in 1714. This scale is mostly used in

England and America for household works and almost in all the countries in clinical

and industrial works.

6 General Science

Fig: 1.2 Mercury Thermometer

Bulb

Mercury

Glass tube

Narrow bore

Relation between the Celsius and the Fahrenheit scales

Suppose, C is the reading on a Celsius scale and F is the reading on a Fahrenheit scale

for the same temperature (Figure 1.3)

Then we get,

The above equation represents the relation between the Celsius and the Fahrenheit

scales.

Example: The normal temperature of a human body is 98.4˚F. What is the

corresponding temperature of 98.4˚ F on a Celsius scale?

So, C= 36.9˚ C (approximately)

Melting point and Boiling point

You know that matter can exist in three states- solid, liquid and gaseous. The most

common example is the three states of water- ice, water and vapour. Ice when

heated, turns into water. Water when heated, turns into vapour. The opposite also

happens. This does not happen to water only. Almost all the matters may have solid,

liquid and gaseous states, if they are heated or cooled enough. Oxygen turns into

liquid at -l83˚C. Even it turns into solid at -219˚C. Iron becomes liquid at 1535˚C

and vapour at 3027˚C.

The process in which a solid turns into a liquid due to heat is called melting point.

Temperature of a solid increases when it is heated. At a certain temperature. the solid

starts to melt. This temperature remains fixed until the whole of the solid turns into

liquid. This fixed temperature is called the melting point of the solid.

Now if heating is continued, the temperature of the liquid continues to increase again.

At a certain temperature, the liquid starts to boil and vaporise. This temperature

remains fixed until the whole of the liquid turns into vapour. This fixed temperature is

called the boiling point of the liquid.

Melting points and boiling points depend on the atmospheric pressure. Melting points

Heat 7

Upper fixed pointLower fixed point

Celsius

Fahrenheit

Fig: 1.3 The relation between the Cclsius and Fahrenheit scales

AX

AB

C-O

100

F-32

180= =

C

5

C-32

9

98.4-32

9= = or,

C

5

66.4

9= = or, c

66.4X5

9

C

5

F-32

9or, =

and boiling points of some substances at normal atmospheric pressure are given below:

Name of the substance Melting point Boiling point

Hydrogen -259˚C -253˚C

Oxygen -219˚C -183˚C

Mercury -39˚C 357˚C

Ice 0˚C 100˚C

Lead 327˚C 1744˚C

Copper 1083˚C 2595˚C

Iron 1535˚C 3027˚C

Maximum and Minimum Thermometer

During the last twenty-four hours, the

maximum and minimum temperatures of

Dhaka were 27˚C and 12.2˚C

respectively. You may have heard this

type of news from radio or television or

may have read in newspaper. Does

someone remain vigilant with a

thermometer throughout the day and

night to measure the maximum and

minimum temperature at a certain time?

No, it does not happen 'so, it needs not.

Because scientists have invented a

special type of thermometer to measure

the maximum and minimum temperature

of a certain time. This thermometer is called the maximum and minimum

thermometer. This thermometer is so constructed that maximum and minimum

temperature of a certain time can be determined by reading the thermometer only

once at the end of the time.

Two separate thermometers may measure the maximum and the minimum

temperatures. But the same thermometer may measure them also. Six's thermometer is

one of those thermometers by which both the maximum and minimum temperatures

of the atmosphere of a certain time can be measured at the same time. It is a U-shaped

narrow glass tube with two bulbs A and B at the two ends (figure 1.4). Bulb A is small

and bulb B is big and long. Bulb B and a part of the tube upto the mark C are filled

8 General Science

A

10

20

B

C

80

70

60

50

40

30

20

10

0

20

10

0

10

20

30

40

50

60

70

80

SD

Alcohol

Mercury

S2

Fig: 1.4 Six's Maximum and Minimum thermometer

Fig: 1.5 Ring and ball experiment

with alcohol. The part of the tube from C to D contains mercury. A part above D and

some part of the bulb A are filled with alcohol. The rest of the bulb A is empty but

contains some vapours of alcohol. There are two metal indicators S1 and S2 at the two

ends of the tube. The indicators are tightly fitted with the walls of the tube by springs.

The instrument is kept vertically in a wooden frame. The two sides of the tube are

fitted with two scales.

Using a strong magnet the indicators are placed in touch with the mercury column

indicating the time of which the maximum and minimum temperature are required to

be determined. Now when temperature increases, alcohol of the bulb B expands and

pushes the mercury column. As a result, the mercury column of the left side rises

upward and pushes the indicator S2 up. Again when temperature decreases, alcohol of

bulb B contracts and pushes the indicator S1 up. But indicator S2 stays in its last

position due to its spring. Positions of S1 and S2 are noted from the scales S1 indicates

the minimum and S2 indicates the maximum temperature of that time.

Thermometer of this type is usually used in forecasting weather and agricultural

research.

Expansion of solids - Coefficient of expansion

Most of the solids expand in length,

area and volume by the application

of heat. Someimes this expansion is

so minor that we cannot see it with

normal vision of our eyes. The

following experiment demonstrates

the expansion of a solid.

Experiment: This experiment is known

as the ring and ball experiment. A brass

ball and a ring are so made that the ball

can simply pass through the ring

when it is cold. But when the ball is heated for a while and placed on the ring, it does

not pass through the ring. This happens because the application of temperature

increases the volume of the ball. After a while, when the ball becomes cold, it will be

found that the ball has hanging passed through the ring again. This proves that when a

solid is heated it expands and when it is cooled it contracts. All solids do not expand

equally even though heat is applied to them to the same extent. Some expand a great

and the others expand a little.

Heat 9

In order to compare the rates of expansion of different solids, the idea of coefficients

of expansion is considered. The expansion of length, area or volume for raising one

degree of temperature is known as coefficient. There are three types of coefficient of

expansion for solid substances:

1. Coefficient of linear expansion: Coefficient of linear expansion is defined as

increase in per unit length of the solid for raising one degree of temperature. It is

denoted by α(Alpha).

2. Coefficient of superficial expansion: Coefficient of superficial expansion is

defined as an increase per unit area of a solid for raising one degree of temperature. It

is expressed by β(Beta).

3. Coefficient of Volumetric expansion: Coefficient of Volumetric expansion is

defined as an increase per unit volume of a solid for raising one degree of

temperature. It is expressed by γ (Gamma).

Coefficient of expansion does not depend on the unit of length, area or volume. It

depends only on the scale of temperature. It means, the unit of coefficient is either per

degree Celsius (/˚C) or per degree Fahrenheit (/˚F). When we say that the coefficient

of linear expansion of an iron rod is 0.000012/˚C, we mean that the iron rod of one

metre will increase by 0.000012 metre in length if the temperature of the rod is

increased by l˚C.

Practical application of expansion of solids

In everyday life, we make many applications of coefficient of expansion due to heat -

1. You may have seen that a cartwheel has an iron tyre fitted on it. How is this iron

tyre fitted on? The diameter of the iron tyre is slightly smaller than the diameter of the

wheel. First the tyre is heated. As a result, it expand in circumference. While it is hot,

it is fitted on the wheel and is cooled by pouring water on it. It contracts due to cold

and fits tightly on the wheel.

2. Sometimes the neck of an inkpot, a medicine bottle or the neck of some glass bottle

is so tightly fitted that it cannot be opened easily. What will you do then? Heat the

neck of the bottle if it is made of metal substances and you will see that it now opens

easily.

3. Sometimes a thick glass breaks into pieces when hot water or tea is poured into it.

The reason is that, the inner part of the glass expands due to the heat of water or tea.

But the outer part does not become so much hot and expand. Due to unequal

expansion inside and outside, the glass breaks into pieces.

10 General Science

Fig. 1.6: Practical applications of expansion of solids

4. You may have seen that there is a gap between two consecutive rails of a rail line.

What is the cause of it? The iron rails turn hot when a train runs on it due to the friction

of wheels with them. As a result, the lines increase in length. If there is no gap left

between the rails the lines will be twisted. This is why gap is kept in between two rails.

5. You may have further observed that telephone, telegraph or electrical wires are

joined loosely between two pillars. This is done with a purpose. During winter or at

night, temperature decreases. As a result, wire contracts and becomes smaller in

length. If the wire is stretched tightly it may be torn due to contraction.

Expansion of liquids

Experiment: Fill up a flask made of

glass with coloured water. Close the

mouth of the flask with a cork and insert

a narrow glass tube through the cork.

You will see that a small amount of

coloured water has come up through the

tube. Make a mark on the tube

indicating the level of water. Now apply

heat to the flask. You will see that water

in the tube falls down first and later

on continues to rise up and finally goes up a little above the mark you made first.

What is its reason? The reason is that the flask gets heated first and expands in

volume. But water does not expand, as it has not yet been hot enough. This causes

the level of water to fall down first. Later water gets heated and expands in

volume. So, the height continues to increase. This experiment further proves that

with the same increase in temperature, the expansion of a liquid is much higher

than the expansion of a solid.

Fitting iron tyre on the cart-wheel Rail line and Fish plate

Heat 11

Fig. 1.7 Expansion of liquid on heating

Mark Line

A liquid has no fixed shape. It has only volume. So, liquids expand only in volume.

Thermometer is constructed using the principle of expansion of liquids.

Expansion of gases

Experiment: Take a big glass flask. Close the mouth of the flask with a cork. Insert a

narrow glass tube through the cork. Drop into the tube a few drop of coloured water.

You will see that the drops of water go down through the tube a little and come to rest

somewhere at A in the tube. Now rub

your hands several times to make your

palms hot and hold the flask tightly in

your hands. You will observe that the

coloured drops of water have gone up in

the tube at A1. This is because heat from

hands made the air inside the flask hot.

As the hot air expands in volume, the

drops of water go up to make room for

the expanded gas. The expansion of

gaseous substance is much higher than

the expansion of a solid or liquid for the same amount of increase in temperature.

Gaseous substances expand in volume only.

Exercise

Multiple choice questions

1. Which one is energy?

a. Coal b. Gas

c. Heat d. Temperature

2. A cup of hot water is taken from a kettle of hot water. For this-

i. the amount of heat of water of both the cup and the kettle will be same.

ii. the temperature of water of the cup and the kettle will be same.

iii. the amount of heat of water of the kettle will be more than the cup.

Which one is correct?

a. i b. i, ii

c. ii, iii d. i, ii, iii

Fig 1.7 Expansion of liquid on heating

Height of LiquidAfter expansion

Before expansion

12 General Science

3. Which one expands more with the same rise in temperature?

a. solid substance b. liquid substance

c. gaseous substance d. all substances expand equally

4 Linear expansion depends

a. on the unit of length

b. on the scale of temperature

c. on both the unit of length and the scale of temperature

d. does not depend on both the unit of length and the scale of temperature

Creative Questions

1.

a. Which relation does the equation above-express?

b. What do you understand by 32?

c. What is the value of F in case of ice with the maximum temperature?

d. Compare the two thermometers with respect to the equation.

2.

Figure

a. What is the co-efficient of linear expansion called ?

b. Explain the cause of the wire's getting loose.

c. How much did the length of the wire get increased ?

d. Explain what is the use of putting gap between the two rail slippers ?

C

5

F-32

9=

before heat after heat

Heat 13

Chapter - 2

Pressure of a Liquid

You have learnt in class six that the weight of an object is the force of attraction with which the earth pulls the object. You have also learnt that pressure refers to a force applied on a unit area. Therefore, any object, which has weight, exerts pressure. Liquid has weight; also it exerts pressure. The force applied by a liquid perpendicularly on a unit area around a point in the liquid is called the pressure of liquid.

Liquid is always kept in a container. Liquid applies pressure on the bottom and walls

of the container.

Characteristics of the pressure of a liquid

There are some characteristics of the pressure of a liquid. A solid substance exerts

only downward pressure but a liquid in addition to its downward pressure also exerts

upward and lateral pressure. We can learn these characteristics by performing some

experiments.

Experiment 1: Take a long can or pot of

tin. Make three holes at different heights up

and down on the same line. Seal the holes

with wax and fill the can with water. Now

open all the holes at a time. You will

observe that jets of water are coming out of

the holes in a position perpendicular to the

wall of the container. However, the jets of

water can-not proceed perpendiculary too

far, since the gravitational force of the earth

pulls the jets downward. You will further

observe that water coming out of the

highest hole has the least speed and falls

nearest to the can. Water coming out of the

lowest hole has the greatest speed and falls

further away.

What's the reason? The reason is that the pressure at any point in a liquid depends on

the depth of water upto the level at that point. The topmost hole has the least depth.

So, the pressure of water at that level is the lowest. So at that level, water pressure is

low. Therefore, water comes out slowly from the topmost hole and falls nearer to the

Fig: 2.1 Pressure increases with thedepth of the liquid

container. The depth of liquid is the greatest at the lowest hole. Therefore, the depth of

water and consequently the pressure at that level is the highest. So, water comes out

with the highest speed from this hole and falls furthermost away.

What did we learn from this experiment? We came to know that the pressure of a

liquid depends on the depth of the liquid. The more the depth, the more the pressure

is. Again water comes out laterally through all the three holes. Therefore, water exerts

lateral pressure. Liquid exerts lateral pressure perpendicular to the wall of the

container.

During the construction of a dam the lower part of the dam is made wider than the

upper part. Can you say why this is done?

Experiment 2: Take a glass funnel. Close the mouth of the funnel tightly with a thin

piece of rubber. Connect one end of a rubber tube with the funnel tube and the other

end with a manometer. Manometer is a simple device for measuring pressure. It is a

U-shaped glass tube. A quantity of liquid, e.g., water or mercury is poured into the

tube. A scale is attached with the manometer. Difference in heights of the liquid

between the two sides is measured with the scale.

Now fill up a large beaker or a glass container with water. Immerse the funnel with its

mouth downward into water. Look at the manometer. What will you see? You will see

that the level of liquid in the left arm

of the manometer with which the

funnel has been connected with the

rubber tube has gone down whereas

the level of liquid has gone up in the

other arm. What is the reason behind

it? The reason is that water exerts

pressure on the rubber attached to the

mouth of the funnel. This presses the

air inside the funnel. The air in turn presses the liquid of the manometer. This causes

the level of liquid goes down. You can find the difference in the heights of liquid in

two arms by readings from the scale.

Now keeping the funnel at the same depth, turn its mouth to all directions, i.e, turn

upward, downward and laterally. Keep your eyes fixed on the manometer. You will see

that there had been no change in heights of the liquid. What does this prove? This

Fig: 2.2 Pressure at any point in a liquid isequal in all directions

Glass tube

Rubber tube

Pressure of a Liquid 15

proves that liquid exerts upward, downward and lateral pressure and that pressure at

any point in a liquid is the same in all directions.

If you submerge the funnel further deep into water, you will see that the difference in

heights of the liquid in two arms increases. This proves that pressure of liquid

increases with the increase in depth.

Now take liquids of different densities in separate containers and repeat the same

experiment. You will see that pressure at the same depth in a liquid depends on the

density of the liquid. The more is the density of a liquid, the more is the pressure.

Experiment 3: Take a glass cylinder

with both ends open. Take also a thin

metal disc with a hook such that it can

close the lower end of the cylinder. Tie a

string to the hook and close the lower

end of the cylinder by pulling the disc

upward. At this stage submerge a part of

the cylinder vertically into water. Release

the string; take care that no water enters

into the cylinder through the sides of the

disc.

It will be observed that the disc does not

fall down even if the string is released;

the disc remains attached to the lower opening of the cylinder. What is the reason?

The reason is that the upward pressure of water keeps the disc stuck to the opening of

the cylinder.

Now take some coloured water in a container and pour it slowly into the cylinder. You

will see that when water inside and outside the cylinder comes to the same level, the disc

falls down spontaneously. Why does this happen? This happens because water inside the

cylinder exerts downward pressure on the disc whereas water outside the cylinder exerts

upward pressure. When water inside and outside the cylinder come to the same level, the

upward and the downward pressure of water on the disc becomes equal. As a result, the

disc falls down due to its own weight. Therefore, we observe that the upward and the

downward pressure of a liquid are equal at any point at the same depth of the liquid.

Fig: 2.3 Upward and downwardpressure of liquid is equal

16 General Science

The specific characteristics, which we came to know from the above experiments are:

1. The pressure at any point in a liquid at rest depends on the depth of the point. If-

depth increases, pressure also increases. If depth decreases, pressure also decreases;

2. All kinds of liquid exert upward, downward and lateral pressure;

3. Liquid, at rest, exerts pressure at a point within it in all directions with equal

magnitude;

4. The pressure at any point in a liquid at rest depends on the density of the liquid. If

the density of a liquid increases, pressure also increases. If the density decreases,

pressure also decreases and

5. The upward pressure at any point in a liquid at rest is equal to the downward

pressure.

Pascal's Law:

Pascal, a French scientist, conducted a

good number of experiments on liquid

pressure. In one of his experiments,

he made a special type of spray. This

spray had a hollow sphere attached to

its one end. Some small holes of

equal size were made on the sphere

and the holes were sealed with wax.

The sphere was filled up with water

and wax was removed from the holes

with a pin. Pressure was then applied

on water by pressing the piston of the

spray. It was observed that water was coming out with equal speed from all the holes.

He concluded from this experiment that the pressure of the piston was distributed in

water equally in all directions. He expressed his conclusion in the form of a law,

which later on was termed as Pascal's Law after his name.

Pascal's Law: Pressure exerted anywhere upon confined liquid is equally transmitted

with undiminished magnitude in all directions and acts at right angles to the surface in

contact with the liquid.

An experimental verification of Pascal's Law:

Experiment: Take a rubber ball. Make a small hole on its surface to pour water

through it. Fill up the ball with water. Make some holes on the ball's surface with a

Fig: 2.4 Pascal's experiment


pin. Now close the first hole with your

thumb and press on other parts of the

ball. You will see that water comes out

through all the holes at right angles to

the surface of the ball and with equal

speed. Therefore, it is proved that if

pressure is exerted upon a confined

liquid at any point, it is transmitted in all

directions equally, i.e., it remains

undiminished and acts at right angles to the surface of the container.

Upward pressure decreases weight

Submerge a football into a bucket of water and keep on holding it under water. It will

appear to you as if someone is pushing the ball upward. Take your hands off; the ball

will jump up and float on water. Can you say what the cause is? The upward pressure

of water is the cause.

Immerse a mug in a bucket of water and fill it up with water. You will feel that the

mug is much lighter when it is held under water. It will appear to be almost

weightless.

If you tie a brick with a rope and immerse it in water, you will feel that the brick is

lighter than before. It is felt heavier when it is pulled up above water. The weight

seems to be much less. We can determine by the following experiment to what extent

the weight of a body decreases when it is immersed in water.

Experiment: Hang a piece of stone or a

brick by the hook of a spring balance.

Determine its weight from the reading

of the balance. Now immerse it into the

water of a container full to the brim

(see figure 2.6). Some water will be

displaced from the container. Store the

displaced water in another container.

The volume of the displaced water

equals the volume of the object. The

weight of the object immersed in water

can be determined from the reading of

the spring balance.

Fig: 2.5 Proof of Pascal's Law

Fig: 2.6 Weight of a solid decreases in liquid

18 General Science

This weight will be less than the weight determined earlier. Now if the weight of the

displaced water is measured it will be observed that it equals the weight lost by the

object immersed in water. The weight of the object decreases to the extent to which

upward pressure of water acts on it. In other words, weight of displaced water =

upward pressure of water = apparent loss of weight.

Therefore, it is observed that when a solid is immersed in a liquid at rest, the liquid

exerts an upward pressure on the solid. As a result, the solid seems to lose some

weight. This apparent loss in the weight of the solid is equal to the weight of the

liquid displaced by the solid. This principle is known as Archimedes Principle.

Archimedes, a famous Greek scientist, discovered this principle about 2200 years ago.

Which object sinks, which object floats

When an object is immersed in a liquid partially or totally, two forces act on it; one is

the weight of the object, which acts downward and the other is the upward pressure of

the liquid that works upward. If the weight of the object becomes lower than the

upward pressure of the liquid, the upward force will push it up. As a result, the object

will float on the liquid partially immersed. On the other hand, if the weight of the

object is greater than the upward pressure of the liquid, the object will sink because of

the greater force downward. However, what will happen if the weight of the object

equals the upward pressure of the liquid? In this state, the object will neither float nor

sink. It will keep on floating totally immersed in the liquid.

The upward pressure of a liquid equals the weight of the liquid displaced by an object.

Therefore, the object floats on the liquid when the weight of the displaced liquid is

greater than the weight of the object. Whereas the object sinks into the liquid when

the weight of the displaced liquid becomes lower than the weight of the object. When

the weight of the displaced liquid equals the weight of the object, the object floats at

any position in the submerged condition.

Now can you say why a piece of iron sinks into water, whereas a ship made of iron

floats? The reason is, when a piece of iron is immersed in water it displaces an

amount of water equal to its volume. Since the weight of iron is greater than the

weight of equivalent volume of water displaced by it, iron sinks into water. A ship,

though made of iron, is hollow and its bottom is much wide, so it displaces a large

volume of water. The weight of this displaced water is much greater than the weight

of the ship. So, the ship floats on water. The weight of a liquid depends on its density.

If density increases, weight also increases. Now say, if a ship enters from sea into a

river, will it float sinking more or less? Sink or float in sinking position?


Hydraulic press

Hydraulic press is an example of a practical application of the Pascal's Law. You know

that the same force increases pressure if it is applied on a small area and decreases

pressure if it is applied on a large area. It can be demonstrated with the help of

Pascal's Law that the same pressure when applied on a larger area increases the force.

In other words applying a low force on one side of a hydraulic press, a high force may

be obtained on the other side. This principle of increasing force has been illustrated in

figure 2.7. This container is a U-shaped cylinder full of water. One arm of the

container is slender and the other is wider. Both arms have watertight pistons.

Suppose, the cross-section of the slender arm, has an area of 10 sq. cm and the cross

section of the wider arm has an area of 1000 sq cm.

We know, pressure = force/area. Therefore, if a weight of 10 Newton is placed on the

slender arm, it will produce 10/10 = 1 Newton/cm2 of downward pressure on the

liquid. According to Pascal's Law, this pressure will be transmitted unchanged through

the liquid in all directions. That is, this pressure will act upward on the wider piston,

consequently the upward pressure on the wide piston will be 1x1000 = 1000 Newton.

Therefore, applying a force of 10 Newton on the slender piston, a force of 1000

Newton, i.e., 100 times more may be produced on the wider piston. Hydraulic Press is

constructed on the basis of this principle of increasing force.

Hydraulic Press: When piston P2 (see fig 2.8) is pulled up by lever L, valve V2

remains closed. But valve V1 opens and water from a reservoir accumulates in

container A. Again when piston P2 is pushed down again by lever L, valve V1 closes

and valve V2 opens due to the pressure of water. As a result, water is transmitted from

Fig: 2.7 Principle of increasing force Fig: 2.8 Hydraulic Press

20 General Science

container A to container B through a metallic tube greater than that of piston P2, the

force produced on piston P1 will be much larger than the force applied on piston P2. In

the process, an extreme pressure against a metal sheet will contract any object placed

on the platform of piston P1. After the completion of a cycle of work, the connecting

key T between the reservoir and the metal tube is opened. At this position, piston P1

comes down and water returns back to the reservoir from container B. This machine is

used for pressing bales of jute, cotton, and clothes etc. extracting oil from oil seeds

and for such other works.

Exercise

Multiple Choice Questions

I. Which kind of pressure does a liquid exert?

a. on1y downward pressure

b. only lateral pressure

c. only upward pressure

d. downward, upward and lateral pressure.

2. What happens when the depth of a liquid increases?

a. Downward pressure increases, upward pressure decreases

b. Upward pressure increases, downward pressure decreases

c. Both the upward pressure and downward pressure increases

d. Both the upward pressure and downward pressure decreases

3. Which one is correct in case of pressure at a point of a liquid?

a. It exerts more downward pressure.

b. It exerts more upward pressure.

c. It exerts more lateral pressure.

d. It exerts same pressure in all directions.

4. Which statement is correct?

a. At all points in a liquid upward pressure and lateral pressure remain same

b. At the time of swimming the weight of displaced water and the weight of

the body become same.

c. The pressure at a point of a stone will be less than the pressure at a point of

a liquid at rest.

d. The weight of a body decreases with the increase in density


From the statement below, answer the questions 5 and 6.

Suman and Rahim made a journey by boat. By sudden storm the boat sank Suman

was able to come to the shore but Rahim drowned because he did not know how to

swim. The villagers rescued Rahim.

5. Rahim drowned because the weight of Rahim was

a. one half of the weight of the displaced water

b. half of the weight of the displaced water

c. equal to the weight of the displaced water

d. more than the weight of the displaced water

6. The weight of the displaced water at the time of swimming was

i. equal to the weight of Suman

ii. more than the weight of Suman

iii. less than the weight of Suman


a. i. b. ii

c. iii d. i, ii & iii

7. The bucket made of iron is easy to pull under water but difficult to pull over water,

because

i. the weight of the bucket on the water is real weight.

ii. the weight of the bucket on the water is more.

iii. the weight of the bucket appears less under the water.


a. i, ii b. i, iii


Creative Questions

1.

Diagram

watertank

22 General Science

1

The key 'K' is opened.

a. Through which pipe will water not come out?

b. Write down the causes of coming out of water from the pipe.

c. Explain what will happen if the second, third and fourth pipe are closed

d. Compare the relative velocities of water coming out of the pipes.

Figure

a. Which law is applied for the figure above?

b. Explain from the figure, the principle of increase of force.

c. Find out the upward pressure on the wide piston for the force 20 Newton.

d. Compare the effect of pressure on piston A2, if the force applied on piston A1

is 50 Newton with the pressure exerted when the force is 20 Newton.

2nd timethe force

is 50Newton

First timethe force

is 20Newton

A1 = 15 square meter A2 = 150 square meter


Chapter - 3

AtmosphereWhat is atmosphere?

You have learnt in class VI what air is. Air is an element. The earth is surrounded by

air. It may be said that we live at the bottom of a vast and deep ocean of air.

If you go up by an aeroplane, will you get the same air as you get on earth? Will the

density, composition and temperature of air remain the same? Scientists have

observed that when one goes up higher, the air becomes lighter and its temperature

and amount of constituent gases changes. At the end, there will be no existence of air.

It is not yet known correctly as to how above the air exists.

This vast and invisible air surrounding the earth is called atmosphere. The

gravitational force of earth attracts the atmosphere around it. The earth rotates along

with the atmosphere attached to it.

Layers of atmosphere

Atmosphere is divided into five layers according to composition, density, temperature

and other characteristics of air. They are- (1) Troposphere, (2) Stratosphere, (3)

Ozonosphere, (4) Ionosphere, (5) Exosphere and Magnetosphere.

1. Troposphere

This is the lowest layer of the atmosphere. It has touched the earth surface. Most of

the air of the atmosphere, about 75% of the atmospheric weight, belongs to this layer.

The air of this layer contains dust particles, moisture and clouds. Natural events such

as wind, rain, storm, lightning etc. occur in this layer. This is why, this layer is also

called the weather layer. There exists difference in temperature of the air in this layer.

When one goes up higher from the earth the density and temperature of air of this

layer also decreases. Temperature falls down by 10˚C per kilometre increase in

altitude. The lowest temperature of the layer is 56˚C. As this layer is the lowest one,

the density and pressure of this layer is the highest. So, this layer is called troposphere

or densed sphere. This layer extends upto 16 kilometres from the earth's surface. We

live in this layer. This layer protects us from the extreme heat of the sun during the

daylight. Again for this layer, cold cannot be severe during night. This layer also

protects us from various fatal rays coming from the space.

2. Stratosphere

The next layer above the troposphere is the stratosphere. This layer extends over about

50 kilometres. The air in this region is very light and almost no moisture exists. For

this, this layer is called the cool sphere. There is no storm, rain or thunder. This layer is

very cold. Temperature ranges from about -56˚C to - 60˚C. The temperature increases

gradually with the increase in altitude. Temperature increases to O˚C at the top of this

sphere. Usually, jet planes fly through this layer.

3. Ozonosphere

This layer extends over 30 kilometres just above stratosphere. This layer mostly

contains Ozone gas. This is why it is known as Ozonosphere. Heat absorption

capacity of Ozone gas is very high. This layer absorbs the solar heat and ultra violet

rays. As a result, this layer has a high temperature. The highest temperature is about

20˚C. But temperature decreases at the top-end and falls down to about -90˚C. Since

Ozone gas in the upper part of the layer is higher, its temperature decreases.

Ultraviolet rays are very harmful to the living world. This layer protects the living

world of the earth by absorbing ultra-violet rays.

4. Ionosphere

Ionosphere lies above the ozonosphere.

This layer extends over 300 kilometres

from the end of the Ozonosphere.

Various gases such as Hydrogen,

Helium etc. exist in this layer in

ionised state. Charged particles are

called ions. This is why, this layer is

termed as Ionosphere. This layer has a

special characteristic of reflecting radio

waves.

Radio waves transmitted from a

radio station are reflected by this

layer and can reach far away locations

Therefore, you, who stay far away from a radio station, can listen to the radio

programmes by receiving them in radio. Aurora or polar lights are seen in this layer.

Temperature of this layer is very high as this layer absorbs various rays coming from

the sun. Its average temperature is about 1100˚C. Shooting stars are seen in this layer.

5. Exosphere and MagnetosphereThese two are the highest layers of atmosphere and exist above the ionosphere. Space

begins at the end of this layer. These layers extend over 400 kms to 750 kms from the

surface of earth. Air is very light in these layers. Mainly hydrogen gas exists in ionized state

and nitrogen and oxygen do not exist in these layers. Here temperature is about 2000˚C.

Fig. 3.1: Different layers of atmosphere

Temperature (Celsius)

Hei

ght

(Kil

om

etre

)

Hei

ght

(Mil

e)Kilometre

Atmosphere 25

Atmospheric Pressure

We know that air has weight. In the case of a particular atmospheric layer, the weight

of the air of other layers above it falls on it. Air exerts force downward due to its

weight. As a result, pressure is created. In this way, the pressure that atmosphere

exerts on any plane attached to it is known as atmospheric pressure.

The word pressure refers to the magnitude of force applied on unit area. So,

atmospheric pressure means a magnitude of pressure applied by the atmosphere on a

unit area of a place. This force is equal to the weight of the air falling vertically on a

unit area.

Air is very light. The density of air is about one thousandth of the density of water.

But atmosphere extends over hundreds of kilometres above the surface of the earth.

This vast and deep atmosphere exerts much pressure on us. The pressure per square

centimetre is about 10 Newton i.e. I kg weight. Since we are accustomed to live in

such a pressure, we do not feel it. Besides we have air inside our body also. The

pressure inside our body maintains equilibrium with the outside pressure. Our blood

has pressure too. This pressure is slightly higher than the atmospheric pressure. The

earth's surface lies at the bottom of atmosphere. So, the atmospheric pressure on the

earth's surface is the highest. The more we go upwards, the more the pressure

diminishes since the amount of air diminishes with the altitude.

Air exerts pressure in all directions

Air does not exert pressure downward only. Like liquids, air exerts pressure in all

directions. We can prove it by experiments.

Experiment-I

Air exerts lateral pressure:

Take a thin tin can. Be sure that its lid

closes the can perfectly airtight. Firstly,

open the lid and pour a little water into the

can. Apply heat to the can till water boils.

water will vaporize and go out of the

mouth. Stop heating before most of the

water turns into vapour. Close the can quickly with the lid and make it airtight. Now

cool the can by pouring water over it. You will observe that the more the can cools, the

more the can buckles inward. What may be the reason for it?

Fig. 3.2: Lateral pressure of air

26 General Science

Experiment- 2:

Air has upward pressure: Fill up a

glass completely with water. Carefully

cover the glass with a card so that

there is no air inside the glass. Hold

the card tight to the glass with one

hand and invert the glass with the

other hand. Now remove your hand

from the card. You will see that the card

sticks to the glass and does not fall down. What did we prove from this experiment?

There is no air but only water inside the glass. This water exerts downward pressure

on the card. Since the card does not fall down due to this downward pressure, there

must be an upward pressure of air on the card. In this case, the upward pressure of air

is higher than the downward pressure of water. So, the experiment proves that air has

upward pressure.

Experiment - 3:Air has downward pressure: Take a

tin can with one end open and the

other end connected with a tube. Tie

the open end of the can with a sheet

of rubber fitting perfectly airtight.

Connect the bottom tube to an air

suction pump.

Now if the pump sucks out the air

from inside the can, the rubber sheet

is found to be depressed inside and

after a while, it bursts with a loud sound.

This happens due to the downward pressure of air. At the beginning, there was air

both inside and out side the can. As air pressures on both sides of the rubber sheet was

equal, the sheet remained stretched. Later when air is sucked out, pressure inside the

can decreases. At this time, the pressure due to outside air depresses the sheet inside

and after a while it bursts out. Thus the experiment proves that air has downward

pressure.

Experiment-4:

Air exerts pressure in all directions- Magdeburg's experiment with hemisphere: In

1654, Otto Von Guericke, Mayor of Magdeburg city of Germany, performed this

experiment to prove the existence of air pressure. He used two hollow and hard metal

Fig. 3.3: Upward pressure of air

Upward pressure

Fig. 3.4: Downward pressure of air

Downward pressure

of air

air

Atmosphere 27

hemispheres. The hemispheres were so made that they could be fitted perfectly

airtight. One of the hemispheres was fitted with a tube through which air could be

sucked out. He fitted the hemispheres airtight and sucked out the air from inside the

hemispheres by a suction pump. It was observed that the hemispheres could not be

separated even when they were strongly pulled by holding their handles. This happens

because there was no outward pressure from inside the hemispheres due to the

sucking out of air inside them. As a result, air pressure from all sides pressed the

hemispheres very hard. When there is air inside the hemispheres, they can be easily

separated. This is because pressure of air inside and outside the hemispheres are equal

at this time. This experiment proves that air pressure in all directions.

Experiment of Air pressure in all directions

Fig. 3.5: Magdeburg's experiment with hemisphere.

Guericke in his original experiment used two hemispheres with the diameter of about

60 centimetres each. When air from inside was exhausted, even a group of sixteen

horses, eight on each side, failed to separate the two hemispheres. Now you may

think, how tremendous the influence of atmospheric pressure is.

Toricelli's experiment:

Torricelli was an Italian scientist. He was the first to measure atmospheric pressure in

1643. He filled up completely a hard glass tube about one metre long and with 1 cm

diameter filled with mercury. Later on, closing the open end with a finger, the tube

was inverted vertically over a trough of mercury. As soon as the finger is removed

from the open end, the mercury column descended a little. The mercury column came

to rest at a height of about 76 cm. The vacant space above mercury surface in the tube

contained no air. This space without air is known as Torricellian vacuum.

Torricelli explained the event stating that atmosphere exerts downward pressure on the

free surface of the mercury in the trough. According to Pascal's theory, this pressure is

transmitted throughout the liquid in all directions with magnitude undiminished. It

thus exerts pressure on the mercury inside the tube in the upward direction. This

pressure keeps the mercury column inside the tube standing.

28 General Science

It is also evident that the space above the mercury column inside the tube is devoid of

air. So, there is no air pressure. As such, the only pressure that exists inside the tube

and acts on the surface of mercury of the trough is the pressure due to the mercury

column only. The outside pressure on the same surface of mercury is the atmospheric

pressure. Since these two pressures are equal, they maintain equilibrium. Therefore, it

is only the atmospheric pressure that keeps the mercury column inside the tube

standing. Had there been no atmospheric pressure, the mercury column inside the tube

would have come down due to the gravitational force that the Torricellian vacuum is

devoid of air can be easily proved. If the tube is gradually inclined keeping its end

open and dipped in mercury, the length of the vacuum decreases but the height of the

mercury column remains the same. If the tube is further inclined, the vacuum is

completely filled up by mercury. This proves that Torricellian vacuum does not

contain air. In fact, the place is not totally vacant- a small amount of mercury vapour

exists there.

The length of the Torricellian vacuum decreases or increases as the tube is gradually

moved up and down keeping its open end dipped inside mercury. But this does not

cause any change in the height of the mercury column inside the tube. This shows that

the height of the mercury column depends on the atmospheric pressure. Mercury

column rises upto the extent as much as atmospheric pressure can support it.

What do we learn from the Torricelli's experiment? We learn that:1. In a one-end open-air free tube, mercury rises up due to atmospheric pressure.

2. The mercury column is high as much as it can be supported by the atmospheric pressure.

76cm 76

cm

Torricelli's experiment

Fig. 3.7: Torricelli's vacuum

Torricelli's vacuum

Ven

tral

posi

tion

Fig. 3.6: Torricelli's experiment

Atmosphere 29

3. Pressure exerted by the mercury column of the tube is equal to the pressure of the

atmosphere.

Though Torricelli used mercury in his experiment, any liquid may be used to perform

the same with the same result. However, the height of the liquid column depends on

the density of the liquid. If the density is low, the height increases. For example, if

water is used, the height of water column will be more than 10 metres because

mercury has a density of 13.6 times more than that of water.

Measurement of atmospheric pressureWe observed from Torricelli's experiment that the atmospheric pressure is equal to the

pressure of mercury column. If the height of the mercury column is 76 cm, the

atmospheric pressure is equal to the pressure of a mercury column of 76 cm. It means

atmospheric pressure is equal to the weight of the mercury of column 76 cm high on a

unit area. This is how Torricelli first measured atmospheric pressure. For this reason,

atmospheric pressure of a place is expressed in terms of the height of the mercury

column. For example, atmospheric pressure is 76 cm of mercury. This means,

atmospheric pressure is the pressure of 76 cm high mercury column.

Normal atmospheric pressureAtmospheric pressure varies from

place to place on the surface of the

earth. Atmospheric pressure at any

place depends on the altitude, latitude

and temperature of the place.

Therefore, standard or normal

atmospheric pressure is considered to

be the atmospheric pressure at a

definite altitude, definite latitude and

definite temperature. This definite

altitude is the altitude of sea level, the

definite latitude is 45˚ and the definite

temperature is 0˚C. In other words, the

pressure exerted by a column of mercury of height 76 cm, at the sea level at. 45˚

latitude at 0˚C temperature is known as the standard or normal atmospheric pressure.

This pressure is called one atmospheric pressure or one 'bar'.

Barometer:

The name of the instrument used to measure atmospheric pressure is Barometer.

Barometer may be of different kinds. Liquid is used in some cases, for example,

mercury barometer, water barometer. In others no liquid is used, for example, aneroid

Fig. 3.8: Simple Mercury Barometer

Vessel

Vacuum

Glass tube

Atmospheric pressure

Mercury

Scale

76 cm

1 Metre

30 General Science

barometer etc. Fortin's barometer is used for accuracy in measurement. Construction

of this type of barometer is a bit complicated. You will know about it in the higher

classes.

The instrument, which Torricelli used in his experiment, was a simple mercury

barometer. It means, it is a glass tube of one square cm diameter and one meter long

with thick walls and closed at one end. The tube is filled with mercury. Care is taken

so that no air bubble exists inside the tube. Later on, closing the open end with a

finger, the tube is inverted and dipped vertically in a trough of mercury. The finger is

removed and mercury falls down a little inside the tube. The height of mercury up to

the point where mercury comes to rest represents the atmospheric pressure. Height is

measured from a scale attached to the tube. Simple mercury barometers are used in

places where there is no need to know the atmospheric pressure minutely. The height

of mercury column increases if pressure increases and the height decreases if pressure

decreases.

ManometerManometer is a device to measure pressure difference. It

is a U shaped glass tube with both ends open. The tube is

partly filled with any liquid. Mercury is used to measure

major differences in pressure, whereas oil, alcohol or

water is used to measure minor differences. When both

ends of the tube are open, pressure on both sides are

equal. So, the height of the liquid at both ends is also

equal. Now if pressure is increased on one side say,by

blowing into it, height of the liquid on that side

decreases, whereas height of the liquid on the other side

increases, as shown in the figure. When the liquid is still, A and B are on the same

level. Therefore, pressure is equal at both A and B. Pressure at B equals to the sum

total of the atmospheric pressure and liquid pressure of height H. The difference of

pressure between the two sides of the tube is calculated from height H.

Change of atmospheric pressure and weather A low pressure has developed in the

Bay of Bengal and as a rasult of which cyclone may occur in the coastal area"- you

might have listened this type of weather forecast from radio, television or have read in

newspapers. It is very important to know about atmospheric pressure to forecast

weather. The condition of weather depends on atmospheric pressure. Because

atmospheric pressure depends on the temperature of air and the amount of moisture in

it. Air becomes lighter when its temperature increases resulting in decrease in air

pressure. Air pressure also decreases due to the increase of moisture since moisture is

lighter than dry air. So, weather may be predicted for a place by observing changes in

its atmospheric pressure.

Fig. 3.9: Manometer

Atmosphere 31

A gradual decrease in atmospheric pressure indicates the increase of moisture in air

and possibility to rain. A sudden decrease in atmospheric pressure indicates the

formation of a low pressure. So, there is a possibility of rushing of air from all around

to that area, which may forecast a storm or cyclone.

If air pressure increases gradually, amount of moisture in air decreases. It means an

improvement in the weather condition. The weather will be dry and the sky will be

clear. This is how weather may be predicted from atmospheric pressure. Atmospheric

pressure is determined from the readings of a barometer. An increase in the height of

the mercury column of a barometer means an increase in atmospheric pressure.

Use of air pressure

There are many practical applications of

air pressure in our day-to-day life.

Often, we use a narrow plastic tube or

straw to drink liquids such as Coca-

cola, seven up or others. What do we do

then? We dip one end of the tube into

the drink and suck from other end the

air inside the tube by our mouth. As a

result, air pressure inside the tube

decreases and that outside the tube forces

the drink into our mouth. Have you ever seen a doctor pushing an injection? In

injections, doctors use a syringe which has a needle with a fine hole in it. When the

needle is dipped in the liquid medicine

and the piston of the syringe is pulled

upward, the area below the piston partly

becomes a vacuum (piston is so tightly

fitted with the walls of the syringe that

air can not pass through the sides).

As a result, pressure inside the syringe decreases and liquid enters into the syringe

through the needle due to air pressure from outside. When the needle is withdrawn

from the liquid, it is again the air pressure which forces the liquid not to come out of

the needle. Needle is then injected into the body and the piston is pressed which

increases the pressure on liquid medicine resulting in the entry of medicine into the

body through the hole of the needle. A spray also works on this principle.

Fig. 3.10: Drinking with a straw

Fig. 3.11: A spray and a syringe

32 General Science

SiphonThe device used to transfer liquid

from one container to another without

moving the containers is known as

siphon. It is a U-shaped (h1, h2) glass,

rubber or plastic tube with both the

ends open. The two sides of the tube

are unequal. The tube is filled up

with the same liquid, which is to be

transferred and the shorter arm is

dipped into the vessel containing the

liquid. The longer arm is dipped into

the other vessel. The first vessel is

kept at a higher level than that of the

second vessel. It is observed that liquid flows through the tube from the upper vessel

to the lower one.

Siphon has many practical applications. For example, siphon technique is used to fill

up a small container with oil from a big drum. Flushing system in a modern toilet

works on the same principle.

A bicycle pump and a vacuum pump represent two more examples of practical

application of air pressure. If you have a bi-cycle or play football, you may have used

the bi-cycle pump. Often the principle of a vacuum pump is used in the brakes of

vehicles.

Exercise


I. Which layer reflects radio waves?

a. Troposphere b. Stratosphere

c. Ozonosphere d. Ionosphere

2. What is the normal atmospheric pressure per square centimeter?

a. 10 Newton b. 100 Newton

c. 1,000 Newton d. 10,000 Newton

3. What does Torricellian vacuum contain?

a. nothing b. air

c. a few mercury vapours d. moistures

h1

h2

C B

A

D

Fig. 3.12: Siphon

Atmosphere 33

Answer the questions 4 and 5 from the following information:

A thin plastic bottle was half filled with hot water. It was kept in the refrigerator. After

sometime it was found that the bottle had been squeezed. But when cold water was

poured in the bottle and the same thing was repeated, the bottle did not squeeze.

4. The bottle squeezed because

a. the pressure outside and inside the bottle is not same

b. due to the atmospheric pressure of the refrigerator.

c. due to sudden cooling.

d. to keep confined in the refrigerator.

5. In case of cold water we cannot see this fact. Because in this case

i. there is no moisture in water.

ii. the pressure of air outside and inside the bottle remain same.

iii. the amount of moisture is very less inside the bottle.


a. i, ii b. ii, iii

c. i, iii d. i, ii, iii

6. If a glass of water is taken at the top of stratosphere then the temperature of water

will be-

a. -56˚C b. -30˚C

c. O˚C d. -40˚C

7.

The cause behind the squeeze of the second balloon in figure is-

a. lateral pressure of air b. upward pressure of air

c. downward pressure of air d. pressure of air in all direction

First balloon Second balloon

34 General Science

Creative Questions

1. A glass tube of one meter is taken, whose one end is open and the other end is

closed. The area of the open end is 1 square centimeter. Making the tube air free,

it is immersed perpendicularly in a vessel filled with mercury. Mercury goes up

inside the tube. Then another tube of the same shape filled with air is immersed.

a. Why does the mercury go up?

b. What will be the maximum possible height of mercury? Explain.

c. Explain the state of mercury after immersing the glass tube with air.

d. Can we use water instead of mercury in this case? Give your opinion.

2. Solim needs to pour water on his head .as he was suffering from fever. His mother

hung a bucket of water as she was busy. She took the measure of pouring water by

connecting a rubber tube and taking out some water through mouth.

a. What is the name of the system of pouring water?

b. Why did his mother take out some water through her mouth?

c. Explain the possible way of pouring water.

d. Discuss the use of this system in our daily life.

3. The height of the Himalayan is about 7 kilometer. The peak is covered with snow.

At the month of April-May, when the temperature at the feet of the Himalayan is

40˚C, a group of mountaineers started to climb the Himalayan.

a. In which layer of atmosphere does the Himalayan stand?

b. Explain what type of air pressure the mountaineers will feel at the time of

climbing.

c. Find out the temperature at the peak of the mountain.

d. Find out from which height snow will start to fall, mentioning the change in

temperature per kilometer.

Atmosphere 35

Chapter- 4

Oxygen

Oxygen is a component of air

In class VI, we have learnt that air is a mixture of different elements. The main two

components of air are oxygen and nitrogen. Besides there, air is a mixture of carbon

dioxide, moisture, inert gases and dust particles also. You have also learnt the

percentage composition of the constituents of air in terms of their volume and weight

in class VI. With the help of different experiments we have come to know that one-

fifth of the volume of air is oxygen and almost four-fifth of the volume is nitrogen.

So, it can be certainly said one of the two main components of air is oxygen and the

other is nitrogen.

General concept about oxygen

In 1774 Joseph Priestly an English Scientist first discovered oxygen by heating. He

prepared this analyzing an oxide of mercury (Mercuric Oxide) by heating. Almost at

the same time, a Swedish scientist named Carlshele and a French scientist Lavoisier

discovered the same gas through individual experiments. French scientist Lavoisier by

analyzing the properties of the gas, named it oxygen. The meaning of oxygen is acid

producer. Scientist Lavoisier thought that oxygen is a component of almost all acids.

That is why he named it oxygen.

Preparation of oxygen

The compounds which contain huge number of oxygen, by heating any of them.

Oxygen can be prepared in the laboratory. In class six, we have learnt that by flowing

electricity through water containing a small quantity of sulphuric acid, we can get

oxygen. But this is not a good process of preparing oxygen. Because this is very slow

and also expensive. Generally, two methods of preparation of oxygen are followed in

the school laboratory; Such as-

(1) Mixing hydrogen peroxide with manganese dioxide.

(2) Heating a mixture of potassium chlorate and manganese dioxide

Let's prepare oxygen in the laboratory by adding hydrogen peroxide with Manganese

dioxide.

Experiment:

For this experiment required apparatus

and materials are: one big test-tube,

5/6 small test-tubes for collecting

gas, one dropper, one delivery tube,

a beaker (full of water), one test-tube

stand, a two hole cork to fit in the

mouth of the wide test tube,

hydrogen peroxide and manganese

dioxide. Take a small quantity of

manganese dioxide (that can contain

upon a 0.5 taka coin) in the clean big test tube.

Fix the two-hole rubber or wooden cork at the mouth of the big test tube. Pull the

hydrogen peroxide with the dropper and insert it into the test tube through one hole of

the cork. Fit the delivery tube as shown in the fig. 4.1 above through the other hole of

the cork and dip the other end of the delivery tube into the beaker which is full of

water. By using wax seal the surface of the cork and the bottom of the dropper as well

as the delivery tube carefully. A test tube filled completely with water set invertedly in

such a way that the end of the delivery tube rests a little inside the test tube.

Now drop a few drops of hydrogen peroxide on the manganese dioxide and observe

closely. What do you see? A chemical reaction has started inside of the test tube.

Bubbles of a gas are going towards the top of the test tube through the delivery tube

and water level of the test tube is going down slowly. When the whole water of the

test tube will go down then you have to realize that the test tube has become filled up

with the gas. Now by using your thumb, at first shut the mouth of the gas filled test

tube and remove it very carefully. Then shut the mouth of the test tube with a cork and

keep it in a test tube stand. In this way, to do different tests of oxygen fill up 5/6 more

test tubes with the prepared gas and store them after shutting their mouth with cork

properly. Now with the collected gas of the test tubes do various experiment to test

different properties of oxygen.

You have observed that the prepared gas pushed down the water of the water filled test

tube. This method of collecting a gas over water is called downward displacement of

water. In this experiment, we took hydrogen peroxide and manganese

Figure 4.1 : Preparation of oxygenfrom hydrogen per-oxide

Dropper

Hydrogen

per-oxide

Manganese

dioxide

Oxygen 37

dioxide in the wide test tube. Is there any change between the two chemical

substances ? Yes, there is a chemical reaction inside the test tube. We can express the

chemical reaction in words in the following way:

Hydrogen peroxide + Manganese dioxide Oxygen + water [Manganese dioxide]

You have seen that hydrogen peroxide has decomposed into oxygen and water.

Manganese dioxide has not taken part in the chemical reaction. If manganese dioxide

does not take part in the chemical reaction, why it should be used ? We will know the

answer of the question after a short while. Before this we will know how oxygen can

be prepared if hydrogen peroxide is not available in the laboratory.

If hydrogen peroxide is not available in the school, then you can prepare oxygen gas

by heating a mixture of potassium chlorate and manganese dioxide. You can get

oxygen by heating potassium chlorate only. But it takes much time and heat. But if a

little quantity of manganese dioxide is mixed with potassium chlorate, may be

produced oxygen in lower temperature and in a short time also.

Experiment:

Mix five parts of potassium chlorate

intimately with one part of manganese

dioxide. Take the mixture into 4 wide

hard glass test tube. Shut the mouth of

the test tube with a one holed rubber

cork and then add a delivery tube.

Now clamp the test tube with a stand

as seen in the figure 4.2. Then

immerse the other end of the

delivery tube into a water filled beaker. Now apply heat to the test tube slowly.

Oxygen will come out through the mouth of the delivery tube in the form of bubbles.

At first allow some gas to come out. Do you know the reason? The inside air of the

test tube will come out too. Now take a water filled test tube, hold it inverted over the

mouth of the delivery tube and collect the oxygen in the test tube. The chemical

reaction that takes place here is as follows:

Potassium chlorate + [Manganese dioxide]

Oxygen + Potassiumchloride + [Manganese dioxide]

Figure 4.2 : Preparation of oxygen from potassium chlorate

Oxygen

Gasjar

38 General Science

Catalyst:

At the time of preparing oxygen from hydrogen peroxide or potassium chlorate we

have used manganese dioxide. We have seen that manganese dioxide was unchanged

at the end of the experiment and it did not take part in any chemical reaction. We have

said earlier that oxygen can be made if only potassium chlorate is heated. Of course, it

takes much time and heat. But addition of a small quantity of manganese dioxide

helps to produce oxygen within very short time and at low temperature.

Similarly, hydrogen peroxide is decomposed and produce oxygen in presence of light.

But this reaction is so slow that it continues for months together. Presence of a small

amount of manganese dioxide decomposed hydrogen peroxide and produces oxygen

quickly. In no case of above, manganese dioxide takes part in the chemical reaction.

In this way, the substance which does not take part in the chemical reactions directly

but its presence accelerates or retards the speed of the chemical reactions and itself

remains unchanged even after the reaction is called a catalyst. In the preparation of

oxygen, manganese dioxide acts as a catalyst. Without being involved in a chemical

reaction directly, it can increase the rate of the reaction. The matter might have

ashtonished you as to how does it happen? Suppose, you have gone to a stadium to

watch a football match between the Abahoni and the Mohamedan. You are a supporter

of one of the two team. Many other supporters of that team like you are encouraging

their players often throwing emotional shout and clap. With this, the mental strength

of the players increase and as a result, the speed of the game also increases. In this

care, you don't play but your presence and encouragement have accelerated the speed

of the game. Here you have also acted as a catalyst.

Properties of Oxygen

Test the properties of oxygen with the gas which you have collected in 5/6 test tubes

following any process that have described above.

(1) Take a test tube filled with the gas and

observe closely. What do you see ? The gas

does not show any colour. That means, it can be

mentioned that oxygen is a colourless gas.

(2) Take off the cork of a test tube and take the

smell of the gas. Have you got any smell? No

smell has been got. Oxygen is an odourless gas.

(3) Take of the cork of another test tube and

check its taste. Have you got any taste of it ?

There is no taste. Oxygen is a tasteless gas.

Fig. 4.3: Oxygen is a colourless and odourless gas

Oxygen 39

(4) Through downwards displacement of water this gas has collected. Oxygen gas is

lighter than water and slightly soluble in it. For this reason, almost the whole gas

comes up as bubbles and stored in the test tube. Oxygen get dissolve in water. With

this oxygen, aquatic plants and animals maintain their breathe. Fish and other aquatic

animals breathe with oxygen through their gills.

(5) (a) Lid a lighted candle with a glass jar. After a little while the candle will be

quenched. The candle was inflamed as long as oxygen exists in the air of the jar.

Lastly after in flaming when total oxygen will be finished the candle will be

quenched.

(b) Put off an inflamed jute stick by a whiff. Put the jute stick into a gas jar filled with

oxygen. What have you seen? The jute stick has inflamed again. So, it can be said that

oxygen helps combustion.

(c) Take a piece of glowing coal in a spoon. Put it into a oxygen filled gas jar. What do

you see ? The coal has inflamed again. That means oxygen supports combustion.

(6) Hold an inflamed jute stick or

match stick in the mouth of a gas

filled test tube. You will see that, the

stick is inflaming more luminously.

But the gas inside the tube is not

burning. From this experiment, it can

be realized that oxygen itself does not

burn but it helps combustion.

(7) Take an oxygen filled test tube and

hold it invertedly over a air filled test

tube. After a short time, put a light

inflamed stick into the second test tube. What happened? The light inflamed stick

inflamed more luminously again. That means oxygen moved down from upper test

tube to lower one and the air of the lower test tube moved up to the upper test tube.

Therefore, it can be said that oxygen is slightly heavier than air.

Oxygen in combustion

Oxygen helps in combustion. Oxygen itself does not burn. But without oxygen it is not

possible to make fire. In class six, we have seen through experiment that an inflamed

candle slowly went out when it was covered with a glass. In fact, the process of

combustion is nothing except addition of oxygen. During combustion of wood or coal,

huge quantity of oxygen is added and produce enormous amount of heat and light.

Flameless sticks blowingFig. 4.4: Oxygen itself does not burn,

it helps combustion

40 General Science

Flameless

blowing stick

oxygen filled

gas jar

Oxygen is a very active element. It combines with metals such as iron, copper,

sodium, magnesium etc. and forms metallic oxides. These metallic oxides are

generally basic.

Oxygen + Metal Metallic oxide

Similarly, oxygen combines with non-metals such as, carbon, nitrogen, sulphur,

phosphorus etc. and forms non-metallic oxides. Generally nonmetallic oxides are

acidic.

Oxygen + non-metal non metallic oxide.

Combustion of non-metals with oxygen

Wood, coal, kerosene, candle, etc. are

all compounds of nonmetals. At the

time of inflaming, molecules of these

compounds combine with oxygen.

This addition evolves in the form of

heat and light. The inflammable

substances like candle, coal, wood,

kerosene contains carbon which

combines with oxygen to form

carbon dioxide.

Experiment:On a combustion spoon take a inflaming candle. Insert it into a oxygen filled gas jar.

What do you see ? The candle is inflaming more luminously. After inflaming a short

time, bring out the combustion spoon with the candle and then shut the mouth of the

jar with a glass lid. Now drop a piece of blue litmus paper soaked in water into the gas

jar. Is there any change of colour of the litmus paper ? The paper appears slightly red.

A property of an acid is that it changes the colour of a blue litmus into red. Therefore,

we can say the combustion of a candle in an oxygen filled gas jar produces such type

of substance that is acidic. Now in that jar pour a small quantity of clear lime water

and shake it. Do you find any change ? The clear lime water becomes milky. As a

result of combustion, carbon of the candle combines with oxygen and produces

carbon dioxide. You know that carbon dioxide turns clear lime water into milky. We

can express this through the following way.

Carbon + Oxygen Combustion Carbon dioxide

Carbon dioxide + Calcium hydroxide Calcium carbonate + Water (clear lime water)

Now examine what happens when a non-metal sulphur burns with oxygen.

Figure 4.5 : Burning candle in oxygen

Oxygen 41

Experiment:

In the science laboratory of your school there must be sulphur powder available. Take

a small quantity of sulphur powder in a combustion spoon and set fire to it. Now insert

the combustion spoon into a oxygen filled gas jar. What have you seen ? Sulphur is

inflaming with a bluish flame. After combustion, drop a blue litmus paper soaked in

water into the jar. Is there any change of the blue litmus paper? During combustion of

sulphur in oxygen, sulphur combines with oxygen and form sulphur dioxide. Sulphur

dioxide is acidic. Acid turns blue litmus to red. That's why blue litmus becomes red.

Sulphur + Oxygen Combustion Sulphur dioxide.

We can come to the conclusion from the experiment that-

(a) Oxides of non-metals form when nonmetals burn with oxygen.

(b) Produced non metal oxides are acidic.

Combustion of metals in oxygen

Metals like- iron, copper, sodium, etc. combines with produces metallic oxides when

they are burn in oxygen.

Experiment:

Take a piece of magnesium ribbon with the help of a

crucible tongue and set it on fire. Now insert it in a

oxygen filled gas jar. What do you see? Magnesium is

inflaming with a much luminous flame and after

combustion white ashes of magnesium oxide have found

in the test tube.

Magnesium + Oxygen Combustion Magnesium oxide

After combustion drop a piece of wet red-litmus paper inside the test tube. Is there

any change of colour of the litmus paper ? Yes, the red colour of the litmus paper turns

into blue. That means, magnesium oxide produced from the combustion of

magnesium in oxygen is basic.

Oxygen for respiration

Oxygen is indispensable for respiration of man and other animals. If there was no

oxygen in the air, human beings and other animals could not live. For weak lungs,

particularly pure oxygen is necessary. In hospitals when the dying patients can not

Fig. 4.6: Combustion of Magnesiumin oxygen

42 General Science

breathe from air than pure oxygen is supplied through their nose. The patients of

asthmatic troubles are some times kept under oxygen tent. The mountain riders, the

astronauts, the aeroplane pilots, the drivers and the crew of submarine use small and

portable oxygen cylinders for breathing. Miners who work deep into the ground for

extracting minerals also carry oxygen cylinders and they use it of the time of their

requirement. Fire fighters also use oxygen masks.

Other uses of oxygen

Besides combustion and respiration

oxygen is used for differrent purposes.

Oxygen is used for various welding

works. Perhaps you might have seen the

welder working in factories. Welders

join together two pieces of metals. For

this they have to melt down the metals.

To do this work, a very hot flame is

necessary. In order to make such type of

very high temperature flame. Oxygen is

burnt with another gas like hydrogen or

acetylene. The flame produced from a mixture of oxygen and hydrogen is called

oxyhydrogen flame. Temperature of this flame is about 2800˚ Celsius. The flame

produced from a mixture of oxygen and acetylene is called oxyacetylene flame.

Temperature of this flame is 3100˚C to 3315˚C.

The heat of oxy-hydrogen or oxy-acetylene flame is well enough for melting, cutting

and welding metals. In rocket and jet planes liquified oxygen is used. Besides this,

oxygen makes our drinking water taste and kills various germs.

Role of oxygen in rust formation on iron:

We know that, there are oxygen and water-vapour in air. If we keep a piece of bright

iron bar or a glazed knife anywhere for a few days, it will be seen that one kind of

layer has been formed on it. This layer is known as rust. Oxygen of the air comes in

contact of iron and make one kind of iron-oxide. This is rust. We have seen earlier that

oxygen reacts with metals to form metallic oxides. Iron is a metal. This metal reacts

with oxygen in presence of water or water-vapour to produce iron-oxides.

Iron + Oxygen + Water-vapour Iron-oxide or rust.

So, oxygen plays the principal role in the process of rusting on iron. In the process of

rusting on iron, the role of oxygen and water or watervapour can be proved by

performing the following experiment.

Fig. 4.7: Oxy-acetylene flame

Oxy-acetylene flame

Oxygen 43

Experiment:

Keep four iron wires one in each of the four test tubes. Marked them as A, B, C and D

and put them as shown in figure below.

'A' open and empty test tube ( in fact full of air).

'B' open and half or partly water filled test tube. The wire is submerged one-half in

water.

'C' Take a test tube filled with water boiled for 20 minutes. The mouth of the tube is

closed with a cork. The cork is sealed tightly with wax so that air can't enter. The

wire is fully immersed.

'D' Open the test tube filled with kerosene. The wire fully immersed into kerosene.

Keep them in this way form a week. After that observed the wires, which one has

rusted? Which one hasn't?

In test tube 'A', there was air that means oxygen. And a very small quantity of water

vapour. So, the wire of 'A' test tube did not get rust within a week but if it is left out

for many more days then it would be rusted.

In test tube 'B,' there were both air or oxygen and water too. So, the wire of test tube

'B' got rust with in one week. Test tube 'C' was air tight. That means there was no

oxygen at all but water was there. The wire kept in it couldn't get rusted. The wire of

test tube 'D' was immersed under kerosene. Here neither oxygen nor water was

present. So, the wire kept in it couldn't get rust too.

Figure 4.8 : Experiment on rusting of Iron

From this experiment we have learnt that for the formation of rust on iron both water

and oxygen are necessary. In the rainy season of our country as air contains enough

water vapour, so all iron made materials like frying-pan, matter, rod, spade, sickle,

plough, etc. get rusted. Rust is an oxide of iron. Iron turns into iron oxide when it gets

rust. Rust is deposited on metal surface like a layer. When the metal is uncovered by

(A) (B)

Water KeroseneBoiled

Water

(D)(C)

44 General Science

removing the rust, it rust again. In this way, the whole metal gradually turns into rust.

Needle, pin, clock-spring, corrugated iron sheet, agricultural equipments, container or

tin, etc. must get rusted if preventive measure is not taken.

Rust prevention

A very wise way to prevent iron from rusting is to add a coating on its surface with

certain material that can protect it from the rust. Such type of coating is made with

paints, enameling materials, grease or molten metals (like coating of zinc or tin). The

coating of molten zinc or tin is called galvanizing. The corrugated iron (C.I) sheets

that are used to make our houses are galvanized with zinc. Milk powder cans are

galvanized with tin. Moreover, the new metallic substance, which is made by mixing

of molten iron with a small quantity of carbon, chromium and nickel, does not rusted

also. This mixture or alloy is called stainless steel.

Exercise


1. What are the products of a reaction between Potassium chlorate and Manganese dioxide?

a. Water and Oxygen

b. Water, Oxygen and Manganese dioxide

c. Potassium chloride and Oxygen

d. Potassium chloride, Water and Oxygen

2. How is Oxygen gas collected in the laboratory?

a. By downward displacement of air

b. By downward displacement of water

c. By upward displacement of air

d. By dissolving in water

3. Which of the following statements expresses the correct property of Oxygen?

a. Oxygen is soluble in water

b. Oxygen itself does not burn, but helps burning

c. Oxygen is lighter than air

d. Oxygen itself neither burns nor support combustion

Oxygen 45

4. Burn out a candle inside a gas jar filled with Oxygen. What will happen if a small

quantity of lime water is poured in this gas jar?

a. Lime water will remain unaltered

b. Lime water will turn milky

c. Lime water will be more clear

d. Lime water will become red

5. Which of the following is necessary for rusting iron?

a. Water b. Oxygen

c. Kerosene oil d. Water and Oxygen

6. What will happen if you burn completely a small piece of Sodium in a jar full of

Oxygen and then drop into the jar a piece of red litmus paper and a piece of blue

litmus paper?

a. Blue litmus will turn red, red litmus will turn blue

b. Red litmus will turn blue, no change of blue litmus

c. No change of red litmus, blue litmus turns red

d. No change of color to litmus.

7. Acidic Oxide-

i. Phosphorous pentaoxide

ii. Zinc oxide

iii Sulpher dioxide


a. i b. i, ii


Answer the questions 8-10 from the following information.

A worker could not join two pieces of iron applying much heat. The other worker took

him to another workshop nearby. He joined those pieces of iron quickly by the flame

he got there. Using another flame, he saw that it more time to join the iron pieces.

8. Which substance is the first flame made of?

a. Oxygen and Acetylene b. Oxygen and Hydrogen

c. Oxygen and Nitrogen d. Oxygen and Ethylene

46 General Science

9. Which substance is the second flame made of?

a. Oxygen and Acetylene b. Oxygen and Hydrogen

c. Oxygen and Nitrogen d. Oxygen and Ethylene

10. What is the temperature of the second flame?

a. 2800˚C b. 3100˚C

c. 3100˚C - 3315˚C d. 3315˚C

Creative Questions

I. Aluminium, Coal, Iron, Wax and Sulphur are burnt in Oxygen. Analyzing the

characteristic of the product it is seen that some are acidic and some are basic.

a. What will be produced if we burn coal?

b. Explain the properties of the product from the combustion of wax.

c. Write down the equation in words if substances except coal and wax take part

in combustion.

d. Explain with reason which oxides are more harmful produced by the combustion

of Iron.

2. (1) Hydrogen per-oxide Water + Oxygen

(2) Potassium chlorate potassium chloride + Oxygen

Sodium is added with Oxygen produced from the reaction (2)

a. What is 'X'?

b. Reduction of which one is easy between reaction (1) and (2)? Explain.

c. Which will be the property of the compound made by the reaction of oxygen

and hydrogen? Explain.

d. Explain the role of' X' in performing the reaction (1) and (2).

X

X

heat

Oxygen 47

Chapter - 5

Hydrogen

Until now 111 elements have been discovered in the world. Scientists have arranged

these elements in a table. This table is called the periodic table. Hydrogen is the first

element of this table.

In 1766, Henry Cavendish first realized the existence of this gas. In contact with fire,

it flames. That is why, he named the gas "flammable air". In 1788, scientist Lavoisier

proved that, hydrogen is an element. Hydrogen produces water when it is burnt in air.

So, he named this gas Hydrogen. 'Hydro' means water. As Hydrogen is the main

element of water, it has been named so. Hydrogen is not available in the earth's crust

or in the atmosphere as an element. But hydrogen exists with a compound. For

example, water, sugar, starch, fat, natural gas, acid and many other substances are

compounds of hydrogen. The major part of our body is water. Moreover, there are

carbon, hydrogen and oxygen compounds and other elements. They are chemically

combined with each other.

Preparation of Hydrogen

Generally, in the laboratory hydrogen is prepared by carrying out chemical reaction of

a dilute acid with a metal. Cavendish himself prepared hydrogen through this process.

Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, etc. all are metals.

But generally zinc is used to prepare hydrogen in the laboratory. Potassium and

sodium are never used for this purpose. Because these two metals react so vigorously

with acids that this may cause of danger. Let us prepare hydrogen gas in the

laboratory and test its properties.

Experiment:

In the laboratory, hydrogen is prepared by reacting zinc with dilute sulphuric acid.

Hydrogen is a component of acid. Zinc displaces hydrogen from acid and form zinc

sulphate, salt and hydrogen.

Zinc + Sulphuric acid = Zinc sulphate + Hydrogen

Procedure

Take some pieces of zinc in a two-nect (Woulfs bottle). Fix a one hole cork in one

mouth of the bottle. Pass a thistle funnel with a long tube through the hole of the cork

in such a way that the end of the long

tube reaches almost the bottom of the

bottle. To the other mouth of the

bottle fix an one-hole cork also.

Through the hole of the cork, pass

one end of a delivery tube as shown in

the figure. Now pour some water

through the funnel so that the long

tube remains dipped in the water but

the mouth of the delivery tube rests well

above the water. Seal carefully the bases of the cork with wax.

To test that the apparatus has become air-

tight, give a puff through the interior-end

of the delivery tube. If the apparatus is

air-tight, water of the bottle will rise

through the tube of the thistle funnel and

if the open end of the delivery tube is shut

with the help either of your thumb or your

tip of tongue the inside layer of the water

will not come down. If the apparatus is

not fully airtight, the evolved gas of the

bottle will leak out and may cause if there

is any flame nearby. So, during prepration

of hydrogen never keep flame in the surroundings. Dip the interior-end of the delivery

tube into a water contained vessel. Now pour some dilute sulphuric acid in the bottle

through the thistle funnel. Zinc and acid will take part in the chemical reaction and

produce gas which will pass through the delivery tube. Firstly, wait for a while so that

the evolved gas and air mixture can completely flushed out from inside of the bottle.

Place a water filled test tube on the delivery tube inverted. You will see that, hydrogen

gas is coming up in the form of bubbles and water of the test tube is going down

gradually. Collect the gas in a few gas jars instead of test tubes. Still keeping

submerged under water, shut the mouth of the jar with a lid, lift it and then put it on

the table with its mouth down. Now test and observe the properties of hydrogen with

the gas collected.

Fig. 5.1 (B): Experiment for air-tightnessin prepataion of Hydrogen

Fig. 5.1 (A): Preparation of Hydrogen

Sulphuric Acid

Hydrogen

Water

ZincChips

Hydrogen 49

Fig. 5.3: Hydrogen burns with bang

Properties of Hydrogen

(1) Hydrogen is a colourless, odourless and tasteless gas. If you look through the test

tube or glass jar filled with hydrogen, it

will appear colourless. Smell the gas, it

has no smell. Have a taste with your

tongue, it has no taste.

(2) Hold a hydrogen gas filled gas jar

invertedly with its mouth down and put

off the lid by your hand. Enter an

inflamed stick into the jar. What have you

seen ? The flame of the stick extinguished

instantly but the hydrogen gas itself

began to turn with a bluish flame at the

mouth of the jar. From this, it can be understood that hydrogen itself burns but does

npt help others to burn.

(3) Hold a hydrogen gas filled test tube

inclined in such a way that a mixture of

hydrogen and air form at the mouth of the

test tube. Now if a inflamed stick is put into

the mouth of the test tube, hydrogen will

instantly inflame making a bang.

(4) Hydrogen is lightest element. Air is

14.4 times heavier than hydrogen. Place

an air-filled jar on the top of a hydrogen

filled jar in a slightly inclined position.

Now remove the lid of the hydrogen

filled jar. Hydrogen is lighter than air

and so within a few moments (say 15

seconds only) hydrogen will go up and

occupy the whole space by pushing the

air from inside the air jar. Air will then

occupy the lower vacant jar.

How would you identify whether the upper jar had been filled up by hydrogen and the

lower jar with air? Insert a inflamed stick into the lower jar. What have you seen? The

stick is inflaming. Similarly pour another inflamed stick into the upper gas jar. What

do you see ? At the same time of entering, the inflamed stick is extinguished and the

Fig. 5.2: Test of inflamability of hydrogen

Fig. 5.4: Hydrogen is lighter than air

Hydrogen

air

50 General Science

gas starts burning at the mouth of the jar. From this experiment it is understood that

hydrogen gas moved from the lower jar to the upper jar. Hydrogen gas is lighter than

air. For this reason, a balloon filled with hydrogen floats up in the sky.

(5) Hydrogen is insoluble in water. It has been collected by the downward

displacement of water. If hydrogen was soluble in water, it would not be possible.

From this, it can be realized that hydrogen is insoluble in water.

(6) Put small pieces of wet red and blue litmus papers into a hydrogen filled gas jar.

Has there been any change to the colours of litmus papers? No, the colours of red and

blue litmus has no change. From this it can be noticed that hydrogen is neither acidic

nor basic. Hydrogen is a neutral gas.

(7) You have analysed water when you were in class VI. Could you remember the

components that you got from analysis of water? You got two parts of hydrogen and

one part of oxygen. That means water is produced from two components of Hydrogen

and oxygen. So, it can be said that hydrogen burn in air to produce water.

Uses of Hydrogen

(I) Hydrogen is the most well known lightest matter. So, balloons filled with hydrogen

can be used for flying. But it is very risky to fly with a balloon filled with hydrogen

gas as it is highly inflammable. So, now a days another light gas called helium instead

of hydrogen is used for flying.

(2) Hydrogen is used in the manufacturing of margarine, banaspati ghee and many

other edible oils like dalda from vegetable oils like cotton seed oil, almond oil,

sunflower seed oil, etc.

(3) Hydrogen is used in the production of artificial petrol or lubricating oil from coal.

The importance of hydrogen will increase when whole reserve of petroleum will be

finished.

(4) Hydrogen is used in the preparation

of nylon, plastic like - p.v.c, etc.

(5) In chemical industries, hydrogen is

used for preparing different chemicals.

Such as ammonia (a compound of

hydrogen and nitrogen). Urea fertilizer

and various explosives are prepared

from ammonia.Figure 5.6 : Use of Hydrogen in

preparing Margarine

Hydrogen 51

(6) This gas is used in the manufacture of Hydro-chloric acid. Besides, hydrogen is

used in the manufacture of various organic compounds and chemicals such as -

Methyl alcohol.

(7) Hydrogen is used as a reducing agent. Removal of oxygen from any substance is

called reduction. The substance which removes oxygen is called a reducing agent.

(8) Hydrogen, mixed with oxygen, form oxy-hydrogen flame. The temperature of this

flame is about 2800˚C and steel can easily cut with this flame. Oxy-hydrogen flame is

used in cutting, melting and welding of metals.

Exercise


1. A inflamed match stick was put into a bottle filled with a gas. The inflamed stick

extinguished and the gas fired itself.

What gas was in the bottle?

a. Hydrogen b. Oxygen

c. Nitrogen d. Carbon dioxide

2. What is produced when dilute Sulphuric acid is mixed with Zinc?

a. Zinc and Sulphuric acid b. Water and Hydrogen

c. Hydrogen and Zinc sulphate d. Water and Zinc sulphate

3. Which of following does not produce Carbon dioxide when burnt?

a. Hydrogen b. Kerosene

c. Wood d. Oil

4. An air tight jar contains two gases. The gases are fired in such a way that air

cannot enter into the jar. The mixture explodes instantly. What are the gases in

the mixture?

a. Hydrogen and Carbon dioxide b. Oxygen and Carbon dioxide

c. Hydrogen and Oxygen d. Nitrogen and Carbon dioxide

5. A test tube filled with a gas was covered with a lid. Inclining the test tube slightly

and removing the cork, a inflamed match stick was put into it. A bang was heard

instantly. What was there in the test tube?

a. Nitrogen b. Hydrogen

c. Oxygen d. Carbon dioxide

52 General Science

6. For what property of hydrogen balloon filled with hydrogen can float a heavy thing?

a. Hydrogen is insoluble in water

b. Hydrogen is an element

c. Hydrogen is the lightest substance

d. Hydrogen is available

7. At present why are balloons filled with hydrogen not used for flying?

a. Hydrogen is not available in air

b. Hydrogen is much expensive

c. Hydrogen is a heavy gas

d. Explosion takes place in Hydrogen gas

Answer the questions 8 and 9 from the following information.

There are two gases in a closed vessel. When the mixture of gases is fired, it is highly

ignited and almost 2800˚C temperature is produced.

8. What are the gases in the mixture?

a. Hydrogen and Carbon dioxide b. Oxygen and Carbon dioxide

c. Hydrogen and Oxygen d. Hydrogen and Carbon dioxide

9. The flame produced by the combustion of the mixture is used in-

i. extracting metal

ii. melting metal

iii. for welding


a. 1 b. i. ii


Creative Questions

If an inflamed match stick is kept at the edge of an inverted test tube filled with a gas, it

is instantly extinguished. But the gas is lighted with blue. flame at the edge of the tube.

A flame of high temperature is produced if we mix the gas with oxygen and burn it.

a. What is the gas in the test tube?

b. How do you understand whether that gas is heavier or lighter than the air?

c. How is this gas used in cutting ships?

d. Explain whether the colour of the litmus will be changed if we keep wetted

litmus at the end of the test tube filled with that gas.?

Hydrogen 53

2.

a. What is the name of the produced gas?

b. Why is the gas jar filled with water?

c. What will happen if we use a gas jar filled with air? Give your reason

d. Explain whether the gas is acidic or basic.

Fig : The production of 'x' gas

Hydrochloric acid

Produced gas

water

zinc

54 General Science

Chapter - 6

Carbon dioxide

Carbon dioxide, a component of air

We have learnt about the components of air. Carbon dioxide is a component of air.

Volumetrically, there are 0.04% carbon dioxide in the air. Carbon dioxide is formed

by exhalation of animals and combustion of fuels. In certain locations it emerges from

deep in the earth's crust. In 1630 A.D scientist Van Helmont first discovered carbon

dioxide.

Generally, carbon dioxide is produced is any carbon containing organic substance

such as wood, coal, oil, paper, etc. is burnt. So, carbon dioxide is always prepared in

various industries and even in kitchens.

Carbon dioxide - a compound

In the previous class we have discussed about elements and compounds. Could you

remember what is called a compound ? When two or more elements combine together

and form a new substance having different properties then the new substance is called

a compound. Like water, carbon dioxide is also a compound. Carbon and oxygen

chemically react to form carbon dioxide.

Carbon + Oxygen = Carbon dioxide.

Preparation of carbon dioxide

Usually, carbon dioxide gas is prepared by reacting carbonate compound with an acid.

In the laboratory, carbon dioxide is prepared by reacting calcium carbonate or marble

chips with dilute hydrochloric acid. The reaction that happens between calcium

carbonate and dilute hydrochloric acid is as follows:

Calcium carbonate + Hydrochloric acid = Carbon dioxide + Calcium chloride + Water

Method of preparation

You have to arrange the same apparatus for preparation of carbon dioxide almost in

the same way as the apparatus you arranged for the preparation of hydrogen gas. As

carbon dioxide is heavier than air, so it is collected by upward displacement of air.

Description of procedure

Take some pieces of calcium carbonate or marble chips in a round bottom flask. Close

the mouth of the flask with a two hole cork. Insert a thistle funnel through one hole

and put at the other hole of the

delivery tube which is bent in right

angle at twice. Place the thistle funnel

in such a way that, the lower end of

the tube reaches almost the bottom of

the flask. Fix the delivery tube in such

way that one end of it rests at a

position very high above the bottom

of the flask and the other end dips into

a gas jar. Check the apparatus carefully

to be sure that it has become airtight.

Pour dilute hydrochloric acid through the thistle funnel so that tip of the funnel dips

under the acid. The moment the acid comes in contact with the calcium carbonate or

the marble chips, chemical reaction will start and the evolving gas will come out

through the delivery tube as bubbles. Carbon dioxide is heavier than air. So by

pushing the air upwards, carbon dioxide collected in the jar. This process of collecting

a gas is collect upward displacement of air. In this way, fill up 4/5 gas jars with carbon

dioxide, close their mouths with lids and then test the properties of the gas.

Properties of carbon dioxide

(1) Carbon dioxide is a colourless gas. Closely observe a carbon dioxide filled gas jar

- the gas is not visible. Because it has no colour.

(2) Have a smell of the gas through your nose and take taste of it with your tongue.

This gas has slightly sweet smell and a sour taste.

(3) Put an inflamed stick into a gas filled jar. What do you see? Is this gas burning

itself? Is this gas helping the stick to burn? The inflamed stick goes out as it is entered

into the gas filled jar and the gas also does not burn.

Therefore, it can be realized that

carbon dioxide does neither burn itself

nor help other things to burn too.

(4) Carbon dioxide is much heavier

than air. For this reason, it can be

poured from one pot to another pot

like water.

'A' is a air filled gas jar. It contains air.

Put an inflamed candle in 'A' jar.

Figure 6.1: Preparation of Carbondioxide gas

Hydrochloric Acid

Carbon dioxideMarble Chips

Figure 6.2: Carbon dioxide doesnot help combustion

56 General Science

What do you see? The candle is burning. 'B' is a gas jar full of carbon dioxide. Tilt the

carbon dioxide filled jar over the air filled jar. What do you see ? The inflamed candle

went out. That means when carbon dioxide filled jar is inclined or turned upside down

on the top of the air jar then carbon dioxide will be stored in 'A' jar. The air will go

out. Carbon dioxide does not burn itself or help others to burn. This is why, the candle

gets extinguished.

As the gas is much heavier than air it is found stored in huge quantity at the bottom of

deep wells, mines or in the decks of big ships. If any person goes down into these

place, he suffers from suffocation within a short time due to lack of oxygen. Even he

can die also.

Figure 6.3: Carbon dioxide is much heavier than air.

(5) Carbon dioxide makes lime water milky. If carbon dioxide is passed through a

clear lime water contained in a test tube then the lime water turns milky. Lime water is

calcium hydroxide. Calcium hydroxide reacts with carbon dioxide to produce

insoluble calcium carbonate.

Figure 6.4 : Carbon dioxide turns clear lime water milky.

A B

B

Carbon dioxide

Lime water

Carbon dioxide 57

Calcium hydroxide + Carbon dioxide = Calcium carbonate + Water (lime Water)

But, if excess carbon dioxide is passed through lime water, insoluble calcium

carbonate will be dissolve with soluble calcium hydrogen carbonate. As a result, the

lime water becomes clear again.

Calcium carbonate + Carbon dioxide + Water = Calcium hydrogen carbonate.

(6) Carbon dioxide is soluble in water. Take some cold water in a container. Cover the

mouth of a gas jar filled with carbon dioxide with a lid and turn the jar upside down

and dip it in the water of the container with the lid and remove the lid. Has the water

risen up in the jar ? Yes, you will see that, water has risen up in the jar. What does it

means? This gas is soluble in water. Close the mouth of the jar with the lid and take

out the jar from water. Now the jar contains water and gas. Drop a piece of blue litmus

paper into it and shake the jar well. Is there any change in the colour of the litmus

paper? Yes, the blue litmus has turned into red.

Carbon dioxide react with water and produce carbonic acid. Blue litmus has turned

into red because of this acid.

Water + Carbon dioxide = Carbonic acid

(7) Take a small piece of Magnesium on a combustion spoon and set it on fire. Put the

combustion spoon with flamed magnesium in the carbon dioxide filled jar. What do

you see? Magnesium is burning. Magnesium flame is spreading out. Black patches of

carbon have developed on the wall of the jar. Magnesium burns in carbondioxide to

produce magnesium oxide and carbon.

Magnesium + Carbon dioxide = Magnesium oxide + Carbon

From this experiment it is seen that carbon dioxide is formed with two elements

carbon and oxygen.

Uses of Carbon dioxide

Carbon dioxide is soluble in water. Normally water

can dissolves carbon dioxide gas of equal volume.

But its solubility increases with the increase of

pressure. By dissolving this gas at excessive pressure,

mineral water, soda water, lemonade, etc. are

prepared. In cold drinks, like- coca cola, fanta, seven-

up, etc. carbon dioxide is used at very high pressure.

Bottles are shut very tightly with metal corks so that

the pressure of inside can not drop. When bottles are

opened, pressure drops and carbon dioxide comesFigure 6.5 : Carbon dioxide in

cool water

58 General Science

out in the form of bubbles. Carbon dioxide can be converted into liquid and solid

forms by cooling it by increasing pressure. In cooled temperature that means at 78˚C,

the gas directly turns into solid without being liquid. As solid carbon dioxide looks

like ice, so it is called dry ice. When kept at normal temperature, it does not melt but

sublimes directly to gas. During this transformation, it absorbs three times more heat

compared to ice.

For this reason, it is used as a refrigerant or cooling agent in refrigerators. Street

hawkers sell ice creams from three wheeler vans. Dry ice is used to keep the ice-

cream cold or frozen foods frozen.

We know that carbon dioxide is heavier than air and this gas does not help to burn. So,

this gas is used in extinguishing fire. Carbon dioxide is specially suitable for

extinguishing fire that breaks out from electrical faults or chemical explosions. In

these case, use of water is dangerous. Perhaps you might have seen fire-extinguishers

in hospitals, big offices, factories and science laboratories. In this equipment, liquid

carbon dioxide is preserved at high pressure. At the time of need, when it is unlocked

the pressure of the equipment decrease. Then carbon dioxide gas comes out at a very

high force and helps to extinguish fire.

Carbon dioxide is one of the raw materials for making foods of the plant kingdom.

Animals release carbon dioxide at the time of exhalation. Plants take carbon dioxide

from air and water from soil and make their own food with the help of chlorophyll in

presence of sun light. This food is prepared in the leaf. This process of making food

by the plants is called photosynthesis.

Chlorophyll Water + Carbon dioxide Glucose + Oxygen Sunlight

Plants make their own food and animals get their foods from plants. As a natural

process' plants get carbon dioxide from air and animals exhale carbon dioxide to air

and the total quantity of carbon dioxide in the air remain almost unchanged. The

quantity of carbon dioxide in the air is 0.036% of total volume of the air. Population

of the world is increasing day by day. As people are increasing so their exhalation of

carbon dioxide is also increasing. Moreover, man has been establishing huge number

of industries. In the industries carbon dioxide is prepared as a waste from burning of

fuels. On the other hand, people are cutting trees for fuels. So, trees are decreasing in

number. It is not possible for trees to produce the same amount of food that they could

make before hand using carbon dioxide. So, the quantity of carbon dioxide in the air

is increasing day by day. Effect of this excess carbon dioxide may be much harmful to

the environment and lives of animals. Carbon dioxide is suffocating and slightly

Carbon dioxide 59

poisonous. We feel uncomfort when air contains more than 5% carbon dioxide. If this

quantity goes to 40% then the animal may die for suffocation. So, in order to maintain

balance in the atmosphere and natural environment it is necessary to control

population on the one hand and to start more plantations on other. For this,

development of awareness among people is necessary.

Exercise


1. Which chemical reaction is wrong?

a. Oxygen + Carbon = Carbon dioxide

b. Oxygen + Hydrogen = Water

c. Carbon + Hydrogen = Dry ice

d. Metal + Oxygen = Metal oxide

2. What are the products from the reaction of Calcium carbonate and Hydrochloric

acid?

a. Carbon dioxide and water

b. Calcium chloride and water

c. Carbon dioxide, Calcium chloride and water

d. Oxygen and water

3. A candle is burning in a gas jar. A student inverts another gas jar filled with carbon

dioxide on the candle. Which two characteristics of carbon dioxide are proved in

this test?

a. Carbon dioxide is a colourless and odourless gas and lighter than the air.

b. Carbon dioxide itself does not burn nor it helps combustion and it is lighter

than the air.

c. Carbon dioxide is heavier than air and it does not bum itself nor helps combustion.

d. Carbon dioxide is heavier than air and does not help combustion, only it bums

itself.

4. Carbon dioxide is bubbled through a fresh solution of lime water. In the first case

the lime water turns milky. After prolonged supply of the gas, the lime water

became a clear solution again. What is the name of the last product?

a. Calcium carbonate b. Calcium hydroxide

c. Calcium chloride d. Calcium hydrogen carbonate

60 General Science

5. What happens when dry ice is heated?

a. Dry ice melts to water

b. Dry ice melts to liquid carbon dioxide

c. Dry ice turns into water vapour

d. Dry ice directly turns into carbon dioxide

6. The reaction from which we get O2 as a byproduct is-

i. Carbon dioxide + Water

ii. Carbon dioxide + Water

iii. Magnesium+carbon-dioxide


a. i b. ii

c. iii d. i, ii, iii

Read the following text and the answer questions 7,8 and 9.

There is some lime water in a jar. If we give a strong blow in it, we can see a whitish

layer on the lime water. But prolonging the blow it is seen that the layer is dissolved

and the solution becomes clear.

7. Which gas is ejected by the blow?

a. Hydrogen b. Oxygen

c. Carbon dioxide d. Vapour

8. What .is the substance present in the layer?

a. Calcium carbonate b. Calcium bicarbonate

c. Carbon dioxide d. Calcium Hydroxide

9. The produced compound for which the solution becomes clear is-

a. Calcium carbonate

b. Calcium bicarbonate

c. Glucose

d. Calcium hydroxide

Creative Questions

1. You are given some edible soda and magnesium carbonate. You have lemon,

potato and red amaranth in your house. Making juice separately from each of

them, first add with edible soda and then with magnesium carbonate.

a. Adding the juice of which one with edible soda you can get carbon dioxide?

chloophull

Sunlight

Carbon dioxide 61

b. Write down the equation in words of that reaction.

c. Describe with equation which gas will be produced if that juice is mixed in

the second case.

d. Is the produced gas acidic or basic? Put your argument.

2. An aluminium foil is ignited in fire with the help of a crucible tongue. Then you

inject the ignited foil into a jar filled with carbon dioxide. Black spots are seen on

the wall of the jar. Now ignite the foil only in the air.

a. Which subtance is responsible for this black spot?

b. Explain the reaction.

c. Write down the chemical reaction of what will happen if the foil is ignited

only in air.

d. From the above experiment, how will you prove that carbon dioxide is a compound

gas?

Figure

In the three balloons above there are three types of gases of same volume. The

balloons are tied with threads.

The gas in the first balloon neither burns itself nor helps to burn others. The gas

in the second balloon burns itself but does not help to burn others.

The gas in the third balloon does not burn itself but helps to burn others.

a. What is the name of the gas in the first balloon?

b. What will be the change if we insert wetted litmus in this gas?

c. Write down the reaction with equation that takes place if this gas is bubbled

through a solution of lime water.

d. What will happen if the balloons are set free? Explain.

62 General Science

1 2 3

Chapter-7

SolutionSolute, Solvent and Solution

you might have drunk sharbat of cane molasses or sugar. You certainly know how

sharbat is prepared from cane molasses or sugar. Take a glass of clear water and add a

few spoonful of sugar to it and stir well. After some time you will find that the sugar

has already disappeared. Sugar grains will not be seen by naked eyes and even with

the help of a microscope. Now the question is where has the sugar gone after it is

mixed with water? Sugar has dissolved in water and disappeared. Have a taste of the

sugar mixed water and it tastes sweet. It proves that the sugar remains in the water.

This mixture is called solution of sugar and water.

Sugar is soluble in water. The solid which dissolves is called solute. Sugar has

dissolved in water. So, sugar is a solute. The liquid that dissolves a substance is called

solvent. In the solution of sugar and water, water has dissolved the sugar. So in this

case, water is a solvent. In this case the homogeneous mixture that is formed as a

result of mixing a solute and a solvent is called a solution. That means-

Solute + Solvent = solution

Do this experiment with common salt, copper sulphate or blue vitriol, sand and flour

instead of sugar and observe. Take a few spoonfuls of salt in a glass of water and stir

well. What do you see? Salt has dissolved in water to make a solution of salt and

water. Similarly, in the water of another beaker, if some copper sulphate or blue vitriol

is mixed and stirred for sometimes, a solution of blue vitriol in water will be prepared.

There will be no sediment on the bottom of the container if a solution of sugar, salt or

copper slullphate is left for a long time. Such clear solution, when filtered with the

help of a filter paper, will not get any residue.

Salt Copper sulphate Sand Flour

Figure 7.1: Preparation of solution of common salt, copper sulphate, sand and flour.

Now take some water in another beaker and mix some sand with it with a glass rod

stir the sand mixed water continuously and observe the mixture keenly. What do you

see? The mixture looks turbid. Now keep the mixture at rest for sometime. You will

find that the sand has settled at the bottom of the beaker and water at the top looks

clear. Now separate the sand and water from the mixture by decantation or filtration

process. You will see, sand is left out in the bottom of the beaker after decantation and

on the filter paper after filtration. From this experiment and observation, we can say

that sand is not soluble in water. So, the mixture of sand and water is not a solution.

Sand is insoluble in water.

In the same way, mix a little flour with water in a beaker and stir continuously. You

will see that flour is not mixing completely with water. A milky mixture of flour and

water has prepared from this mixture flour and water can be separated by filtration or

stirring ? This mixture also should not be called a solution. Flour is also insoluble in

water like sand. So, we can say that if a solute does not mix up thoroughly to make a

homogeneous mixture then it can not be called a solution. From the above discussion,

we can define solute, solvent and solution by the following way:

Solution

The homogeneous mixture of two or more substances, of which relative quantity of

the components can be changed up to a certain limit is called a solution. A solution

has two parts viz- solute and solvent. The substance which is dissolved, is called the

solute and which dissolves the solute is called the solvent. In the solution of sugar and

water, sugar is the solute and water is the solvent. That means:

Solution = Solute + Solvent

Solute

The component of a solution that exists in smaller quantity and which produces a

homogeneous mixture by dissolving in a solvent is called solute. If a solution is

prepared by dissolving sugar or salt in water then the sugar or the salt will be the

solute of that solution.

Solvent

In a solution, the component which exists at a higher proportion and dissolves a solid,

liquid or gaseous substance producing a homogeneous mixture is called a solvent.

Water is an excellent solvent.

Characteristics of a solution

The principal characteristics of a solution are:

(a) Solution is a homogeneous mixture. That means, the components, the composition

and the properties of different portions of the solution will remain the same.

64 General Science

(b) Molecules of both the solute and the solvent in a solution retain their

individual properties.

(c) Particles of the solute can not be separated from the solution by filtration.

(d) Particles of solute do not settle at the bottom if the solution is left out.

(e) Solute and solvent can be separated from solid-liquid solution by distillation process.

(f) Proportion of the solute in a solution can be decreased or increased by changing

its the temperature.

(g) Solute in a solution remains so thoroughly that these substances can't be seen

even by a standard microscope.

Water is an excellent solventAmong the thousand types of liquids in the world, the quantity of water is highest. It

is not easy to measure the quantity of water present in lakes, canals, rivers and

oceans., Water is available on demand, that means, among the various types of liquids,

water is the cheapest and the most easily available. The most important property of

water is - water can dissolve the highest number of substances. That is why we do not

get pure water in nature. Because, rivers originating from mountains flow over rocks

and soils and dissolves all soluble substances at the time of flowing. These substances

dissolves in water. For this reason, there are iron and lime as mixture in certain river

water. In rain water many components of air remain dissolved. Spring or tube-well

water when percolating through underground, striates carries soluble mineral

substances. For this reason, spring and tube well water gives taste and smell of certain

mineral substances such as calcium, magnesium, iron, etc. The salt, that we use in our

diet remains dissolved in large quantity in sea water. So, in most cases salt is prepared

by drying of sea water. Various gaseous substances are also dissolved in water.

Among the various types of solvents, water is the most easily available and the

cheapest one. Water dissolves the highest variety of substances. That is why water is

called an excellent solvent or greatest solvent or universal solvent.

Preparation of saturated and unsaturated solution with salt and waterTake some water in a beaker and add some spoonful of salt and stir the mixture. What

do you see? The salt quickly dissolves in water making a homogeneous solution. At

this condition the solution of salt and water that has been prepared is called an

unsaturated solution. In this solution if more solute is added, it will be dissolved. So at

a particular temperature, a solvent of a solution dissolves less solute than the

maximum quantity of it can be dissolved, then this solution is called an unsaturated

solution. By adding more solute to an unsaturated solution, it can be turned into a

saturated one at the same temperature. Let us prepare a saturated solution by adding

more salt to the unsaturated one.

Soluton 65

Add salt in tea spoonful quantity with the unsaturated solution of salt and water one

after another and keep stirring. You will see that the salt has dissolved. In this way

continue the process of adding some salt and stirring. You will see that a moment will

come when no more salt dissolves. Stop stirring. You will see some excess salt settled

at the bottom of the beaker when the water of the beaker becomes stationary. The

reason is that, a fixed quantity of water can dissolve a fixed quantity of salt of definite

temperature. With the increase of water you will see that the salt remained un-

dissolved at the bottom of the beaker has dissolved. Again the solute that remains un-

dissolved at the bottom of the beaker can be dissolved by raising the temperature of

the solution. From this, it is proved that in a particular solvent, the quantity of a solute

that can be dissolved at a particular temperature is limited. That means, a fixed

quantity of solvent can dissolve at a particular temperature only a definite quantity of

solute.

Figure 7.2: Preparation of saturated solution of salt and water

So, the solution which contains the maximum possible quantity of the solute at a

particular temperature and can not dissolve more solute if added, is called a saturated

solution. And the solution which can dissolve more solute if added at a particular

temperature is called an unsaturated solution.

We can compare the terms saturated solution and unsaturated solution with our meals.

When we feel hungry, we take foods. We go on eating as much as we can. When our

appetite is satisfied, then we do not want to take more food. We take food as long as

we have appetite. This condition can be compared with an unsaturated solution. But

we do not like to eat when our appetite becomes satisfied. This condition can be

compared with that of a saturated solution.

You have observed earlier that dissolving power of a solvent depends upon heat or

temperature. At a particular temperature, a definite quantity of a saturated solution

dissolves a definite quantity of solute. If the temperature of the solution is increased

66 General Science

then the solute dissolving power of the solvent also increase. As a result, a saturated

solution turns into an unsaturated solution at a higher temperature. On the other hand,

some quantity of the solute slowly separates out and settles at the bottom of the

container if the temperature of the solution is decreased gradually. Without adding any

more solute, an unsaturated solution can be turned saturated by two ways.

(i) Solubility of a solution goes down on cooling it. So, an unsaturated solution turns

into a saturated solution if it is cooled.

(ii) If a solution is heated then some solvents evaporate out. As a result, the quantity of

the solvent in the solution reduces. In this circumstances, the solution will be

converted into a saturated one.

Distinction between saturated and unsaturated solution

You have seen that a solution is prepared by adding solute to a solvent. Now, is there

any change the volume of a solvent when a solution is made by adding sugar or salt to

a definite volume of that solvent?

Make a saturated solution in 100 ml water by dissolving salt or sugar and measure the

solution again with the help of a measuring cylinder. You will see that, there is no

change in the volume of the solvent after making the solution. As a result of

dissolving the sugar or salt in the water, the density of water has increased.

Unsaturated solution

1. An unsaturated solution can

dissolve more solute until it gets

saturated.

2. When the temperature of

unsaturated solution is raised, it

becomes more unsaturated.

3. On, cooling, an unsaturated

solution slowly turns into a

saturated solution

4. An unsaturated solution becomes

more unsaturated or dilute if more

water is added.

Saturated solution

1. If more solute is added to a saturated

solution, it will not be dissolved, it will settle

at the bottom of the container.

2. When the temperature of a saturated

solution IS raised, it turns into an unsaturated

solution.

3. On cooling a saturated solution the dissolved

solute slowly comes out as solid and settles at

the bottom. Because, the solute accommodation

capacity of a solvent decreases when its

temperature is decreased.

4. If more water is added to a saturated

solution then, it becomes an unsaturated

solution. At the same temperature more

solvent can dissolve more solute.

Soluton 67

Unsaturated solutions can be divided into two groups viz- dilute solutions and

concentrated solutions. A solution which contains comparatively less quantity of

solute is called a dilute solution. On the other hand, the solution, which contains

comparatively more solute is called a concentrated solution.

In a definite quantity of a liquid, all substances to the same extent can not dissolved to

the some extent. Do an experiment to see, in 100 ml water the quantity sugar that can

be dissolved, in same volume of water, the quantity of salt that can dissolved is the

same or not.

Experiment 7.1:

Take 100 ml water in a beaker with the help of a measuring cylinder. Now mix sugar

in tea spoonful quantity one after another with the water of the beaker and keep

stirring. In this way, write in a table as below to make a saturated solution of sugar

and water as to how many spoonfuls of sugar required.

Figure 7.3 : Solution of salt and sugar

Similarly, take 100 ml of water in a beaker and by using the same spoon add spoonful

of salt with it one after another and keep stirring. In this way make a saturated

solution of salt and water. Write down in the table as to how many spoonfuls of salt

needed to make the solution. What have you seen ? Are the quantities of sugar and salt

same to make saturated solution in both the cases? Have a look at your table. More tea

spoonfuls of sugar were required than the number of tea spoonfuls of salt needed to

Solvent

Water 100 ml

Number of tea spoonful of sugar

required for making a saturated

solution of sugar and water

Number of tea spoonful of salt

required to make a saturated

solution of salt and water

68 General Science

prepare a saturated solution. That means, sugar is more soluble in water than salt. At

normal temperature sugar is six times more soluble than salt. The reason of this is that

solubility of sugar is much higher than that of salt. Therefore, it can be said that a

definite quantity of a solute can be dissolved in 100 gram of solvent at a fixed

temperature forming a saturated solution, the quantity of the solute is called the

solubility of the solvent at that temperature.

Preparation of various types of solutionSolution can be divided into a number of classes. Such as-

(a) Solid-liquid solutionCommon salt, sugar, blue. vitriol etc. are solid substances, can be dissolved in water

making an aqueous solution of the substances. In this solution, water is called the

solvent and the solid substances like salt, sugar, blue vitriol, etc. are called solutes. In

a solution of sugar and water, water is the solvent and sugar is the solute.

(b) Liquid-liquid solutionIn such a solution both the solute and the solvent are liquids. Here the liquid which

exists in higher quantity is known as solvent and the one in lower quantity is known as

the solute. For example, a solution of milk and water, is made by mixing 100 ml water

with 1 litre milk. In this solution the quantity of milk is higher, so milk is a solvent

and the quantity of water is lower, that is why water is solute.

Soda water is an aqueous solution of carbon dioxide. Here, water is the solvent and

carbon dioxide is the solute.

(d) Gaseous solution

Two or more gases which do not mutually react can mix together at any ratio to form

a saturated solution. Air is mainly a solution of nitrogen and oxygen. Here, the

quantity of nitrogen is higher and so it is the solvent and of other gases being less in

ornately are mentioned as solute.

(e) Solid solutionBronze is a mixture of copper and zinc and brass is a mixture of copper and tin.

Separation of solid solute from solution

We know that there are two parts of any solution- viz- solute and solvent. In a solution

of salt and water, salt is the solute and water is the solvent. Solid solute and liquid

solvent can not be separated from a solution by means of decantation and filtration. If

a solution of salt and water is left out, it remains unchanged, solute and solvent do not

get separated. Even a solvent can not be separated by filtering through a filter paper.

Components of a solution can be separated by a vaporation and distillation.

Soluton 69

Taking a solution in a container if heated, then the liquid turns into vapour and the

solid solute settled at the bottom of the container in dry form. This process is called

evaporation. During evaporation, the liquid solvent turns into vapour and escapes.

That is why the liquid part is not returned.

On heating, water or a liquid first turns into a vapour. That vapour can be cooled back

to liquid again. The process by which a liquid first turns into vapour and on cooling it

turns again into the same liquid is called distillation. That means-

Distillation = Evaporation + Condensation

Through distillation, both salt and water can be recovered from a solution of salt in

water. Besides, evaporation and distillation processes, a solute may also be partially

separated from a solution by crystallization process.

Separation of solute by crystallization

Make a saturated aqueous solution of sugar, salt or blue vitriol in a beaker. It will turn

into an unsaturated solution if heated. At this condition, extra solute is added will also

be dissolved. Filter this hot and concentrated solution with the help of a filter paper

and then cool the filtrate. Observe after a while. is anything being found ? Yes, some

granular substance, has settled at the bottom of the solution. Such solid substance is

called crystals. And their process is called crystallization. Crystals of certain

substances look very beautiful. Perhaps you have seen sugar candy. Granules of sugar

candy are also such crystals. The crystals of salt, sugar, candy, blue-vitriol, alum, etc.

look beautiful.

Preparation of sugar-candy

You can make sugar candy that we use to prepare drinks, you can make it at home.

Take some water in a pot. Add sugar through stirring and heat the solution. Keep on

mixing the sugar until some un-

dissolved sugar settle in the bottom of

the pot. Filter the solution heating

after and divide the concentrated

solution of sugar into two parts. Put

them in two separate cups. Drop a

small grain of sugar candy in a cup. In

the other cup, hang a small grain

sugar candy with the help of a thread.

Keep the two cups with sugar solution

in a cool place. After a few hours,

when the solution is cool, you will see

water

sugar

Fig. 7.4: Preparation of sugar candy

70 General Science

that large number of candy crystals are settled at the bottom of the first cup and in the

second cup, the candy crystal has grown very big. Grains of sugar from the solution

have settled on the grain of the candy that is hung with thread. As a result, it becomes

bigger in size.

Preparation of salt from sea water

Everyday we take salt. Every living being needs salt to live. We can not live without

salt. Vegetables, fruits, fish, meat, drinks etc. contains salt. Plants absorbs salts from

the soil with the help of its roots. We take these salts with foods. We suffer from

various diseases due to deficiency of salt. Salts help to clean our blood. Salt is used to

keep fish, meat, etc. fresh and preserve leather. Food becomes delicious if appropriate

amount of salt is used. But food becomes inedible if excessive salt is used. So, you

might have understood how useful the salt is to us and in preparing food an

appropriate quantity of salt is required. Now the question is - where do we get the salt

from ? Yes, the salt that we take in our food can be collected from two sources. For

example - from salt mine and from sea water. Salt mine is not available in all the

countries of the world. Salt mine is not available in our country also. That is why? We

get salt from sea water in our country.

Sea water tastes very saline when it is takes in the mouth. We can not drink this water.

Originally sea water is a solution of salt. Due to the heat of the sun the water of the

sea evaporates out but the salt dissolved in water remains in the sea. The sea water

evaporates to form clouds. Clouds are stored on the hill side as snow. Again clouds

from rain. Water from molten snow and rain streams into the sea. You have learnt that

water is an excellent solvent. Water dissolves various salts when it flows over the soil,

rock, etc. of the surface of the earth. These dissolved salts flow and store into the sea.

In this way, the concentration of salts in the sea-water is increasing day by day. You

might have understood the reason of the sea water being salty. Of this vast quantity of

sea water, 26 parts out of one thousand is salt. That is why, common salt is prepared

from sea water.

You already know that water evaporates away from salt water solution when heated

and salt separates out as grains or crystals. Try to recollect the name of the process by

which a solute can be isolated from a solution. It is called the evaporation process. By

using this evaporation process, it is not any to prepared common salt from sea water.

Salt preparation process is very interesting. To prepare a huge quantity of salt, a vast

Soluton 71

quantity of sea water has to be evaporated. In our houses, salt can be prepared by

heating sea water in a hearth. Every year thousands of mounds of salt are prepared in

this way in the district of Chittagong of Bangladesh. Those who produce salt are

called salt cultivators.

For preparing salt, in coastal grounds sea water is accumulated by making dykes. At

the time of high tide sea water enters to these places which are surrounded by the

dykes. Sometimes canals are dug to bring sea water into the area. Then the canals are

closed. As a result, water can not get out or get in of the dams. Then water of the

dooms evaporates away due to sun shine. A five or six inches thick layer of salt forms

on the bed of the dooms. To prepare salt in this way, needs 20/25 days. The salt thus

produced contains several dirt materials. So, after refining the salt with the help of

machines, it is sold in the market. Machines used for refining of the salt are called

refining machine or salt clarifier. Human body needs a very small quantity of iodine

everyday. Due to lack of iodine thyroid glands does not work properly. So, the person

may be attacked by goitre. In case of pregnant women, are to lack of small quantity of

iodine in the body, the pregnant woman sometimes procreate disabled and dumb

babies. So, proportionate quantity of iodine is mixed with the salt at the time of

purification with the help of refining machine. Now a days, we use iodine mixed salt.

You will use iodine mixed salt is cooking and as a table salt.

Liquid and gas solution

In this chapter we have discussed about solution of solid solute and liquid solvent. In

the classification of solution, you have come to know that a solution is made not only

of a solid solute and a liquid solvent but also by mixing a liquid with another liquid.

Solution may be prepared by mixing liquid with gas and gas with gas also. Try to give

an example of a solution which is made by gas solute land liquid solvent. Perhaps you

will say - soda water, lemonade, etc. We know that all animals need oxygen for

respiration. The creatures living under water like- fish, aquatic plants, etc. need

oxygen for breathing but can you say from where do they get it ? Yes, oxygen is

slightly soluble in water. That means oxygen is solute and water is solvent. Oxygen

and water mix together to make a homogeneous solution. As oxygen remains

dissolved in water, it helps aquatic plants and animals to breath. Fish, with the help of

gill and aquatic plants, with the help of leaves, take dissolved oxygen for their

respiration.

72 General Science

ExerciseMultiple Choice Questions

1. Which of the following statements expresses the right characteristic of a solution?

a. The components, formation and properties of different parts of a solution are

not the same.

b. Components can be separated from the solution by sieving or filtration.

c. When solution is left out, solute settles at the bottom as sediment.

d. Molecules of solution, solute and solvent retain their properties.

2. What happens when extra water is added to a saturated solution of water and blue

vitriol?

a. The solution will be more saturated

b. The solution will be unsaturated

c. Crystals of blue vitriol will remain as sediment

d. The solution will not be changed

3. Which statement of the following is appropriate in case of sugar in water and salt

in water?

a. Water dissolves all substances equally

b. Salt dissolves more than sugar in water

c. Sugar dissolves much more than salt in water

d. Salt and sugar dissolve equally in water

4. Which one is the solute if 20gm of sugar is dissolved in 250 ml water?

a. Sugar

b. Water

c. Both sugar and water

d. None of them

5. What will be the volume of the solution if 25gm of sugar is dissolved in 250 ml

water?

a. 250 ml. b. 275 ml.

c. 300 ml. d. 350 ml.

A saturated solution is made by dissolving 10gm salt in 250 ml water in a beaker of

volume 500 ml Furthermore, some salt is added.

Answer the questions 6 and 7 from the above information.

6. If the solution is heated-

i. the salt will remain as sediment

ii. the additional salt will be dissolved

iii. no change will occur in solution

Soluton 73


a. i b. ii

c. iii d. i, iii

7. What will be the volume of the saturated solution?

a. 250 ml. b. 260 ml.

c. 500 ml. d. 750 ml.

Creative Questions

1.

a. What is the name of the solute in the solution of figure 'a'?

b. Explain in which case more solute will be dissolved.

c. What will happen in case of figure 'b' if more heat is applied?

d. Give your opinion, from which we can get back the solute rapidly between

figure 'a' and 'b'.

2. Mr. Answar prepares date molasses. Every morning he collects date juice. Then

he gives heat to the juice in an open spread pot made of tin. After heating for a

long time, he gets date molasses at the bottom of the pot.

a. What is the chemical name of date juice?

b. Why does Mr. Answar give heat to the open spread pot made of tin?

c. Explain the result of heating the pot.

d. Write down the significance of this process in our daily life.

5 gm. blue vitriol + 100 ml waterFigure 'a'

10 gm sugar + 100 ml water Figure 'b'

74 General Science

Chapter - 8

Morphology of Plants:

Leaf, Stem, Flower and Fruit

Leaf is an organ of a plant. We have learnt in class-VI about leaf, its characteristics,

classification and different parts; like leaf base, petiole and lamina. Now in class

seven, we will learn about the longevity, shape and classification of leaves based on

the capacity to get sunlight; modified leaves and stems; flower and its parts;

classification of flower, inflorescence and pollination and origin of ftuits.

Kinds of leaves

Leaves are classified on the basis of longevity into three groups; such as-

I) Caducous: The leaves that fall off after they bloom from the leaf buds are called

caducous. Example: Opuntia (Phonimonsha), Acacia (Akash moni) etc.

2) Deciduous: The leaves that remain on the plant for one season or one year and fall

at last are called deciduous leaves. Example: Silk cotton (Shimul), Sal etc. Plants with

deciduous leaves are called deciduous plants. Forests of deciduous plants are called

deciduous forests.

3) Persistent: The leaves which remain on the plant for long time are called persistent

leaves; such as Polyalthia (Debdaru). The plants with persistent leaves are called

evergreen plants. Forests with evergreen plants are called evergreen forests. Example

of evergreen forests are the Sundarbans, forests of Chittagong Hill Tracts etc.

Fig.8.1 :Types of leaves based on shape and capacity to get the sun light

Centric leaf

Isobilateral leaf

Dorsiventral leaf

Based on shape and ability to get the sun light; leaves are classified into following

three types:

1) Dorsiventral leaf: When the leaf exists horizontal to the ground is called

dorsiventral leaf. The leaves do not get sun light equally on both sides. So, the upper

surface is deep green and the lower surface is light green. Example: Leaves of Mango,

Jackfruit, Banyan tree etc.

2) Isobilateral leaf: When the flat green leaf blade remain more or less vertical, then it

is called isobilateral leaf. Sun light falls more or less equally on both sides, so both the

surfaces of the leaf are equally green. Example: leaves of Paddy, Grass, Sugarcane etc.

3) Centric leaf: This leaf is cylindrical in shape and remain in vertical position and

gets equal light all around the leaf surface. Example: Onion leaves.

Modified leaves

When leaves or leaflets of some

plants are modified to perform some

specific functions; are called modified

leaves. Example: Pea, Opuntia, Onion

etc. Some types of modified leaves

are discussed below:

Leaf tendril: In Pea plants the entire

leaf blade or some of its parts are

modified to tendrils. It helps the plant

in climbing.

Leaf spine: Leaves of some plants

modify into sharp spines; such as

Opuntia, Date palm etc. These spines

protect them from attack of herbivorous

animals.

Scale leaves: Onion, Garlic, Turmeric,

Zinger develop underground modified

stems. The leaf blades of these plants

are modified into thin and non green

leaves. Scale leaves may be dry and

thin or juicy and fleshy. Example:

Onion and Garlic. Its function is to

protect the axillary bud. Leaves of

76 General Science

Fig. 8.2: Leaf tendril (garden pea)

Tendril

Fig. 8.3: Pitcher plant

Lid (leaf apex)

Pitcher

(modified lamina)

Fig. 8.4: Pitcher (modified lamina)

Modified to pitcher

(lamina)

Root

some plants are modified to perform physiological functions like storage of water

storange of food, reproduction and trapping insects etc.

Leaves of Pitcher plant (kolshi), Dodder (Surjia shishir), Bladder wart (Jhanjhi)

etc.are modified into insect traps for collection of protein foods from !he body of the

insects. These plants are called insectivorous plants. The leaf blade of a pitcher plant

is modified into a pitcher and the leaf apex functions as a lid. When an insect enters

into the 'pitcher, the lid is closed. The insect gradually dies and its body is digested by

secreting different types of enzymes.

The leaf blades of Dischidia, an epiphytic plant, is modified into a pitcher for storage

of water. Roots developed from the nodes enter into the pitcher and absorb water for

the plant. These plants are found in the hilly areas of Sylhet region.

Leaves of Aloe vera (Ghreto kumari), Onion, Garlic etc. store foods and become thick

and juicy. Leaves of Bryophyllum (Pathar kuchi) also store food and become thick and

juicy. Some buds develop from it which help in vegetative reproduction.

Modified Stems

In class-VI we have learnt about which is stem; its different parts, structures and

functions. In this class we shall learn about modifications of stems. The stems whose

shapes are not as like as a normal stem; then it is called a modified stem. Example:

tuber of Potato, rhizome of Zinger etc. Modified stems are mainly of three types

(a)Under ground modified stems, (b) Sub aerial modified stems and (c) Aerial

modified stems.

A. Under ground modified stems

Normally stems remain above the ground level. But when the stem stay under the

ground then it is called underground modified stems. They are colourless or grey

coloured and have no green leaves as they remain under the ground. Still they are

different from roots because they show characteristic features of stems; such as scale

Fig. 8.5: Aloe vera

Juicy leaf

leaf bud

Fig. 8.6: Bryophyllam

Morphology of Plants: Leaf, Stem, Flower and Fruit 77

leaves, nodes and internodes, axillary and apical buds etc. Their main functions are to

protect plants from unfavourable environment, vegetative reproduction and storage of

food. They are of four types; such as: 1) Rhizome, 2) Tuber, 3) Corm and 4) Bulb.

1) Rhizome: Stems that stay and

grow horizontally and become

swollen due to storage of food, are

called rhizome. They bear nodes,

internodes, scale leaves and apical

and axillary buds. Every year in

favourable environment the aerial

leaves are produced. When these

leaves dry up and fall, a scar is

produced on the rhizome.

Adventitious roots are developed

from the underside of the rhizome.

Example: Zinger, Turmeric, Indian arrow root (Shati), etc.

2) Tuber: When apical portion of underground branches swell up and form a

spherical or egg like fleshy structure then they are called tuber; e.g.Potato. The 'eye' of

Potato bears scale leaf and axillary bud.

3) Corm: The underground stem that is more or less like a round and thick stump and

grows vertically under the soil is called a corm. It has roots, buds, nodes and

internodes. Example: Telega potato (Ol kachu).

4) Bulb: The underground stem that looks like a small conical disc is called a bulb.

Example- Onion, Garlic etc. The place from where adventitious roots develop in

Onion and Garlic is the bulb stem. Many nodes and internodes are compressed

together in bulb stem.

Fig. 8.7: Rhizome (Zinger)

Scale leaf

Apical bud

Node

root

Fig. 8.8: Tuber (Potato)

Bud

Daughter corm

RootEye

Fig. 8.9: Corm (Ole)

78 General Science

Scale leaves may be dry or juicy. Bulb is of two kinds; such as

(a) Tunicated bulb: The bulb where one scale leaf surrounds the other and the outer

leaf becomes dry, is called tunicated bulb; such as Onion.

(b) Scaly bulb: The bulb where the fleshy scale leaf does not surround another scale

leaf, rather it remains free, is called a scally bulb. Example: Tulip, Lilly, Garlic etc.

B. Sub aerial modified stemsIn some plants a portion of stem remains above the ground; while the rest remain in

the soil; Example:- Indian pennywort (Thankuni), Arum (Kachu), Water hyacinth

(Kochuripana), Mint (Pudina) etc. These stems are called sub aerial modified stems.

Their main function is vegetative reproduction. These are of four types; such as-

1) Runner 2) Offset 3) Stolon and 4) Sucker.

1) Runner: It grows horizontally on

the soil surface arising from the

ground level axillary buds. Its

internodes are very long. Adventitious

roots are developed from undersurface

of nodal region and buds are

developed on the upper surface of

these nodes. This buds develop as new

plants. Example: Oxalis, Indian

pennywort etc.

2) Offset: It is like a runner but its

internodes are thick and short. It is

normally found in floating plants;

such as Water hyacinth (Kachuripana),

Topa pana etc.

Fig. 8.10: Scaly bulb (garlic)

Dry Scale leaf

Fleshy scale leaf

Dry scale leaf

Apical bud

Stem (bulb)

Fibrous roots

Fibrous roots

Fig. 8.11: Tunicated bulb (Onion)

Fig. 8.12: Runner (Oxalis)

Fig. 8.13: Offset (Water hyacinth)

Root pocket

RunnerMother plant

OffsetRoot


3) Stolon: It develops from the lower

buds near the soil surface. It grows

parallel to the ground. Its internodes

bends like an arch and remains

partially under the soil surface and

partially above the ground. The

nodes touch the ground after

travelling for certain distance; such

as Arum (Kachu).

4) Sucker: These subaerial stems

(branches) are developed from

underground buds of normal stems

and grow obliquely above the ground

level and grow like a normal branch.

Roots grow from the undersurface of

these branches. Example: Mint

(Pudina), Plantain (Kanchakola),

Chrysanthemum etc.

C. Aerial modified stems

Some aerial stems or their parts modify in such a way that it becomes difficult to

identify them as stems. These are aerial modified stems. They perform functions like

manufacture of food, self defense, help climbing and vegetative reproduction. They are

of five types; such as 1. Phylloclade 2. Cladode 3. Thorn 4. Stem tendril and 5. Bulbil.

1) Phylloclade : When an aerial stem

modifies and become flat and green; it

is called Phylloclade. Its leaves fall

when in bud or modify into spines. It

has nodes and inter nodes and buds.

These buds develop into new branches.

A mature Phyloclade poduces flowers

in appropriate time Example- Opuntia

(Phonimonosha).

Fig. 8.14: Stolon (Arun)

Mother plant

Scale leafStolon

Fig.8.15:Sucker (Chrysenthimum )

Mother plant

New shoot

sucker

Adventitious roots

Fig.8.16: Phylloclade (Opuntia)

Flower

spice

Node

Inter node

80 General Science

2) Cladode: When aerial stems modified

to needle like one inter node structure; it

is called a Cladode. The leaves fall off at

bud stage or modify into spines; such as

Asparagus (Satamuli).

3) Thorn: In some cases the axillary

bud of an aerial stem does not bear

leaves, branches and flowers. It becomes

modified into a strong and sharp thorny

structure, called Thorn. Example- Wood

apple, Thorn apple (Dhutura) etc.

4) Stem tendril: When axillary buds

modify into spiral string like structure,

it is called a stem tendril. It helps the

plant in climbing. Example- Passiflora(

Jhumko lata) etc.

5) Bulbil: When axillary buds do not

grow normally and take the shape of a

rounded fleshy structure; called

bulbil. These bulbils give rise to new

plants when get detached from the

mother plant and fall on the ground..

Example: Dioscorea ( Gach aloo ).

The Flower

Flower is an important organ of a plant. We see various kinds of flowers around us.

Usually we see the flower blooming at the apex of a branch. Moreover, some flowers

grow from the branches of a plant but in Jackfruit we can see flowers growing on the

main stem also. Like other organs of a plant, the flower has different parts. The parts

are 1) Floral axis, 2) Calyx, 3) Corolla, 4) Androecium and 5) Gynoecium.

Now we have learnt that the part with floral axis, calyx, corolla, androecium and

gynoecium is called a flower. Now let us know about different parts of a typical

flower in details.

Fig.8.17: Cladode (Asperagus)

Cladode

Fig.8.18: Thorn (Wood apple)

Thom

Leaf

Fig.8.19: Stem tendril

Leaf

Stem

Stem Tendril

Fig.8.20: Bulbil (Dioscorea)

Leaf

Bulbil


Different parts of a complete flower

The flower with floral axis, calyx,

corolla, androecium and .gynoecium

is called a complete flower. Example:

China rose, Thorn apple, Mustard etc.

A complete flower has five parts such

as 1) Floral axis, 2) Calyx, 3) Corolla,

4) Androecium and 5) Gynoecium.

1. Floral axis: The axis on which calyx, corolla, androecium and gynoecium are

arranged is called floral axis. It has two parts; such as a) Pedicel and b) Thalamus.

a) Pedicel: A stick like slender structure under the flower is called pedicel. It is the

stalk of a flower. Flower with pedicel is called pedicellate flower. When the pedicel is

absent, the flower is termed sessile. China rose is a pedicellate flower and Tube rose is

an example of sessile flower.

b)Thalamus: It is the apical swollen end of pedicel. Calyx, corolla, androecium and

gynoecium are called floral leaves. The floral leaves are arranged on it. This swollen

and flat portion of the floral axis is called thalumus.

Function of thalamus: To keep the flower attached to the stem;. bear the whorls of

floral leaves and exchange of food materials between the stem and floral leaves.

2. Calyx: The outer most whorl is

called calyx. It is green and are

divided into small. parts. Each part is

known as sepal. In exceptional cases

the calyx may be of different colour;

e.g. Mussaenda. Sepals when united

together is called gamosepalous calyx.

Example: Thorn apple (Dhutura). But

when each part is free, it is called polysepalous. Example: Mustard. Sometimes extra

whorl is found below the calyx; called epicalyx. Example: China rose (Joba).

Functions of calyx: Main function of calyx is to protect the flower bud from rain,

storm, temperature and diseases. Green calyx can manufacture food like normal

leaves. Persistent calyx protects the flower, while a colourful calyx helps pollination.

3. Corolla: The whorl next to calyx is the corolla. Each part of corolla is called a

petal. Total number of petals makes a corolla. Petals may remain united or separate. In

Gourd and Thorn apple (Dhutura ), petals are united. But in Radish and Mustard

flowers petals are separate. It may be white, yellow, red, blue and other colour.

Usually corolla is bright and fragrant.

Fig.8.21: Different parts of a complete flower(Longitudinal section)

Carpel

Stamen

Petal

Calyx

Floral axis (Thalamus)

Fig.8.22: Polysepalous calyx Fig.8.23: GamousepalousCalyx

82 General Science

Functions of corolla: Corolla protects androecium and gynoecium. Bright coloured

and fragrant corolla attracts insects resulting in pollination.

4. Androecium: Whorl inside the corolla is called androecium. Each part of the

androecium is called stamen. Stamen has three parts; such as a) Filament, b) Anther

and c) Connective.

(a) Filament: The slender and soft stalk like part of the androecium is the filament.

(b) Anther: The swollen structure on the

filament is the anther. Anther produces

large Number of pollen grains inside.

Usually anther is divided lengthwise into

two chambers. Each chamber is termed as

pollen chamber. In China rose anther is

one chambered. Stamen is the male

reproductive organ of the flower. Pollen

grains, produced in pollen sack inside the anther, take part in reproduction through

pollination.

c) Connective: The portion of filament attached to the anther is called connective.

Functions of Androecium: Production of pollen grains is the main function of

androecium.

5) Gynoecium : The inner most whorl of

flower is called gynoceium. It consists of one or

more carpels. Each carpel has three parts; such

as (a) Ovary, (b) Style and (c) Stigma.

(a) Ovary : The swollen part of gynoecium

found on the thalamus is the ovary. It contains

ovules and each ovule produce egg cells inside.

(b) Style: The thin long tube like part above the

ovary is called the style.

Fig. 8.24: Gamopetalous corolla

Petal

Fig. 8.25: Corolla Fig. 8.26: Polypetalous corolla

Fig. 8.27: Stamen

Anther

Connective

Filament

Fig. 8.28: Gynoecium

Stigma

Style

Ovary


(c) Stigma: The swollen part at top of the style is the stigma.

Functions of gynoecium: Main function of gynoecium is to produce eggs inside the

ovules. After fertilization ovary turns into a fruit and the ovules into seeds.

Classification of flowers

Based on structure, flowers are variously classified. Out of these only two types of

classifications are discussed below.

(1) Complete and incomplete flower

a) Complete flower: A flower is called comptete when it bears all the four whorls of

floral leaves like calyx, corolla, androecium and gynoecium. Example- Rose, China

rose, Thorn apple (Dhtura), Mustard etc.

b) Incomplete flower: When a flower

bears less than four floral whorls; it is

called an incomplete flower. Here one

or two whorls are absent. Example-

Bottle gourd (Lau), Gourd (Misti

kumra) etc.

(2) Unisexual and bisexual flower

a) Unisexual flower: When a flower has either androecium or gynoecium, it is called

unisexual flower; such as- Bottle gourd (Lau), Gourd (Misti kumra) etc. All unisexual

flowers are incomplete flowers. Unisexual flowers are of two types: i) Male flowers

and ii) Female flowers.

i) Male flower: When a flower bears only androecium, it is called male flower.

ii) Female flower: When a flower bears only gynoecium, it is called a female flower.

b) Bisexual flower: When both androecium and gynoecium are present in the same flower;

it is called a bisexual flower. Example- China rose, Thorn apple (Dhutura), Rose etc.

Longitudinal section

Fig. 8.29: Polypetalous corolla

Corolla

Stamen

Carpel

Sepal

Ovay

Fig. 8.30: Incompete and unisexual flower (Gourd)

Male flower

Stamen after removal of corolla

Female flower

Gynoecium after removal of corolla

84 General Science

Inflorescence

In China rose, Thorn apple (Dhutura) and Rose plants; a single flower grows at the

leaf axil. Marigold, Sunflower, Plantain etc. produce a large number of flowers in a

bunch. These are not a single flower but actually a bunch of small flowers. It is an

inflorescence. The special branch or branch system bearing a cluster of flowers is

called an inflorescence.

Kinds of Inflorescence

Depending on the position of the peduncle on the branch or stem, inflorescences are

of two types: i) Apical inflorescence and ii) Axillary inflorescence.

i) Apical inflorescence: The inflorescence that develops from the apical bud of the

stem or branches; is called apical inflorescence; such as-Mustard.

ii) Axillary inflorescence: The inflorescence that grows from the axillary bud, is

called axillary inflorescence. Example- Coconut, Pamyra palm etc.

Based on the growth of apical bud of the peduncle, inflorescences are of two types:

a) Recemose inflorescence and b) Cymose inflorescence.

This classification was proposed by Greek scientist Theophrastus, some hundred

years before Christ.

a) Recemose inflorescence

The peduncle that grows indeflnitely and the flowers bloom in acropetal or centripetal

succession, is called recemose inflorescence. Example: Mustard, Mango, Paddy,

Arum,Plantain, etc.

Characteristics of recemose type

a) Flowers bloom from the base towards the apex of the peduncle. It is known as

acropetal succession.

b) Flowers bloom from periphery to the

center of the receptacle. It is called

centripetal succession.

c) In this inflorescence, the position of the

youngest flower is at the apex of the

peduncle or at the centre of the

receptacle.

d) Growth of the peduncle is not limited. Fig. 8.31: Recemose inflorescence

Youngest flower

Older flower

Fruit


2. Cymose inflorescence

The inflorescence in which the flowers bloom at basipetal (from top towards base) or

centrifugal (from centre towards the periphery) succession is called cymose

inflorescence. Example-Heliotropium (Hatisur), Clerodendrone(Vant), Night Jasmine

(Sheuli), Arabian Jasmine (Beli), etc.

Characteristics of cymose inflorescence

a) Flowers bloom from top towards the

base. It is called basipetal succession.

b) When flowers are arranged on the

receptacle the blooming starts from

centre towards periphery. It is called

centrifugal succession.

c) In this inflorescence the youngest

flower is formed at the base while the

oldest one on the top of the peduncle.

d) Growth of the peduncle is limited.

Differences between recemose inflorescence and cymose inflorescence.

Pollination

You might have seen that butterfly, honey bee and various other insects fly from flower

to flower when the flowers bloom. These insects are attracted by fragrance and colour

of the flowers. They also collect honey from the flowers. While visiting flowers these

insects carry pollen grains from anther of one flower to stigma of another flower. This

transference of pollen grains from anther to the stigma is the pollination.

When mature pollen from the anther transferred to the stigma of the same flower or

another flower of the same plant or the flower of another plant of same species; it is

called pollination.

Oldest flower

Youngest flower

Fig. 8.32: Cymose inflorescence

Cymose

1. Youngest flower at the base

2. Growth of the peduncle is limited

3. Flowers bloom successively from

apex towards the base or from the centre

towards the periphery.

Recemose

1. Youngest flower at the top

2. Growth of the peduncle is not limited.

3. Flowers bloom successively from base

towards the apex or from the periphery

towards the centre.

86 General Science

Classification of pollination

Basically Pollination is of two types; such as a) Self pollination and b) Cross

pollination.

a) Self pollination: When pollen grains from stigma of the same flower or different

flower of the same plant is transferred to the stigma; it is called self pollination.

Example- Cotton, Tomato etc.

b) Cross pollination: When pollen grains from the anther transferred to the stigma of

a flower of another plant of the same species, it is called cross pollination. Example-

Paddy, Silk cotton etc.

Agents of cross pollination

To transfer pollen grains from the anther of one flower to the stigma of another flower

requires some agents, called pollinating agent. Usually these agents are wind, insects,

animals and water. The process is named according to the nature of agents; such as

1. Aenemophily: The flowers, where pollination takes place through wind are called

anemophilous flowers. The process is anemophily. Example- Paddy, Wheat, Sugar

cane, Maize etc.

2. Entomophily: When the pollinating agent

is insect, the flower is called an entomophilous

flower and the process is entomophily.

Example: Jasmine (Jui) Sunflower, Rose,

Mustard, Lotus, Water lily etc. While the

insects travel from flower to flower attracted

by colour, fragrance and honey; pollen grains

are transferred to the stigma of another flower.

3) Zoophily: When pollinating agent is

varieties of animals (such as sorrel, bat, bird),

the flowers are called zoophilous flower and

the process is zoophily. Example - Kadamba,

Banana, Arum, Silk Cotton, Polash etc.

4) Hydrophily: When pollination occurs

through water, the pollination process is

hydrophily and the flowers are called

hydrophilous flowers. Example Vallisnaria, Hydrilla etc.

Fig. 8.33: Entomophily

Fig. 8.34: Hydrophily


Fertilization

In this chapter we have learnt about different parts of a flower; such as stamen, carpel,

ovary, ovule, style, stigma, pollen grain, anther etc. We have learnt that eggs are produced

inside ovules. Male gametes are

produced from pollen grains. When

pollen grains are transferred to the

stigma; its outer covering burst open

and a tube like structure, called pollen

tube, comes out. Pollen tube travels

through the style and enters into the

ovule inside the ovary. Male gamete

and female gamete (egg) unite and

fuse together. This process of fusion

of male and female gametes is called

fertilization. Male and female gametes differ in size, shape and physiologic behaviour.

Importance of fertilization

After fertilization, ovules become seeds; while ovary turns into a fruit. Seeds ensure

the continuity of species. Animals, birds and man live on fruits and seeds of plants.

Mango, Jambu, Jackfruits, Litchi, Rice, Wheat, Maize, orange, Sugar cane etc. give us

food. These are the out come err fertilization. So, fertilization is very important

process in a plants life.

Origin of fruit

We get fruit from the flower. Do all parts of a flower turn into a fruit? Not all but few

parts of the flower form the fruit. Usually the ovary modifies into a fruit. This

modification of the ovary does not occur automatically but through fertilization process.


1. Jasmine is a-

a. Zoophilous flower b. Anemophilous flowerc. Hydrophilous flower d. Entomophilous flower

2. How many whorls are there in a complete flower?

a. Two b. Three

c. Four d. Five

Fig. 8.35: Fertilization

Floral azis(Thaliamus)

Male gametes

Eggs

Anther

Stigms

Ovary

Ovale

Petal

Calyx

Pollan grain

88 General Science

3. Which of the following plant sheds leaves yearly?

a. Debdaru(pine) b. Date palm

c. Shal d. Mango tree 01740941057

4. What is the function of the label A of above diagram?

i. Holds pollen grains

ii. Attracts insect

iii. Converts into fruit


a. i b. ii

c. iii d. i, iii

5. Which of the following set is called underground modified stem?

a. Ginger, Potato, Onion, Garlic

b. Oal, Amrul, Aurum, Cactus

c. Bryophyllum, Ghritakumari, Potato, Ginger

d. Lily, Tulip, Ginger, Aurum

Answer the questions 6 and 7 with the help of the diagram.

6. The part of the diagram above is a plant's-

a. stem b. sac

c. leaf blade d. flower


Figure

A

X

7. By the part 'X' a plant-

i. catches insects

ii. collects protein type-of food

iii. collects.water


a. i b. ii

c. i, ii d. ii, iii

8. Which one is the major function of calyx?

a. Exchange food b. Protect bud

c. Attract insects d. Collect pollen grains

Creative Questions

1.

a. How many whorls are there in a complete flower?

b. Describe the functions of part A.

c. Complete the above incomplete diagram and label its different parts.

d. Discuss the importance of the part C

2.

a. What is modified stem?

b. What is the function of the part A in diagram?

c. Drawing a diagram of Ginger compare part B and D of diagram above

d. Explain for which part of the diagram, it is a modified stem?

B

A

C

D

90 General Science

A

C

Chapter - 9

Microbial World

Microbes

'Microbes' or 'germs' mean very small

organisms. There are many such organisms

in soil, water, air and environment; which

cannot be seen by unaided eyes. Its

existence is realized only when seen under

a microscope.

These minute organisms are termed as

microbes. Rickettsia, Bacteria, Fungi,

Protozoa etc. are the microbes. Most of

them are parasites and produce diseases in

the host body. Virus is also a special kind of

microbe.

Virus

Virus is a microbe or an entity in between

living and nonliving. Their size is very

small; about 12-400 nanometer (one

nanometer is equivalent to one millionth of

a millimeter). It cannot be seen without the

help of an electron microscope. They cannot

lead independent life. They live inside a host

body as a parasite. The body of a virus

particle does not have any cell membrane,

cell wall, organised nucleus, cytoplasm

and different cytoplasmic inclusions. It a

coating of protein having nucleic acid (DNA or RNA) inside. In some virus this

structure bears a tail like projection. In this case the virus particle looks like T.

Example- T2 or T4- phage virus. Few fibrous growths are visible at open end of the

tail.

Outside the body of the host, the virus does not show any sign of life, like- respiration,

nutrition, metabolism, reproduction etc. Virus particles can be crystallized out and can

be preserved for long time. When they again get inside the living body of the host,

then it can multiply like other microbes.

Tebacco Mosaic Virus

Poliomyeletis Virus

Influnza Virus

T2 Bacteriophage

Fig. 9.1: Different types of virus

Because of absence of normal functioning of life activities and ability to become

crystallized they seem to be non living. But when activities like ability to multiply and

presence of DNA are shown; they seem to be living entities. Due to this dual

characteristics the virus is placed in between living and nonliving. They are treated as

microbes because they are microscopic and show some symptoms of life.

Harmful effect of virus

Virus harms us mainly by causing diseases; such as:

a) Diseases of plants: Virus is responsible for mosaic diseases of tobacco, beans,

papaya etc.

b) Diseases of man: The most fatal disease of man is the AIDS disease. It is caused

by HIV virus. Besides, Small Pox, Measles, Poliomyelitis, Influenza, Mumps, Herpes,

Hepatitis etc. are also caused by virus.

c) Diseases of animal and birds: Foot and mouth disease of cattle, hydrophobia of

cats and dogs; Foot and mouth diseases of cattle IBD or Infectious Bursal Disease

(Ranikhet) of poultry etc. are caused by virus.

d) Destroying beneficial bacteria: Some virus destroy some beneficial bacteria.

Rickettsia

Rickettsia looks like a rod or an egg. They are 350 nanometers long and 25

nanometers wide. It can be seen under a powerful optical microscope; with a great

difficulty. An electron microscope show their structures very similar-to a bacteria. But

its size is very small. All rickettsia except one species do not show any sign of life

outside the host body. They are treated as an intermediate micro organism in between

virus and bacteria.

Different types of rickettsia live in the elementary canal of spider, insects and other

arthropods. Few species cause disease in man. In cold countries the louse transmitted

typhus fever is worth mentioning disease.

Bacteria

Bacteria are microbial organisms. It is larger than the virus and rickettsia. Even then

they are not visible with unaided eyes. They are visible only under a microscope. The

size of bacteria varies from 1-5 micron (one micron is one thousandth part of a milli

meter). Bacteria are unicellular. Bacterial cells are not like normal plant cells. These

cells are protocells or primitive cells. These cells do not have mitochondria, plastids,

golgi bodies etc. and their nuclei are not organized. It is bounded by a cell wall. The

cytoplasm of the cell bears only one DNA molecule and there is no nuclear

membrane. So, it is called nucleoid and not nucleus.

92 General Science

Classification of bacteria based on shape

Bacterial cells are of various shapes. According to shape these are classified into four

groups. They are as follows:

1. When the cell shape is round or egg shaped, they are called coccus (plu-cocci).

2. Rod like bacterial group is called bacillus (plu.- bacilli).

3. When the shape is like comma (,), it is called vibrio.

4. When bacterial cells are twisted like a loose spring, it is called spirillum.

When coccus bacteria remain attached to each other in a row; is called streptococcus

and when coccus bactaria remain attached to each other in an irregular group, is called

staphylococcus. Few bacteria are parasitic. They are responsible for diseases of man,

animal and plants. Few other bacteria get .energy from decomposed dead remains of

organisms. In this way, they keep the environment clean. Nitrogen fixing bacteria fix

atmospheric free oxygen to the soil. As a result, soil becomes fertile. Some bacteria

can get energy from inorganic substances like iron, sulfur, and nitrogen.

Importance of bacteria

Bacteria has both harmful and useful

roles. It has much contribution to

our economy. Now let us discuss

how bacteria play the useful and

harmful roles.

Beneficial effects of bacteria

I. Increase of soil fertility:

Bacteria decompose dead bodies of animals and plants and increases soil fertility.

Example-Nitrobacter. Besides, few soil bacteria fix nitrogen from the atmosphere to

the soil. This process increase fertility of soil. Example- Azotobacter, Clostridium etc.

In leguminous plants some symbiotic bacteria live in the roots and form root nodules.

These bacteria convert free nitrogen of the air to nitrates and make the soil fertile.

2. In jute rotting: Jute has a great impact on the economy of Bangladesh. Through

export of jute Bangladesh earns huge amount of foreign exchange. Jute stems after

cutting, immerged in water for few days for loosening of fibres. Some bacteria play

roles in this rotting process.

3. In curing process: Bacteria have important role in processing of tea,

coffee,tobacco etc.

4. In leather industry: Bacteria help to remove hair from raw skin of cattle.

Fig. 9.2: Different shapes of bacteria

Microbial World 93

5. Production of milk product: Bacteria play its role in the production of curd,

butter, cheese etc. from milk.

6. Preparation of medicine: Vaccines for cholera, typhoid, tuber-culosis, etc. are

produced from bacteria.

7. Decomposing garbage: Bacteria decompose garbage in drains and canals and help

to keep the environment clean.

Harmful Effects of bacteria

1. Diseases

a) Human diseases: Bacteria are responsible for fatal human diseases like Cholera,

Typhoid, Tuberculosis, Pneumonia, Diphtheria, Tetanus, Syphilis etc.

b) Diseases of animals and birds: Some of the diseases caused by bacteria are

Tuberculosis of cattle, Cholera of poultry etc.

c) Diseases of plants: Blight of rice, Gummosis of sugar cane, Canker of tomato,

Brown rot of potato etc. are caused by bacteria.

2. Rotting of food: Bacteria do harm to us by rotting of fruits, vegetables, fish, meat,

milk and cooked food staffs etc.

3. Denitrification: Few soil bacteria produce free nitrogen gas from nitrates of the

soil and leave in the environment. As a result, the fertility of soil decreases.

Fungi

Fungi (sing-Fungus ) are usually multicellular organism. Unicellular fungi are also

available. Previously they were treated as a member of plant kingdom; now they are

treated in a separate kingdom. They have no chlorophyll so they are nongreen. and

cannot manufacture their own food. They cannot take solid foods like animals.

They live only on liquid food from the body of other organisms, dead or alive. So,

fungi are either saprophytes or parasites. Fungi that live on decomposed dead remains

Fig.9.3: Mucor Fig.9.4:Mushroom

94 General Science

of organisms are called saprophytes; such as Penicillium, Mucor, Yeast, Agaricus etc.

While other fungi that draw their food from living body of the host, are called

parasites; such as Phytophthora, Puccinia etc.

Importance of fungi

Fungi have many economic importances. They have both beneficial as well as harmful

effects.

Beneficial Effects

a) Production of medicine: The most important antibiotic drug is Penicillin. It is

extracted mainly from Penicillium notatum. Penicillin is the medicine used for throat

pain, septic ulceration, tonsillitis etc. Microbiologist Alexander Flemming discovered

Penicillin in 1928 from Penicillium fungi.

b) Soil fertility: Fungi living in the soil decompose different organic matters and

increase fertility of the soil.

c) Production of alcohol or ethanol: Fungus plays an important role in production of

alcohol or ethanol. Yeast, an unicellular saprophytic fungus has a role in its

production.

d) Bakery products: Yeast is necessary in the production of bread. Yeasts make

breads swollen up and soft.

e) As food: Some fungi such as Mushrooms, are nutritious food. People of many

countries are fond of this food.

Harmful effect

a) Diseases

1. Human diseases : Fungi cause diseases like Ring worm, Lentego etc.

2. Diseases of plants: Fungi cause various diseases of important crop plants like

Paddy, Potato, Wheat, Sugar cane etc.

3. Spoil of food: Fungi spoil bread, fish, meat, cooked food, vegetables, fruits, jelly,

jam, pickles etc.

4. Destroying Furniture, leather and optical goods: Fungi spoil wooden furniture,

leather goods, camera, microscopes etc.

5. Poisonous action: Some mushrooms are very poisonous. When it is taken, it may

cause even death.

Microbial World 95

Cause of dysentery

Dysentery is a dangerous disease of man. It is of two types. (1) Bacillary dysentery

and (2) Amoebic dysentery. Bacillary dysentery is caused by bacteria and amoebic

dysentery is caused by one type of amoeba, called Entamoeba hystolytica.

These parasites get into human body through food and water. Entamoeba lives in large

intestine and sucks nutrition from the intestinal wall. It produces ulceration on the wall.

As a result, blood and mucous are released with stool. This symptom may accompany

with fever. The affected person feels pain at the abdomen. In some cases the intestinal

wall may develop pores. This parasite can spread to the liver and other parts of the

body including brain through blood. In extreme case the patient may die even.

What is Entamoeba?

Entamoeba is a member of phylum protozoa of animal kingdom. They are unicellular

animals, very small in size and cannot be seen with unaided eyes. Entamoeba

histolytica is the human parasite living in the large intestine. They produce dysentery.

Six other species of Entamoeba live in the human body as heterophytes but they do

not cause diseases.

Morphology of Entamoeba

The Entamoeba has no definite shape.

Its shape is changeable with the

formation of pseudopodia. It is a

unicellular animal. The cell membrane

is living, thin, and elastic. The cell is

divided into two parts; such as

cytoplasm and nucleus. It moves by

producing pseudopodia from

cytoplasm. Its cytoplasm contains

different types of vacuoles.

Nucleus regulates different physiological activities of the cell.

Life history: In unfavourable conditions Entamoeba develops a covering around its

body with the help of cytoplasm. This stage is called a cyst. Large number of cysts

come out with stool of infected persons. Drinking polluted water or taking unwashed

raw vegetables are the ways these cysts enter into the human digestive tract. They may

enter into the human body with other foods. Cysts travel through stomach to the small

intestine. Here in small intestine the outer covering of the cyst is lost and they enter

into the large intestine, leading to the production of new cells. The new cells gradually

become mature and secrete enzymes that digest epithelial cell layer of large intestine

wall. Entamoeba cells take pieces of epithelial cells and red blood cells as their food.

Fig. 9.5: Entomoeba

Food vacuole

Nucleus

96 General Science

As a result, small wounds are created on the wall of large intestine. Due to these

wounds small pieces of epithelial layer come out with stool as mucous.

Preventive and curative measures against dysenteryThe following preventive and curative measures may be taken against dysentery .

1. Drinking water must be boiled properly.

2. Food must be covered to avoid contamination through fly or cockroach.

3. Fruits and vegetables must be washed thoroughly before eating.

4. Hands should be washed thoroughly before taking anything or clean spoon should

be used.

5. Nails should not be allowed to grow long.

6. Use of clean and sanitary latrine should be ensured.

7. Hands should be washed well after defecation.

8. A sick person must take medicine as per advice of a physician.


1. Which one of the following lies in between the non-living and the living objects?

a. Bacteria b. Fungi

c. Virus d. Entamoeba

2. Which one of the following does not belong to micro organism?

a. Fungi b. Bacteria

c. Rickettsia d. Algae

3. Which is viral disease?

a. Cholera b. Diphtheria

c. Newcastle (Ranikhet) d. Syphilis

4. Which one of the following diagrams is non-cellular?

a. Fig: 1 b. Fig: 2 c. Fig: 3 d. Fig: 4

Microbial World 97

5. Which increases the fertility of land?

i. Mucor

ii. Nitrobactor

iii. Rhizobium


a. i b. ii, iii

c. ii d. iii

Creative Questions

1. While returning home from school Arif bought some 'Chotpati' from road side and

ate up. After coming back home within a few minutes he got pain in his stomach and

suffered from fever. He started going to toilet frequently and became sick. Parents

took him to the doctor.

a. What is the name of the disease of Arif?

b. Describe how the disease spreads.

c. Explain the cause of pain in the stomach of Arif.

d. What would happen if Arif did not go to the doctor?

2.

a. What is the name of the micro organism of fig B?

b. Explain the two main characteristics of the cell of micro Aorganism of fig

c. Explain why the organism of fig B needs another living body to remain

alive?

d. Discuss how the organism of fig A plays important role to maintain the balance

of environment?

Figure A Figure B

98 General Science

Chapter -10

Animal Kingdom

Many different kinds of animals live in different environments in the world. Some of the

animals are very minute while some are huge. Some live on land while others live in

water. Some of the animals spread out their wings and fly about in the sky and some

crawl on land. Many members of the animal kingdom are helpful for human beings,

while others are harmful. Depending on the presence or absence of vertebral column, the

animal kingdom is divided into two major categories. Such as-

1. Animals that do not have vertebral column are called invertebrates. For example:

earthworm, spider, crab, scorpion and others.

2. Animals that have vertebral column or spinal cord are called vertebrates. For example:

human, cow, goat, fish and others.

Invertebrate AnimalThere are different types of invertebrate animals in this world. Depending on the number

of cells, external covering of the body, body segments, excretory system, circulatory

system, perforations, shape and so on, the animals are divided into several groups. These

groups are known as phylum (plural: phyla). Animal kingdom is divided into ten major

phyla, of which nine are for the invertebrates. The vertebrate animals belong to the

phylum chordata. The following is a brief description with examples of the different

phyla in animal kingdom.

1. Name of the Phylum: Protozoa: Example-amoeba, entamoeba, and malarial parasite.

Major feature(1) Unicellular animal, all the body

functions is carried out by one cell

(2) Can be seen only with the help of a

microscope

(3) Moves around with the help of

pseu-dopodia

(4) Lives individually or in groups

(5) Some species live independently, others are parasitic in nature

(6) The cell contains a nucleus

(7) Varies in shape - round, oval, elongated, irregular

2. Name of the Phylum: Porifera: Animals in this phylum contain numerous

perforations on their body. This is why they are called porifera or animal with multiple

porosity Example: leucosolenia

Fig. 10.1: Amoeba

Major feature(1) Multicellular aquatic animal

(2) Numerous pores on the body

(3) Water along with nutrients and

oxygen enter and the waste is

excreted through the pores

(4) Cannot move independently

(5) Mainly marine, with a few fresh-

water species

(6) Commonly known as "sponge"

3. Name of the Phylum: Cnidaria or Coelenterata: Animals in this phylum have a

hollow body that acts as both body cavity and digestive organ. The hollow body is

also known as coelenteron. For example: coral, jellyfish, hydra, and so on.

Major feature(1) Body is hollow like a pipe

(2) Has a mouth but no anus

(3) Multicellular animal

(4) Body is made up of two layers of

cells, a jelly-like substance lies

between two layers

(5) Has hand-like projections or

tentacles used for locomotion and

ingestion of food

4. Name of the Phylum: Platyhelminthes or flatworms: Flat bodied worm-like

animals belong to this phylum. For example: tapeworm and liver fluke.

Major feature

(I) Multicellular animal

(2) Parasitic animal

(3) Body is flat like a ribbon and has

no body cavity

(4) Head has a sucker

(5) Has mouth but no anus

(6) Some of these have externally

segmented body

Fig: 10.2: Leucosolenia

Fig: 10.3: Hydra and Jellyfish

Temtacle

Hypostoms

Bud

Sole of foot

Fig: 10.4: Tapeworm

100 General Science

5. Name of the Phylum: Nematoda or roundworms: Round shaped worms are included in this phylum. For example: pinworm, hookworm, ascaris, filarial worm

Major feature(l) The body is cylindrical, round, tapering and unsegmented

(2) Has both mouth and anus

(3) Lives independently in soil and in water

(4) Many live as parasites in plants or animals

6. Name of the Phylum: Annelida: These animals have ring-shaped segmented bodies. For example: earthworm, leech, and so on.

Major feature

(I) Have ring-shaped segmented body

(2) Each body segment has bristles called setae that help in locomotion

(3) Multicellular animal

(4) Have mouth and anus

(5) Live in water or moist soil

7. Name of the Phylum: Arthropoda or arthropods: Animals in this group have at least three pairs of legs. For example: prawn, crab, scorpion, cockroach, fly and so on.

Major feature

(I) Segmented body consisting of head, thorax and abdomen

(2) To most segments is attached a pair of articulated accessory organ or legs

(3) Skin has an compound substance called chitin, sometimes the skin is very hard

(4) Eats different types of food and so its jaw is adapted accordingly

Fig: 10.6: Earthworm

Fig: 10.5: Roundworm

Fig: 10.7: Crab and cockroach

Cephalothorax Thorax

Wing

Leg

Abdomen

Animal Kingdom 101

8. Name of the Phylum: Mollusca: Animals in this group are softbodied covered by

a shell. For example: snail, oyster, octopus, and so on.

Major feature

(1) Body is soft, is not segmented

(2) Body is covered with a hard shell

(3) Lower (ventral) part of the body

has a muscular foot and the upper part

contains the intestine and other organs

(4) Most live in sea and freshwater, while some live on land

9. Name of the Phylum: Echinodermata: All animals of this phylum are marine. For

example: starfish, sea urchin, sea lily and so on.

Major feature

(I) The skin is covered by small hard

sheets formed of lime like substance

(2) Numerous big and small spiny

projections are present on the external

surface of the body

(3) Like spokes limbs project outward

from the centre of the body

(4) Most live freely in the bottom of the sea over sand, mud and rocks. The sea lily is

attached with a stalk to the bottom of the sea and cannot move.

10. Name of the Phylum: Chordata: The vertebrates are included in this phylum.

What are vertebrate animals and their classification

Animals that have vertebral column on the dorsal aspect of the body are called

vertebrates. For example: fish, frog, crocodile, bird, tiger, human and so on. Some

animals with simpler form of spinal cord, mostly marine, are also included in this

phylum.

Major feature of phylum chordata

1. In the embryonic stage, the dorsal aspect contains a elastic column called notocord.

The notocord later develops into the vertebral column

2 Pharynx (behind the head) contains the respiratory hole

3. Dorsally there is a hollow bundle of nervous tissue

4. A tail lies behind the anus

Fig: 10.8 Oyster and Snail

Fig: 10.9 Starfish

102 General Science

Toad Golden Frog Tree Frog

Fig: 10.11: Amphibans

Depending on the external features, life style, locomotion, and so on, the vertebrates

are classified into five classes that are described below with examples.

1. Name of class : Pisces or fishes:All fishes living in salt- or freshwater are included in this class. Examples include:

hilsa, ruhi (salmon), catla, mrigel, puti, koi, boal, shul, gojar, shing, prawn, magur,

ayer, tangra, meni, roopchanda (pomfret), ritha, coral, shark, and so on.

Fig: 10.10: Fishes

Feature(1) With the exception of a few, most fish have scales. For example: shing, boal,

magur, tangra and so on

(2) Gills are present, lung is absent

(3) Odd or even number of fins are present in the body

(4) Eggs are laid that hatch into baby fish

Name of class: AmphibiaSome vertebrates can live in both water and land and are called amphibians. The eggs

and larvae of the amphibians do not survive without water. For example: toad, golden

frog, tree frog and so on.

PutiShing

Hilsha

Ruhi Catla

Animal Kingdom 103

Fig: 10.12: Reptiles

Lizard

Chanmeleon

Crocodile

Fig: 10.13: Birds

Bulbuli

Indian myna

Sparrow Robin

Weaver bird Warbling bird

Feature(1) Hairless, featherless, scaleless body with soft skin

(2) Has four legs but no nail

(3) Lung and gill present

(4) No neck, the head is connected to the trunk

(5) Catch their prey with the tongue and swallow it

(6) Cold-blooded animal

3. Name of class: ReptiliaVertebrates that move on the ventral aspect of the body are called reptiles. For example: lizard, tortoise, turtle, snake, crocodile, ghorial, guishap, chameleon, and so on.

Feature(1) Moves on the ventral aspect of the body

(2) Rough skin that may be scaly or hard

(3) Lung present

(4) Cold-blooded animal

(5) Usually they have four legs with nails but snake legs are dispelled

(6) Lay eggs

4. Name of class: Aves or birdsVertebrates that have feathers are called birds. For example: duck, hen, pigeon, sparrow, weaver bird, warbling bird, king fisher, crow, cuckoo, moyna, parrot, kite, vulture, eagle, Indian moyna, stork, crane, peacock, kiwi, emu, ostrich and so on. The

104 General Science

kiwi, emu, and ostrich cannot fly as their wings are underdeveloped but can run as

their legs are strong and well formed.

Feature(I) Body is covered with feathers

(2) Beak is present but tooth is absent

(3) Lung and air sac are present

(4) Warm-blooded animal

(5) Big bones are hollow and light

5. Name of class: Mammalia or mammals Vertebrates that give birth to live young ones or babies that feed on mothers' milk are called mammals. For example: human, cat, cow, buffalo, dog, elephant, horse, tiger, bear, monkey, mongoose, blue whale, hedgehog, rat, deer, and so on.

Feature

(I) Body is covered with hairs

(2) Gives birth to young ones that feed on mothers' milk

(3) Four-footed animal, except for human beings who are bipedal

(4) Fingers have nails, sometimes the nails are transformed into claws or hooves

(5) Warm-blooded animal

Introduction to Earthworm and Lizard

EarthwormEarthworms usually live in moist soil and cannot survive in dry soil. They cannot tolerate light and heat and so during the daytime they stay underground and look for food at night. Earthworms are usually 7-8 inches long, with segmented rounded body tapering towards the ends. They have no eye, ear, or lung. Breathing is carried out through the skin, which is also very sensitive. The body is divided

Cow

Man

RatFig: 10.14 Mammals

Fig: 10.14 Earthworm

Animal Kingdom 105

into about 150 ring-like segments. Each body segment has four pairs of minute setae or bristles that help in locomotion. Earthworms burrow into the soil as a result air and fertilizer easily enter the soil. This makes the soil fertile and soft. As the earthworms

help to increase soil fertility, they are called "natural plough".

Lizard

Lizard is a variety of reptile. The

body is covered with small scales and

is divided into four parts-head, neck,

trunk and tail. They are cold-blooded

animals having red coloured blood.

They have four legs, two eyes and

many small teeth. The soles of the

feet are padded with soft muscle that

help them to move freely on straight

walls and under surface of roofs. If

the tail of lizard falls off, it

regenerates over time. Lizards can lay

up to 20 eggs at a time. They live on insects. A lizard can move its two eyes

independently and so has a greater visual field. Lizards eat many of the harmful

insects and thus are beneficial to human beings.

Comparison between invertebrate (earthworm) and vertebrate (lizard)

Earthworm Lizard

1) Lives in moist soil 1) Lives in dry places

2) The skin in soft and moist 2) Body is covered by small scales

3) Nocturnal animal 3) Moves around in both light and dark

4) The body is segmented 4) Body is divided into head, neck, trunk and tail

5) Has no teeth 5) Has small teeth

6) Has no leg or tail 6) Has four legs and a tail

7) It respires through the skin 7) It respires with the help of lungs

8) Has no eye 8) Has two eye

9) Has no vertebral column 9) Has vertebral column

Fig: 10.15 Lizard

106 General Science

Exercise


1. Whicb one is the vertebrate animal?

a. Amoeba b. Mongoose

c. Sponge d. Fly

2. Which one has sucker on the head?

a. Tape worm b. Thread worm

c. Annelida d. Joint footed animal

3. Which animal is called the plough of nature?

a. Earthworm b. Protozoa

c. Cockroach d. Thread worm

4. Which class the blue whale belongs to?

a. Mammalia b. Aves

c. Pisces d. Reptilia

5. Which class of vertebrate animals has no tooth?

a. Aves b. Amphibia

c. Pisces d. Reptilia

6. The characteristics of Mollusca are

i. mostly live in saline water

ii. body is covered with hard shells

iii. body is segmented


a. i b. i, ii

c. ii, ii d. i, ii & iii

7. The amphibian animals are those who-

i. have lung and air sac

ii. have lung and gills

iii. have gills but no lung


a. i b. ii


Animal Kingdom 107

Fig.

8. The phylum of the animal of the fig is

a. Arthropoda b. Annelida

c. Platyhelminthes d. Nematoda

9. Which is applicable for the animal in figure?

i. Setae are present in each body segment

ii. Sucker present on the head

iii. Has mouth but no anus


a. i b. ii


Creative Questions

1. Fig: A Fig: B Fig: C

a Which class Fig: B belongs to?

b. Why is the animal of (Fig:c) different from the other two?

c. Explain why you would keep Dolphin in a specified group with one of the

animals above.

d. Although ostrich and fig: B belong to the same class, ostrich is little bit different.

Discuss.

108 General Science

2.

a. Which kind of animal is fig: A?

b. Explain a special characteristic of the animal of fig: B.

c. Two types, of respiratory system are found in the life cycle of the animal of

fig: B- explain.

d. How the animal of fig: A depends on the animal of fig: B and analyze its effects

on environment.

A: B:

Animal Kingdom 109

Chapter - 11

Interdependency of Plants and Animals

Water, Carbon and Oxygen Cycles

Water, carbon and oxygen are essential for the survival of life. The protoplasm of

living cells contains 75-90 percent water. So, cells cannot survive without water.

Food is necessary for survival of all life. Compound of carbon, carbon dioxide, is

needed for the preparation of food. Again, oxygen is needed for respiration and life

cannot survive even for a short time without oxygen.

Hence, it is seen that no life can exist without anyone of water, carbon and oxygen.

These substances are used continuously but the amount of these substances in the

environment is not decreasing significantly. This is because on one hand these are

being used up, while on the other these are being produced too. Now we shall try to

know how water, carbon, and oxygen cycles are going on in nature.

Water Cycle

Water is essential for the survival of

every life. No living thing can exist

without water. About 75 percent of

our body is water.

Oceans are the main source of water on

the earth's surface. Besides, numerous

rivers, canals, lakes and so on are also

sources of water. Due to sun heat the

water from these water bodies evaporate

and enter the atmosphere.

In the atmosphere the water vapour condense to form cloud. Later, rain falls from

these clouds. Thus, the water comes back to earth surface. The soil absorbs this water.

Some of this absorbed water is taken up by the plant roots. Rest of the water flows

underground to reach the rivers, other water bodies and finally the ocean. Through

transpiration process of the plants, the water as water vapour is again passed into the

atmosphere. This water vapour forms cloud and again comes back to the earth as rain

water. In this way, water evaporates from different water bodies to form water vapour

and thence cloud and then returns to earth as rain. This continuous recycling of water

is called water cycle.

Fig: 11.1 Water cycle

Flow with wind fromocean to overland

Condensation & rainfallRainfall

Evaporation

Falling into

Evaporation

Ocean

60100

775

875

Importance of Water Cycle

(1) One of the main components of environment is water. Water has unlimited

influence on weather. Water plays a major role in maintaining the humidity of

atmosphere. Transpiration from plants keeps the air humid and contributes towards

rainfall. This not only keeps the environment cool but also increases production of

vegetation. Side by side, it also maintains the equilibrium of water in the environment.

(2) As there are no plants in the deserts, there is scanty rainfall. Also the concentration

of carbon dioxide in the environment is relatively more in the deserts leading to

increased temperature.

Carbon Cycle

Carbon has an important role in the formation of living bodies. No organic compound is

formed without carbon. In other word, where there is life, there is carbon. The main source

of carbon in the atmosphere is carbon dioxide gas. Normally, the air contains about 0.036

percent carbon dioxide. The carbon dioxide in air comes from different sources.

During photosynthesis the green plants directly take up carbon dioxide from the

atmosphere to produce carbohydrate type of food. During this process the carbon

dioxide is converted into organic compound, which is stored in the plant body. No

animal can produce its own food. Every animal, directly or indirectly takes the food

produced by the plants. That is, by taking in carbon compound, the animals meet the

demand of carbon.

Fig: 11.2 Carbon cycle

Living things receive carbon as carbon dioxide from different environments. On the

other hand, during respiration, the carbon compound is broken down to produce

carbon dioxide. Thus the carbon is returned to the environment.

Atmosheric CO2

Due to burning of oil, coal & wood

Due to volcanic eruption

By respiration

Fossll layer

New fossll layer

Due todecomposition

Photosynthes

ised

glucose

Gaseous exchangebetween air & water

Interdependency of Plants and Animals 111

The carbon compound in the dead plants and animals are decomposed by bacteria and

fungus to release carbondioxide, which is returned to the environment. In this way the.

exchange of carbon between living things and the environment goes on. Besides, due

to burning of different fuel such as wood, coal, paper, kerosene, petrol, diesel and so

on also produce carbon dioxide that is passed to the atmosphere. Carbon dioxide is

also produced during volcanic eruption.

The above discussion shows that equilibrium of carbon in the environment is

maintained by the carbon cycle. Thus, the process by which exchange of carbon

occurs between living things and the environment to maintain the equilibrium of

carbon in nature is called carbon cycle.

Importance of Carbon Cycle

(1) For food, all living beings are dependent on photosynthesis. Green plants

manufacture food by photosynthesis. Carbon dioxide is used in the preparation of

food. If there is depletion of carbon dioxide in the environment, plants will not be able

to manufacture food. As a result, all living beings will die due to lack of food.

(2) On the other hand, it would be impossible for living beings to survive if the carbon

dioxide in the environment increased. For this reason, the carbon cycle plays an

important role in maintaining the equilibrium of carbon dioxide in the environment.

As only the green vegetation takes up carbon dioxide to produce food and keeps the

environment healthy, so we should plant more trees to increase the number of plants.

Oxygen Cycle

Oxygen is one of the components of atmosphere. The content of oxygen in the

atmosphere is about 21 percent. Oxygen is also present in dissolved form in water.

Both air and water are the sources of

oxygen for living things. Oxygen is

essential for the survival of life. No

aquatic or terrestrial living thing can

survive without oxygen, as oxygen is

needed for respiration. For respiration,

terrestrial plants take in oxygen from

air., while the aquatic plants take in

oxygen from water. Besides respiration

of living things, oxygen is also needed

for combustion of anything.

It may appear that due to constant use

of oxygen, the oxygen content of theFig: 11.3 Oxygen cycle

Photosynthesis

Photosynthesis

Respiration

Respiration

Respira

tion

Respiration

112 General Science

environment may be depleted some day. As a result, life will not survive. Apparently it

may seem so, but it is true that the environment will never face oxygen depletion, for

on one hand as oxygen is being depleted, on the other hand in the same way there is

mechanism to replenish the deficiency.

During respiration, reaction of food with oxygen occurs. As a result, carbon dioxide

and water is formed. Both of these are compounds of oxygen. By this reaction the free

oxygen is transformed into oxygen compound and returned to the environment.

Through photosynthesis, green plants with the help of water and carbon dioxide

produce carbohydrate like food. In the process, oxygen is released as by-product,

which then pass back into the environment. As plants produce oxygen by

photosynthetic process, there should be plenty of green plants in the environment.

Compared to the rural areas, there is less number of plants in the urban areas. So,

there is less atmospheric oxygen in urban than in rural settings. So, measures to

increase the supply of oxygen in urban areas can be done by planting more trees or

cultivating more vegetable.

The above discussion shows that during respiration plants take up oxygen. On the

other hand, release oxygen during photosynthesis. Taking up of oxygen on one hand

and releasing it on the other keeps the oxygen content unaltered in the environment.

This is called oxygen cycle.

Importance of Oxygen Cycle

(1) The oxygen equilibrium in the environment is maintained through photosynthesis.

(2) The necessary balance of oxygen and carbon dioxide essential for life is

maintained through this cycle.

Photosynthesis

Whoever has life is living. Living organisms include plants, animals, and

microorganisms. All living beings take food for survival. No living being can survive

without food. As food helps to form the structure of body and supply energy for

different organic activities. No animal can produce its own food. Directly or indirectly

animals have to depend on plants for food. Plants are not dependent on animals for

food, they produce their own food, that is they are autotrophic.

The process by which chlorophyll containing cells of green plants in the presence of

sunlight convert carbon dioxide and water into carbohydrate type of food and releases

oxygen as by product is called photosynthesis.

Carbon dioxide + water Glucose + OxygenSunlight

Chlorophyll


By photosynthesis, carbon dioxide reacts with water to produce food. In a factory, just

as raw material is needed to be supplied to produce anything, in the same way plants

need supply of raw materials to produce food. The raw materials needed in the plant

body to produce food are water and carbon dioxide. As a result of the chemical

reaction of water and carbon dioxide in the presence of sunlight within chlorophyll

containing cell, carbohydrate type of food is produced. In factories while producing

something from raw material, some additional substance is also produced. These are

called by-products, for example, glycerine is formed as a by-product during soap

production. In the same way, oxygen is produced as a by-product during

photosynthesis.

During daytime, green plants carry on the process of photosynthesis as long as they get

sunlight and continue to release oxygen into the atmosphere. This is the way by which

oxygen in increased in the atmosphere. It is only through photosynthesis in green

plants that oxygen increases in the atmosphere.

So, in the forest the air content of oxygen is more and carbon dioxide is less. Because

there are less trees in the urban compared to rural areas, the atmosphere in urban areas

is more polluted, that is, urban areas contain more carbon dioxide and less oxygen.

By Photosynthetic Process Plants Release Oxygen

By photosynthetic process plants

release oxygen. We can set up the

experiment ourselves. Listen how we

shall do the experiment.

For this experiment, some fresh

aquatic plants or parts of plant need

to be collected. In addition, a funnel, a

test tube, a beaker, and a matchbox is

needed. It is possible to collect algae

from nearby pond.

The plants are placed in a beaker and covered with an inverted funnel. The beaker is

then filled with water in such a way that the nozzle of the funnel is immersed. Now, a

test tube is filled completely with water and its mouth is closed with the fingertip and

inverted over the nozzle of the funnel. This has to be done very carefully so that the

water in the test tube does not spill out. The beaker is then placed in sunlight and

watched for sometime. After sometime it will be seen that a gas is being produced in

the beaker which collects in the test tube and water from the test tube enters the beaker.

When the gas fills the test tube, the open end of the tube is closed with a fingertip and

is brought out of the beaker in the inverted position. A flameless burning matchstick is

held near the mouth of the test tube. It will be seen that the matchstick burns with a

bright flame. It is possible for the stick to burn thus as the test tube contains oxygen.

Fig: 11.4 Release of oxygen duringphotosynthetic process

Oxygen gas

Test tube

Funnel

Hydrilla

Bubble

Beaker

114 General Science

It can be proved in this way that green plants give out oxygen through the process of

photosynthesis.

Carbon Dioxide and Oxygen balance through PhotosynthesisThe atmospheric content of oxygen is about 21 percent and carbon dioxide is 0.036

percent. During respiration, all living things take up oxygen from the environment

and give out carbon dioxide in the environment. This decreases the amount of oxygen

and increases carbon dioxide in the atmosphere. Apparently, it appears that oxygen

content in the environment will be depleted and carbon dioxide will fill up the

environment. But this does not happen. The reason is that during the process of

photosynthesis all green plants take up carbon dioxide from the atmosphere and give

out oxygen. In this way the balance of carbon dioxide and oxygen is maintained in

the environment.

RespirationEnergy is required to accomplish all organic functions in living bodies. The organic

functions that go on in the living body, such as production of new cells, transportation

of food to the different parts of the body, production of flowers and seeds in plants

and so on all require energy. In living things, this energy is derived from carbohydrate

type of food. Green plants produce this food through photosynthesis.

Sun is the source of all energy. When plants produce carbohydrate type of food

through photosynthesis, the solar energy is converted to chemical energy and stored as

potential energy in food.

In cells, oxygen reacts chemically with food. As a result, the stored potential

energy in food is converted to a form that can be used for organic functions in the

body. This process is called respiration. In other words, it can be said that the

process of production of usable energy as a result of metabolism of food in the cell

is called respiration.

Energy produced through respiration is used for all functions of the body

Out of interest you plant a sapling in a tub. Next, this will require careful tending,

such as watering it and keeping it in sunlight. If any one of water or sunlight is not

available, the plant will not be able to produce food and will die due to lack of food.

If a person does not eat day after day, he will gradually lose strength and vigor. He

will also lose the ability to move around. And if this continues for long, he will die.

This shows that no life can survive without food, as without food the necessary energy

is not produced.

During respiration in plant and animal cells, food and oxygen react chemically. As a

result of this reaction, that is, due to respiration usable energy is produced from food.

All activity of living body is accomplished with the help of this energy.


Interdependency of Plants and AnimalsThe plants and animals together constitute the living world. One cannot survive

without the help of the other. Though it may appear that plant and animal have

different life styles, yet for survival they are dependent on each other. Few examples

of interdependency of plant and animal are given below:

(1) Plant cells contain a type of green pigment called chlorophyll, which is essential

for the production of food. So, plant can produce food. But since animal cells do not

have chlorophyll they cannot produce food. All animals directly depend on plants for

their food. Some animals directly take plants as food, while some animals indirectly

take food from plants. Some plants are insectivores, for example pitcher plant, which

depend on insects to meet the need of protein food.

(2) Plants and animals are dependent on each other for oxygen and carbon dioxide.

During respiration, plants and animals take in oxygen and give out carbon dioxide.

Green plants, by the process of photosynthesis take in carbon dioxide from the

atmosphere and release oxygen. Thus, the plants and animals are dependent on each

other through the balance of oxygen and carbon dioxide in the atmosphere.

(3) Animal excreta, carcass and so on, mix with soil and increase its fertility which is

essential for plants.

(4) Some plants are dependent on insects for pollination. When insects fly from flower

to flower in search of honey, pollination occurs. In some flowers, birds help in

pollination.

(5) Human beings and other animals carry mature and sweet fruits from one place to

another and after eating the fruit, throw away the seeds. Thus the animals help in the

spread of seeds. At times, some animals eat the seeds along with the fruit. The seeds

may not be digested in the animal stomach and passed out with faeces. Thus, animals

help to dissemirate the seeds of plants.

(6) Red ants build their habitat on trees like mango and litchi. They survive on the

juice of fruits on the tree and save the plant from its enemies.

From the above discussion it is understood that plants and animals depend on each

other for survival.

Food ChainDifferent living things in this world are dependent of each other. No one can survive

as an individual entity. Let us consider energy. Energy is needed for all activities that

are necessary for survival by each and every living thing. This energy is derived from

food. But all living things cannot produce food for themselves. They eat plants or

other animals for survival. They are called consumers. In this way, in different

environments, a group of animal depends on another group for energy. Let us discuss

the producer-consumer relationship among the different living things in a pond.

116 General Science

Floating algae grow in the pond. They are called phytoplankton. They are green

coloured, so in the presence of sunlight they can produce food by photosynthesis, a

process which animals cannot accomplish. They are called producers. Very small

animals consume these producers. These animals also float. They are called

zooplankton. These animals that eat the producers are called first level of consumer.

They are in turn eaten by small fish or insects. Those who eat the first level of

consumers are called second level of consumer. Again those who eat the second level

of consumers are called third level of consumer. In this way in different environment,

the chain of flow of energy collected by the producer passes from producer to the

highest 1evel of consumer is the food chain.

Phytoplankton Zooplankton Small fish Big fish

(Producer) (1st level of consumer) (2nd level of consumer) (3rd level of consumer)

The part of energy that the producer receives from sunlight decreases by 80-90

percent as it passes through the different consumer levels. So, in a pond the amount of

carnivore fish (shing, magur) grown will be much less than the herbivore fish (ruhi,

catla, mrigel, boat, and so on).

Here a simple food chain in terrestrial environment is shown.

Paddy-plant Grasshopper or pamry insect Frog Snake Hawk

Food Web

Many types of food chains are seen among living things. These food chains are inter-

connected by different animals. These food chains together form the food web. That is,

all the food chains in which a living species is included together form the food web.

Fig: 11.5 Food web

Snake

Hawk

Grass

Rat

Rabbit

Grasshopper

Chameleon


A

BPhotosynthesis Respiration

Photosynthesis Respiration

Exercise


1. What is the percentage of water in our somatic cell?

a. 60% b. 65%

c. 70% d. 80%

2. Which one is important to maintain the humidity of air?

a. Wind b. Water

c. Sun light d. Pressure of air

3. What is the type of food that gives energy to living being?

a. Protein b. Carbohydrate

c. Salt d. Vitamin

4. Secondary consumer of a pond is

a. Magur fish b. Phytoplankton

c. Mola fish d. Zooplankton

5. The more the steps of consumer the energy will be

a. More b. Less

c. Equal d. Slightly increased

118 General Science

6. What is the name of the gas which is labeled as A?

a. Oxygen b. Carbon dioxide

c. Carbon mono-oxide d. Ozone

7. Phytoplankton and zooplankton

i. Float on the pond

ii. Produce food

iii. Primary consumer


a. i b. ii

c. iii d. i, ii

8.

Fig:

There is in Fig A and Fig B respectively-

a. Glucose and water

b. Water and glucose

c. Chlorophyll and water

d. Water and chlorophyll

9. Which of the following food chain supply more energy to Hawk?

a. Grass Grass hopper Lizard Hawk

b. Herb Rabbit Hawk

c. Rice plant Hispa Frog Snake Hawk

d. Young leaf Rat Snake Hawk

Oxygen

Carbon

dioxide

A

B


Creative Questions

1.

a. What is the name of the gas in figure labeled as G2?

b. Explain whether the process will run or not, if there is another animal except

man in the diagram.

c. Explain the cyclic rotation of gases G1 and G2.

d. Describe the importance of the gas G1 in environment.

2. (1) Chicken Man Wheat

(2) Man Wheat Pigeon

(3) Goat Man Grass

a. What is food chain?

b. Write down the difference between the food chain and food web?

c. Arrange correctly the above food chains.

d. Make a food web with respect to those food chains.

Fig:

G1

G2

120 General Science

Chapter - 12

The Vertebrates

Food is needed for survival. Of the six components of food, protein is the main

component. Protein is needed for growth and repair of wear and tear of the body. Fish

is the major sources of protein. Besides protein, fish is the source of mineral salts and

vitamins.

Fish provides about 75 percent of protein requirement of human beings in the world.

For long, people in Bangladesh have been dependent on fish as a source of protein.

Among general people in the country, about 80 percent of protein requirement is met

by fish. Fish is not only easily available in this land of rivers but also easily digestible

and relatively cheap.

Fish is an aquatic vertebrate animal. It is available in different water bodies-ponds,

lakes, shallow water bodies, nala, haor-baor, beel, rivers, canals, oceans, and so on.

Fish is a Vertebrate Animal

The following features of vertebrates are present in fish, so fish is a vertebrate animal.

1) Fish has vertebral column

2) Fish body is covered by scales

3) Has different types of fins that help in swimming

4) Has gills for respiration

5) Has a fleshy tail behind the anus

External Feature of Salmon (ruhi)

Salmon is one of the most well known fish. It is a vertebrate having scales on its body.

It is mainly herbivorous and lives chiefly in fresh water. In Bangladesh, salmon fish is

abundantly found in rivers, canals, beel, haor-baor, ponds and other water bodies.

Salmon fish usually live in the mid-level of the depth of the water bodies. Algae,

rotten leaves, and so on are their favourite food.

External Structure

Shape and Colour: The body of salmon is elongated and flattened from two sides.

They resemble pointed gourd (a kitchen-vegetable shaped like a cylinder tapering at

both ends), that is the mid-part is expanded and tapering towards both ends. Its ventral

surface is silver white, while the dorsal surface and adjoining parts is blackish-brown.

The fins are reddish in colour. Salmon is about a metre long, the body is divided into

three parts-head, trunk and tail.

Fig: 12.1 External feature of salmon

Pelvic fin

Poctoral fin

Head

Dorsal fin

TrunkCaudal fin

Tail

Lateral line

Head: The head of salmon is relatively smaller than the trunk. The mouth is situated

at the anterior end of the head a little below the top. It is lined by two full lips, the

upper lip has a pair of barbell. Salmon has a pair of nostrils above their mouth. Behind

each nostril is a rounded eye. On either side of the posterior part of the head is the

half-moon shaped operculum. Under each operculum is four comb-shaped gills.

Trunk: The part of the body that extends from the end of the operculum to the anus is

called trunk. Compared to the head and tail, this part is bulkier. On the ventral aspect

of the tail end of the trunk there is a slight swelling that contains the anus.

Tail: The rest of the body behind the anus is the tail. The tail is compressed laterally.

Scale: The body of salmon has a layer of scales covering their skin. Scales are

smooth, rounded plates. In salmon, the scales do not cover the head and fins. The

scales are arranged in rows such as that one scale covers a part of the scale behind it.

The slippery mucus secreted by the body helps in self-protection.

Lateral line: A lateral line lies below the skin along the two lateral sides of the fish.

This lateral line helps the salmon to maintain equilibrium in the water.

Fin: The body of salmon has two pairs of

paired fins and three unpaired fins making a

total of seven fins.

Paired fin: The two paired fins are (a) a

pair of pectoral fin and (b) a pair of pelvic

fin.

(a) Pectoral fin: These fins are situated

behind the operculum on either side of the

body. These contain many stick-like spines

called fin-rays.

Fig: 12.2 Pectoral fin

Fig: 12.3 Pelvic fin

122 General Science

(b) Pelvic fin: The second paired fin is the

pelvic fins. They lie behind the pectoral fin

and just in front of the anus. These have

finrays too.

Unpaired fin: The unpaired fins are (a)

dorsal fin (b) anal fin and (c) caudal fin.

(a) Dorsal fin: Behind the head on the back

at about the middle of the trunk lies the

dorsal fin. The fin is quite big and looks

like a triangle. This has many fin-rays.

(b) Anal fin: The fin lies behind the anus

on the ventral surface of the trunk. This has

few fin-rays.

(c) Caudal fin: This is the third unpaired

fin. It is situated at the end of the tail. It is

divided into two equal halves. It has many

fin-rays.

Function of fin: With the help of fins, salmon moves and maintains body equilibrium

in water.

Necessity of fish as food

(1) Source of Protein: Protein is necessary for body growth and repair of wear and

tear in the body. Fish is a major source of animal protein. It is easily digestible. It also

contains mineral salts and vitamins. The added advantage is that fish can be dried or

salted for preservation and can be taken as food after a long time.

(2) Restorative Food: Different fishes, like pabda, tangra, shing, magur are used as

restorative food for the ill persons. These fishes are easily digestible and help the body

to regain lost energy.

(3) Source of Vitamin: The oil derived from the shark's liver is a rich source of

vitamins A and D. This oil helps in the growth of teeth and bones in children.

(4) Source of Mineral Salt: Fish contains lots of mineral salt that help in the

structural formation of bones and teeth and increase vision. The bones of small fishes

help to restore the calcium level in the body.

(5) Preparation of Soup: In China and Philippines the fins of shark are used to

prepare soup, which is considered a delicacy in those countries.

Fig: 12.4 Dorsal fin

Fig: 12.5 Anal fin

Fig: 12.6 Caudal fin

The Vertebrates 123

Economic Importance of Fish

Bangladesh is a land of rivers. Fish contribute a great deal in the economy of

Bangladesh.

In our country, about 4 percent of the national income and 11 percent of the export

earnings come from fish resource. It is not only a source of food but also contributes

greatly to the social and economic development of the country through foreign

exchange earnings and reducing unemployment.

Besides, employment of a large segment of the population, fish cultivation helps to

improve the socio-economic condition of the people.

(1) Earning Foreign Exchange: There is world-wide demand for fish. The

production of fish can be increased in the suitable water bodies, including restored

abandoned ponds. This would lead to export of fish after meeting the domestic

demand to bring in considerable foreign exchange. Besides, trinkets and other

adornments prepared from fish scales can also be exported to earn further foreign

currency.

(2) Food for Domestic Animal: The scales, fins and intestine are a good source of

food for the domestic animals. Powdered dry fish is a good source of nutrient for the

domestic fowl.

(3) Fertilizer: Powdered dry fish is used as fertilizer for certain crop production.

(4) Mosquito Control: In stagnant water, fishes like koi, bhada, foli, gappi eat the

larvae and pupae of mosquitoes and thus help in the control of mosquitoes.

(5) Whip Production: The tail of shankar fish is used to make whip.

(6) Shoe Production: Shark skin is used to make shoes and bags. It is also used in the

preparation of sand-paper.

(7) Hobby: Many people have the hobby of keeping fishes in aquariums. This not

only serves as recreation but also brightens up the environment.

(8) Spending Leisure Time: Many people spend leisure time by catching fish.

(9) Soap Production: The oil extracted from certain fishes is used as a raw material

in soap production.

(10) Employment: Starting from fish cultivation to fish catching; production and sale of

fishing net, fishing instruments; preservation of fish and exporting; all provide employment

to a large section of the population. This reduces unemployment to some extent.

124 General Science

Exercise


1. Which fish lives in the middle layer of water?

a. Catla b. Salmon (Rui)c. Mrigal d. Calibaus

2. What is the shape of operculum of Rui fish?

a. Round b. Oval

c. Moon shaped d. Crescent

3. Which vitamin is available in the oil we get from Shark liver?

a. A b. B

c. C d. E

4. Fins of which fish are used to prepare soup in China and Philippines?

a. Coral b. Hilsa

c. Shark d. Ritha

5. What is the size of the head of Rui fish in comparison to trunk?

a. Big b. Little bit bigger

c. Equal d. Small

6. Which organ of the following diagram helps to maintain the balance of fish?

a. A b. B

c. C d. D

B

A

D

C

The Vertebrates 125

7. Applicable for Rui fish

i. Live in upper layer of water

ii. Has three impair fins

iii. Number of gills is eight

Which one of the following is correct?

a. i b. ii

c. ii, iii d. i, ii & iii

8. Bones of Mola fish are rich in-

a. Vitamin A b. Vitamin D

c. Calcium d. Phosphorus

Creative Questions

1.

Fig:

a. Which type of water the fish of the diagram lives in?

b. Explain the function of part A of the above fish.

c. Label the parts of figure B, C and F, drawing a fish of that class familiar to you.

d. Describe the role of this fish in the ecosystem of a pond.

A

B DE

C

F

126 General Science

Chapter - 13

Light

You are reading this book, could you do it if there were no light? Certainly not only

books but also it would not be possible for us to see anything in this world without

light. But can we see light? Look all around you. Chairs, tables, houses, plants or

other things would come to your sight. But do you see anything called light? No, you

do not. Because light cannot be seen. The quality of light is this that it makes objects

visible but it itself remains invisible. We see a lighted object but we do not see light.

We can see an object only when light from the object reaches our eyes. When light

enters into our eyes, our nervous system transmits the message to our brain. Then it

gives us the sensation to see the object that is we can see the object.

Earlier you have learnt that light is a form of energy. Light is such a form of energy

which enters into our eyes and gives us the sensation to see.

The sun is the main source of light. During the day hours, we get light from the sun.

The sun is a natural source of light. At night, we get light from electric bulb, candle

lantern, hurricane light, etc. These are man made sources of light.

How does light from the sun come to the earth? You have learnt from chapter three

that there is only space beyond our atmosphere. Therefore, sunlight travels through

this space. It means that light does not require any medium. It can pass through

vacuum. Again it can also pass through some substances. For example, sunlight passes

through atmosphere to reach the earth. Light passes through glass and water too. Air,

glass, water, etc. and such other media through which light can pass easily are known

as transparent media, On the other hand, there are some substances through which

light cannot passs. These substances are known as opaque medium. For example,

wood, iron, brick, paper, etc. Take a piece of paper and soak its centre with a drop of

oil. Now hold the piece of paper in front of a light source. You will see that some light

is coming through the oiled place of the paper. Such substances through which light

passes partially are known as translucent media. When a transparent glass is grazed it

becomes translucent.

Light travels in a straight line in a transparent medium. This path of light along a

straight line is called a ray of light.

Shadow, Umbra and PenumbraHold your hand in front of a light source such as a torch light, a candle or an electric

bulb. Now if you hold a sheet of white paper behind your hand you will see that a part

of the paper appears to be dark or black. The shape of this black area is similar to the

shape of your hand. This is said to be a shadow of your hand on the paper.

Reasons for the formation of shadows

I) Light cannot travel through opaque

substances.

2) Light travels through a transparent

medium in a straight line and

3) Light rays do not bend and glide

along the edges of opaque substances.

Your hand is an opaque substance. Light can neither penetrate through the hand nor

glide along the edges of the hand, since it travels in a straight line. So, light cannot

reach the area behind your hand. As a result, a dark region resembling the shape of a

hand is formed at the back of the hand. This is known as the shadow of a hand.

Generally, when an opaque object is placed in between a source of light and a screen,

certain regions of the screen behind the object do not receive any light. So, the region

turns dark or black. This dark or black region is known as the shadow of the opaque.

An opaque object has a shadow and the edges of a shadow are similar to the edges of

the object. This fact proves that light travels in a straight line.

Shadows are of two types_ the umbra and the penumbra. Place a football or any

opaque object, round in shape, in front of a point source of light. Now if you place a

white screen behind the object, it will cast a black shadow, round in shape on the

screen. This shadow is termed as the umbra (Fig 13.2). The light ray, PA that comes

from point P travels along edge A of the football and falls on the screen at point A1.

Similarly another ray PB falls on the screen at point B1 The football is an opaque

object. So, rays between P A and PB cannot travel through the football and fall on the

screen. Thus no light ray falls on any point inside the area A1B1 of the screen. The

area remains completely dark. This deep dark or black area, round in shape, is known

as the umbra of the football.

In most cases a source of light is not a point source, rather a bit bigger than it, such as

an electric bulb, a candle or the sun. Figure 13.3 shows such a source of light. This

source is extended from P1 to P2 and may be conceived as a collection of innumerable

point sources. Thus source of this type is termed as an extended source of light. Thus

light ray P1A travels along the edge A of the football and falls on the screen at point

D. Another ray P1B falls on the screen at point F. It means rays from point P1 will

form a shadow on the area DF of the screen being obstructed by opaque AB. In the

Fig: 13.1: Shadow

128 General Science

same way rays from point P2 will form a shadow on the area EC of the screen. P1 is

the highest point and P2 is the lowest point on the source. Therefore, rays from all

points in between P1 P2 are the lowest point in the source. Therefore, rays from all

points in between P1 P2 will form a shadow of the object covering an area CF of the

screen. Now if you pay attention, you will observe that light cannot reach area DE of

the screen from any of the points of the source. As a result, a deep black shadow is

formed there; this shadow is called the umbra. The area CD of the screen does not

receive light from points lying on the lower part of the source but receives light from

the upper part of the source. Similarly, the area EF receives light from the lower part,

though it does not receive light from the upper part of the source. Therefore, a

partially lighted shadow is formed in areas CD and EF of the screen. This light black

or gray shadow is called penumbra.

So, we observe that a shadow has two parts- the umbra and the penumbra. The part of

the shadow which does not receive light at all, that is, the part which is completely

dark, is known as the umbra. The part of the shadow which receives some light and

remains partially lighted or dark, is known as the penumbra. No area of the source of

light can be viewed from the region of umbra but the source of light can be partially

viewed from the region of penumbra. Now look at figures 13.2 and 13.3. You will

observe that-

1. As the screen is moved away, both umbra and penumbra increase in size but they

decrease when the screen is moved nearer.

2. As the source is moved away, both umbra and penumbra decrease in size but they

increase when the source is moved narer.

3. If the object is moved nearer to the screen that is if moved away from the source

both umbra and penumbra decrease in size but they increase when the object is moved

away from the screen.

Fig: 13.2: Umbra Fig: 13.3: Penumbra

Light 129

A

BB1

A1P1

P2

A

B

F

E

D

C

P

Solar eclipse

A total solar eclipse was observed from some regions of Bangladesh on 24th October

1995. It was the last total solar eclipse of the century viewed in Bangladesh. It was a

rare incident. It roused interest and excitement among all . Many travelled far away up

to the Hiron Point to see the total solar eclipse. You might have seen it yourself. It

seems as if a black shadow gradually engulfs the sun. This solar eclipse is a natural

phenomenon of forming umbra and penumbra.

You know that the earth is a planet of the sun and the moon is a satellite of the earth.

The earth moves continuously round the sun and so does the moon round the earth.

The sun is considered to be an extended source of light. The earth and the moon are

opaque bodies. Sunlight brightens the moon. So, we can see the moon. We cannot see

the moon during the night of a new moon. At this time, the moon on its journey around

the earth comes in between the sun and the earth. Therefore, the sunlight which falls on

the other side of the moon, which cannot be seen from the earth. Occasionally, the sun,

the moon and the earth lie on a straight line during a new moon. At this time, some

parts of the earth are shadowed by the moon (Fig. 13.4). As a result, the sun cannot be

viewed in full from those regions. This is known as the solar eclipse.

The sun is an extended source of

light and the moon is a large

opaque body. So, both kinds of

shadow, the umbra and the

penumbra are formed. The sun is

not seen at all from those

regions of the earth on which

umbra is formed. This region is

deep and dark. The eclipse in

this region is known as the total

solar eclipse (Region CD of Fig 13.4). The sun can be partially viewed from the

regions of penumbra. So, the eclipse in these regions is known as the partial solar

eclipse. (Region EF of Fig 13.4).

The region where total solar eclipse takes place becomes as dark as night and stars are

seen in the sky.

Lunar eclipseThe earth moves round the sun and the moon moves round the earth. While moving,

sometimes the earth comes between the sun and the moon. During this time the part of

the moon which receives sunlight, can be viewed in full from a particular region of the

earth. This fully lighted moon is known as the full moon i.e., full moon is seen on

Sun

Fig: 13.4: Solar eclipse

partialeclipse

Totaleclipse

Moon Earth

130 General Science

that day. Occasionally the sun, the moon and the earth come to lie in a straight line in

a full moon. At this time, the shadow of the earth falls on the moon and the moon

cannot be viewed totally or partially. This is known as the lunar eclipse (Fig 13.5).

The sun is the largest in size. The earth is smaller than the sun. So, the umbra of the

earth is bigger than the moon. When the moon completely enters into the earth's

umbra, it cannot be viewed since the moon has no light of its own. This is known as

the total lunar eclipse. When a part of the moon enters into the umbra, that particular

part only is not viewed. This is known as the partial lunar eclipse. The moon lies in

the penumbra region immediately before and after the lunar eclipse. During this time,

the moon diminishes in brightness and becomes pale.

Surely you have a question now in your mind, why solar eclipse and lunar eclipse do

not occur every new moon and full moon?

The reason is that the orbit of the earth and the orbit of the moon do not lie in the

same plane. The plane of the earth's orbit of the earth is inclined at an angle of about

5˚ to that of the moon. So the sun, the moon and the earth do not come to lie in a

straight line in every new moon. Lunar eclipse happens to occur only in that full moon

in which the sun, the moon and the earth lie in a straight line.

Reflection of Light

How do you see this book? Light from a source comes through the air and falls on the

book. The book is an opaque substance, so light cannot pass through it. so being

obstructed by the opaque object; book, it returns to your eyes. In this way, you can see

the book. Similarly, when a beam of light travelling in one transparent medium falls

on a second medium, part or whole of the incident light is turned back to the first

medium. This phenomenon is known as reflection. The surface from which light is

reflected is called a reflector. Glass or any other smooth or shiny object serves as a

good reflector.

sun

Fig: 13.5: Lunar eclipse

moon

Light 131

earth

The smoothness and nature of an object determine to what extent light will be reflected

when it falls on the object. When light falls on an opaque substance, such as a book, a

part of the light is absorbed and most of it is reflected. If the surface of the opaque

body is very smooth or bright, most of the light will be reflected. But if the opaque is

very smooth and bright, most of the light will be reflected. But if the opaque body is

black in colour, most of the light will be absorbed and only a little of it will be

reflected. As light is fully absorbed by a deep black object, there is no occurrence of

reflection. On the other hand, if the object is completely white, there will be no

absorption but a complete reflection of light. It means the object which does not reflect

light looks black. The more an object reflects light, the more white it is to look at.

If the second medium is a transparent one, such as water or glass, only a little part of

the light is reflected and most of it travels through the transparent medium. We can

see glass or water as it reflects some part of the light. We cannot see air as it does not

reflect light. In fact, an object comes into sight only when it reflects light.

Regular and diffused reflection

Drop a football straight downward in the field. If the surface of the ground is

smooth, the football will come back straight upward. But if the surface of the ground

is uneven or rough, the ball will not come back straight upward, rather it will go in

any direction.

Fig. 13.6: Regular reflection Fig 13.7: Diffused reflection

Similarly, when a beam of light falls on a smooth surface, it is reflected in a fixed

direction according to some laws. This type of reflection is known as regular

reflection. Regular reflection occurs only from smooth surfaces, such as looking

glass, polished metals or any other smooth surface. In a regular reflection, an image

of the source of light is formed. You can see your image in a mirror due to regular

reflection on it.

When a group of parallel rays fall on a non-smooth surface, the rays are not reflected

parallel in a fixed direction, rather they are scattered in different directions. Reflection

of this type is known as the diffused reflection. Diffused reflection occurs from the

132 General Science

most of the objects we see around us, such as the walls of a room, rough papers,

ground glass, etc. No image is formed in the diffused reflection. Only the object

which reflects, is seen. Diffused reflection occurs from the pages of your book. As a

result, you can see your book. All of you may have enjoyed a film show. Have you

noticed the colour and smoothness of the screen? You will see that the screen is white

and unpolished. What may be the reason for it? The reason is: if the screen is

polished, there would be regular reflection and spectators of a particular corner only

would be able to see the film. As the screen is unpolished, diffused reflection occurs

i.e., light is scattered in all directions. As a result, spectators can see the film from all

comers. Moreover, as the screen is white and maximum reflection occurs from white

objects, the film appears to be bright on the screen.

Laws of reflection

Regular reflection of light observes

some laws. Regular reflection takes

place when a beam of light falls on a

plane mirror or glass.

Suppose a ray of light. AO falls directly

on mirror M1 M2 at point O and is

reflected on the direction of OB.

The ray of light, which falls on the surface of the reflector, is called the incident ray.

Here AO is the incident ray. The point at which the incident ray falls is called the point

of incidence. O is the point of incidence. The ray of light, which is reflected back from

the surface of the reflector, is known as the reflected ray. Here OB is the reflected ray.

The perpendicular drawn at the point of incidence is known as the normal. Here ON is

the normal. The angle between the incident ray and the normal is called the angle of

incidence. Angle AON is the angle of incidence between the reflected ray and the

normal is called the angle of reflection. Here BON is the angle of reflection.

Two laws of reflection of light

First law: The incident ray, the reflected ray and the normal drawn on the reflecting

surface at the point of incidence, all lie in the same plane.

Second law: The angle of incidence is always equal to the angle of reflection. Do you

want to see the reflection of light? Hold a mirror facing the sun and move it in

different directions. Note how sunlight gets reflected.

Fig: 13.8 Reflection of light

Incident rayPoint of incident

Reflected on

Normal

Angle of

reflection

Angle of

incidence

Light 133

Image

What do you see when you stand

before a mirror? You see your replica.

It seems as if you are standing behind

the mirror. This is your image.

We know that we can see an object only

object only when rays of light from the object come to our eyes. In this case, rays of

light from your body do not come to your eyes directly. Rays of light from each point

of your body fall on the mirror and reflect back to your eyes. So, what you see is not

"You"- but your image. In other words, when rays starting from a point on your body

suffer a change due to reflection and appear to diverge from a second point, then the

second point is called the virtual image of the first point. This image appears to be

formed at the back of the mirror but cannot be cast on a screen. This image has no

existence in reality, since light cannot pass through the mirror to reach a point at the

rear. This is why, this image has no real existence and so it is known as the virtual

image. But the picture you see in a cinema is an image formed on the screen. The

image of a film is cast on the screen with the help of a projector. In fact, rays of light

converge on the screen here. This image is, therefore, called the real image.

So, it can be said that when rays of light meet at a second point being reflected or it

seems that the rays come back from the second point, that second point is called the

image of the particular first point. Images are of two types- the real image and the

virtual image. A real image is formed by actual convergence of the rays and can be

cast on a screen. A virtual image is not formed by actual convergence of the rays,

rather appears to come from an image. A virtual image cannot be cast on the screen.

Image in a plane mirror

A mirror is a polished surface from

which regular reflection occurs

according to the laws of reflection. The

most familiar mirror to us is a looking

glass. This mirror is constructed by

coating a thin polished glass with

silver. The mirror that has a plane

reflecting surface is known as a plane

mirror.

Figure 13.10 shows how an image of a point source is formed in a plane mirror. AB is

a plane mirror and P is a point source. The point source P emits rays of light in all

Fig. 13.9: Image

you

Mirror

Your image

Fig. 13.10: Formation of image in a plane mirror

134 General Science

directions. One of the rays PQ falls on the mirror perpendicularly. As the angle of

incidence, in this case, is zero, the angle of reflection will also be zero according to

the laws of reflection. It means the ray will retrace its initial path in the QP direction.

Two other rays, PO and PR fall on the mirror obliquely. According to the laws of

reflection, these two rays will be reflected along OS and RT respectively. Now if OS

and RT are extended backward, they would converge at point P1. This means, the

reflected rays appear to diverge from point P1 behind the mirror. Therefore, point P1 is

a virtual image of point P.

The relative positions of object mirror and image

You can determine the position of an image on a plane mirror with the help of an

experiment.

Experiment: Fix a white sheet of

paper with a pin on a drawing board

or a plane table. Draw a straight line

XY on the paper. Place a mirror

vertically on the line such that the

reflecting surface of the mirror

touches the line XY. Fix a pin vertically

at the point O in front of the mirror.

Now looking from one side, vertically fix three other pins at A, B and C such that the

image I at point O and the images of these three pins appears to be in a straight line.

In the same way, fix three other pins L, M, N and three pins P, Q, R such that L, M, N

and I lie on one straight line and P, Q, R and I lie on another straight line. Now

remove the mirror and draw three straight lines along CBA, NML and RQP.

All the three lines are in the direction of I. So, the lines will converge at the point I

when these are extended backward. Thus we see that the pin at point O has formed an

image at the point I. Now it can be proved by measuring with a scale that OM = IM.

This means that the distance of the object from the mirror is equal to the distance of

the image from the mirror.

Uses of light

We need light to see things. Besides this important use of light there are other uses of

light also. Most of you have seen that red, yellow and green signals of light are used

to control traffic at crossroads of a city. Those who have traveled by trains during

night hours might have noticed that the train leaves a station on receiving a signal

from the guard who whistles and swings a green light simultaneously. Often in seas

two ships use light signals to exchange messages. There are many such examples of

Fig: 13.11 Formation to an image in a plane mirror

Light 135

the use of a light signal. You need light to take your photo with the help of a camera.

Photographs cannot be taken without light. You have learnt from chapter eleven that

plants prepare food through the process of photosynthesis. Sunlight is required for this

process. Generation of electricity is another important use of light. There are some

special kinds of elements which generate electricity when light falls on them. Silicon

is an example of such an element. Silicon is used to make solar cells. Solar cells

transform light energy into electrical energy. Nowadays solar cells are used in

electronic calculators, watches and other equipment.

Exercise


1. When does a lunar eclipse take place?

a. in every new moon

b. in every full moon

c. in a new moon when the sun, the earth and the moon remain in a straight line

d. in a full moon when the sun, the earth and the moon remain in a straight line

2. If we hold our hand in front of white paper, a part of the paper appears to be dark,

because-

i. light rays do not bend at the edge of the hand.

ii. the size of the umbra and penumbra will be smaller if we take the source away

and it will be larger if we take the source closer.

iii. light cannot travel through the hand.


a. i b. i, ii

c. i, iii d. i, ii & iii

3. What do you see in a mirror?

a. Umbra b. Penumbra

c. Virtual image d. Real image

4. An incident ray makes an angle 300 with the plane of a mirror. What is the magnitude

of the angle of reflection?

a. 300 b. 450

c. 600 d. 900

136 General Science

Creative Question

1.

Figure

a. Give the definition of the place marked 'a'.

b. Explain the cause of the shadow.

c. Describe with figure how the shadows marked 'b' and 'c' are formed.

d. In the figure what type of eclipse is shown? Describe diagram.

2.

Figure

a. In the figure above what type of reflection is shown?

b. In this case, why is this type of reflection taking place?

c. Find out the value of the angle PON.

d. Stanaing at point P can a man watch his image by the mirror MM1?

Explain with figure.

Light 137

Chapter -14

Magnet

You have learnt in class VI that magnet is a kind of substance, which has attractive

and directive properties A magnet attracts magnetic substances due to its attractive

property The two ends of a freely suspended magnet direct north and south due to its

directive property. Magnetic substances can be transformed into magnets by rubbing

method and electrical method.

Is there any other method for transforming magnetic substances into magnets? How

far does the attractive force of a magnet work? Why does a magnet always indicate

north and south? You will get answers to these questions in this chapter.

Magnetic Induction

Experiment: Take an iron nail and

some iron fillings. Put the nail in the

iron filling. You will see that the nail

does not attract the iron fillings. Now

take a bar magnet and bring one of its

poles near to or in contact with the

nail. This time the iron fillings will

stick to the nail, i.e. the nail will

attract the iron fillings [Fig 14.1 (a)].

Take away the bar magnet, the iron

fillings will fall off from the nail. It

means that the iron nail turned into a

temporary magnet due to the influence

of the bar magnet. If iron fillings are

not available, you may use some

iron nails or iron paper clips to conduct this experiment. When the nails or iron paper

clips are kept together, they do not attract one another. Take a bar magnet near to the

nails or to the clips. You will see that the nearest nail or clip jumps to stick to the

magnet.

Now if you move the magnet slowly upward, you will see that another nail or clip

sticks to the free end of the first nail or another clip below the previous one. In this

Fig, 14.1(a): Demonstration of magnetic induction.

Iron bar

Iron filings

way, a chain of nails or clips will

be formed [Fig 14.1 (b)]. This

shows that each of the nails or clips

has turned into a magnet. Now hold

the first nail or clip in one hand and

take away the bar magnet. You will

see that the nails or clips fall off.

This means that the nails were

transformed into temporary

magnets due to the influence of the

bar magnet.

Iron is a magnetic substance. So, we have observed that when a magnetic substance is

brought near to or in contact with a magnet, the magnetic substance turns into a

temporary magnet. This phenomenon is known as magnetic induction, i.e. the

phenomenon of temporary transformtion of a magnetic substance in the presence of a

magnet is known as magnetic induction. This type of temporary magnetism exists as

long as the permanent magnet remains to be present. When the magnet is taken away,

this magnetic substance loses its magnetism.

The magnet which causes magnetic induction is called inducting magnet. Its

magnetism is known as inducting magnetism. The magnetic substance, which is

converted into a temporary magnet due to induction is called inducted magnet. Its

magnetism is known as inducted magnetism.

We can know about the nature of the poles of an induced magnet by performing

another experiment.

Experiment: Take a needle magnet. It is at rest indicating north and south. Take also

a bar magnet. Place the bar magnet at such a distance with its north pole facing the

north pole of the needle so that the bar magnet does not influence the needle magnet,

i.e. the bar magnet does not repel the north pole of the needle. Now place a soft iron

bar in between the two magnets. You will observe that the north pole of the needle

magnet has moved away. That is, iron bar repels the bar magnet. Repulsion causes

only between two similar poles. Therefore, the iron bar has developed a north pole at

the end which is nearer to the needle. The other end which is nearer to the north pole

of the bar magnet has developed a south pole.

Fig: 14.1(b): Demonstration of magnetic induction

Magnet

Paper clip of Iron

Magnet 139

Fig. 14.2: Polarity of an induced magnet

Hence, we can conclude that in magnetic induction the inducing pole induces opposite

polarity at the end of the induced pole near to it and a similar polarity at the far end.

Induction precedes attraction

When a magnet is brought near to a magnetic substance, a force of attraction takes

place between the two. Now can you say what the reason is? The reason is that the

magnetic substance first turns into a temporary magnet due to magnetic induction and

the end of the magnetic substance near to the inducting pole develops an opposite

pole. Since, opposite poles attract each other, a force of attraction develops between

the magnet and the magnetic substance. So, it is said that induction is followed by

attraction.

Magnetic fieldWe know that a force is a push or pull.

You push or pull the door to open it. You

need to touch the door to apply a force on

it. You cannot apply force from a

distance. But if you place an iron nail

near to a magnet, the magnet attracts the

nail without touching it. There is no need

for a touch to apply magnetic force. This

force acts from a distance. This force attracts any nail kept in the confounding area of

a magnet. This area is called magnetic field.

If you want to observe the influence of a magnetic field, you need to put a small

needle magnet near to another magnet. You will see that the needle turns away. Slowly

move the needle away, the magnitude of its turning decreases gradually. The

Fig: 14.3: The magnetic field of a bar magnet

(a)

(b)

Needle magnet

Bar magnet

NS

B

A

140 General Science

needle does not turn at all when it reaches a fixed distance. This means there is no

more influence of magnetic force at this point. Magnetic force remains active in a

certain surrounding area of a magnet.

Therefore, the space surrounding a magnet within which the influence of the magnet

is observed is called its magnetic field. When a magnetic substance is placed

anywhere within a magnetic field, it experiences the attractive force of the magnet.

You cannot see a magnetic force but you can see its effects. Place a bar magnet on a

white sheet of paper and sprinkle some

iron fillings over the paper. Now gently

tap the paper and you will see that the

iron fillings arrange themselves in a

special pattern as shown in the figure.

At some places in the iron fillings are

found to be dense while at others

these are much light. This pattern

represents the magnetic field pattern

of a bar magnet. It was Michael

Faraday, a British scientist, who first

used the term magnetic field in 1845.

Magnetic lines of force

Now you need a bit imagination. Suppose, a free unit North Pole is brought near to

the north pole of a bar magnet. What will happen? Since, similar poles repel each

other, the unit north pole moves away from the north pole or the bar magnet. But the

South Pole of the bar magnet attracts the unit north pole. This force of attraction is

weaker than the force of repulsion because of the greater distance of the unit north

pole from the south pole of the magnet. As a result, the unit pole instead of moving

away in a straight line moves in a slightly curved line towards the South Pole. In this

way the more the unit pole goes away, the weaker the repulsion force becomes and the

stronger is the attraction force. Consequently, the free North Pole moves from the

north pole of the bar magnet to the South Pole along a curved line as shown in the

figure. This curved path is known as magnetic lines of force. In other words, the path

in a magnetic field along which an isolated and free north pole travels is called

magnetic lines of force. A magnetic lines of force starts from North Pole to the

direction of South Pole.

In reality there is no existence of a free and isolated north pole. Therefore, magnetic lines of

force do not really exist. These are imaginary. You can perform an experiment to hav.e an

idea as to how would these have looked like if it was possible to see these imaginary lines.

Compass

Fig: 14.4: Magnetic lines of force

Magnet 141

Experiment: Fix a white sheet of paper on a drawing board with pin. Place a bar

magnet, at the middle of the paper and draw the outlines of the magnet with a pencil.

Place a small compass needle very close to the north pole of the magnet. When the

compass needle comes to rest, mark its two ends with dots. Next place again the south

pole of the needle on the dot mark made earlier for the North Pole. Put another dot

mark on the north end of the needle. Continue this process of marking by moving the

compass in such a way that on each move the south pole of the needle is placed on the

north pole mark of the preceding move. Take away the compass when it reaches the

south pole of the magnet and draw a line through the dot marks with a pencil. The

curved line drawn in this way from the North Pole to the South Pole of the magnet

represents a magnetic line of force. Next bring the needle compass again to the north

pole of the magnet and place it near the mark where it was placed at the start.

Following the same procedure draw another magnetic line of force. In this way you

can draw a number of magnetic 'lines of force as shown in figure 14.4. This pattern of

the magnetic lines of force is called the magnetic field pattern.

Properties of the magnetic lines of force

1. Two magnetic lines of force never intersect each other.

2. The intensity or strength of a magnetic field is stronger at points where the lines of

force are close to one another. It is weaker at points where these lines are away from

one another.

3. Magnetic lines of force start vertically from North Pole and meet at the south Pole.

Terrestrial magnet

We know that magnets have a

directive property. It means that when

a bar magnet is suspended at its centre

by a string, it is always found to come

to rest in the direction of north and

south of the earth. This indicates that

a strong magnet must have influenced

the bar magnet, otherwise it would

have been pointing to any direction.

But there is no magnet nearby. Then

where is this invisible magnet? Is it

that the earth itself is a magnet?

Indeed it is. Dr Gilbert, a British

scientist, came to the conclusion by

Fig: 14.5: Influence of terrestrial magnet at different

places of the earth

Terrestrial north pole

Terrestrial south pole

Geographical north pole

Geographical south pole

Equator

142 General Science

performing different experiments that the earth is a big magnet .It may be imagined that

there is long bar magnet lying hidden under the earth from one pole to the other. This

imaginary magnet is called the terrestrial magnet. The terrestrial magnet has its north

pole along the geographical South Pole and South Pole along the geographical North

Pole. Figure 14.5 shows the lines of direction in which a freely suspended needle

magnet indicates at different places of the earth. It is found that the needle remains

parallel to the surface of the earth near the equator. If the needle is taken gradually

towards the north, its north pole sinks or dips more and more towards the surface of the

earth. The needle sinks vertically downward west to the geographical north pole. This

indicates that the terrestrial south pole is located at the west to the geographical north

pole. This is why the terrestrial south pole attracts the north pole of the needle.

Similarly, if the needle is taken gradually towards the south pole, its south pole dips

more and more towards the surface of the earth. From the east the geographical south

pole sinks vertically downward. Therefore, the terrestrial north pole is located east to

the geographical south pole. The imaginary line joining the two poles of the terrestrial

magnet is called the axis of the terrestrial magnet.

Neutral points

If an isolated and free north pole is placed in the magnetic field of a bar magnet, it

Fig. 14.6: The pattern of magnetic lines of force; location of neutral points

will be influenced by the bar magnet as well as by the terrestrial magnet. At places

near a magnet, the influence of the magnet is stronger than that of the terrestrial

magnet. As the distance from a magnet increases gradually, its influence decreases

and that of the terrestrial magnet increases, there are two points at equal distance from

the centre of the bar magnet where the influence of the bar magnet and the terrestrial

North pole pointing north North pole pointing south

Magnet 143

agent is equal and opposite. It means, if the bar magnet attracts the free North Pole,

the terrestrial magnet repels it with equal force. So, these two equal and opposite

forces eliminate each other and become neutral. This is why these two points are

called neutral points. The free north pole remains undisturbed and steady at these two

points. In other words, a magnet does not exert any influence at neutral points, i.e. the

influence is zero.

In the figure above locations of neutral points have been shown in situations when a

bar magnet is placed in the terrestrial magnetic field with its north pole pointing north

and south pole pointing south. Here the cross marks (x) indicate the neutral points.

Uses of magnet

We talk and sing in front of a microphone. A microphone transforms sound energy

into electrical energy. The speaker of the microphone again transforms the electrical

energy into sound energy. Thus we can hear words or music. Magnets are used in the

microphone and the speaker to transform energy from one kind to another. The most

important and essential use of magnets is found in the production of electricity. In

1831, Michael Faraday, a British scientist, discovered that when a magnet is quickly

moved in and out of a coiled wire, electricity is produced in the wire. This principle

provides the foundation for the production of electricity. The machine which produces

electricity with the help of the magnet is called a dynamo or generator.

You may have seen a crane lifting up heavy loads. Cranes often use magnets to lift up

heavy objects such as motorcars, which are made of magnetic substances. Attractive

property of magnet is used in cranes.

Exercise


1. The area surrounding a magnet within which its influence is felt is called

a. the magnetic field b. the magnetic lines of force

c. the terrestrial magnet d. the neutral point

2. The magnet which causes magnetic induction is called

a. an inducting magnet b. an induced magnet

c. a bar magnet d. a needle magnet

3. If the distance between the lines of force decrease

a. the density increases b. the density decreases

c. the magnetic field become strong d. the neutral point is found

144 General Science

4. The magnet used in microphone

a. turns the sound energy into heat energy

b. turns the electric energy into sound energy

c. turns the electric energy into heat energy

d. turns the sound energy into electric energy

From the figure below, answer the questions 5 and 6

Figure

5. In the figure, at the two ends of the magnet-

i. Magnetic force will be maximum

ii. Magnetic intensity will be more

iii. Magnetic field will be strong


a. i b. i, ii

c. ii & iii d. i, ii & iii

6. The arrow in the figure indicates

a. direction of the magnetic field

b. direction of the lines of force

c. direction of the force of attraction

d. direction of the force of repulsion

Creative Questions

1.

Figure-l

N S S N

Magnet 145

The two pieces of bar magnet in the figure are kept close to each other.

a. What is the meaning of 'N' in figure?

b. Explain the magnetic lines of force for the first magnet.

c. Explain the effect of lines of force created by the two pieces of bar magnet.

d. Describe the position of neutral point drawing the lines of force for the second

magnet.

2.

Figure

a. What is the needle magnet called?

b. Explain how induction will occur before attraction takes in the figure.

c. Explain the effect of lines of force drawn from the bar magnet to the needle

magnet.

d. Explain how the bar magnet will be affected by the terrestrial magnet.

Bar magnet Needle magnet

146 General Science

Chapter - 15

Electricity

We have learnt in class VI that there are two kinds of electricity - the static electricity

and the current electricity. The kind of electricity that remains confined in an object is

called static electricity. The electricity that continuously flows in a particular direction

through a conductor is called the current electricity. The most used form of electricity

is the current electricity.

We will know more about current electricity from this chapter.

Flow of electricity and current electricity

You may have seen the flow of water in a river. The running of water from one place

to another is called flow of water. Similarly, the continuous movement of electric

charge or charged particles in a fixed direction is called the flow of electricity.

Water flows from higher level to lower. This source of river water is ice melting water

of mountains. Mountains are high, so water flows downward. As the sea level lies

down, so river water flows into the sea.

Charges are of two kinds- positive charge and negative charge. A positive charged

particle is compared to water situated at a higher level. On the other hand, a negative

charged particle is compared to water situated at a lower level. The higher is the level

of water, the more is the pressure. Water flows from a place of higher pressure to a

place of lower pressure. The electrical pressure is high with the positive charged

objects whereas it is low with the negative charged objects. So, when a conductor

connects a positive charged object with a negative charged object, positive charges

start flowing towards the negative charged object. This flowing of charges is known as

the current electricity.

So we came to know

The flow of positve charges along a

conductor in a fixed direction is called

an electirc current. The continuous

flow of electric current is called the

current electircity. When a positive

charged object is connected with a

negative charged object, electric current is generated.

Here, in fact, we need to know that positive charges do not flow along a conductor,

rather there occurs flow of electrons. An electron is a very small particle. There are

Fig: 15.1: Structure of an atom

rotating electron

nucleus

one or more electrons in each atom and all matters are composed of atoms. An atom is

the smallest particle of an element. It has nucleus at the centre. Electrons move round

the nucleus in fixed shells or orbits in the same way as the planets move around the

sun. The nucleus is composed of two kinds of particles- proton and neutron. An

electron is negatively charged and a proton is positively charged, while a neutron has

no charge. An electron and a proton carry equal but opposite type of charges.

Normally, an atom contains equal

number of electrons and protons. So,

it remains neutral in charge. If an

object contains more electrons, it

becomes negative charged. On the

other hand, if the object is deficient in

electrons, it becomes positive charged.

Electrons lie in the exterior region of

an atom and so can move easily.

For this reason, if a positive charged body is connected with a negative charged body

by a conductor, electron starts flowing towards the positive charged object. This flow

of electrons is what we call an electric current. But conventionally, direction of the

flow of an electric current is assumed to be from the positive charged object to the

negative charged object, i.e., opposite to the direction of the flow of electrons. (Fig.

15.2).

History of the invention of an electric cellLuigi Galvani was a professor of physiology at the University of Bologona in Italy.

One day in 1786, he was performing an experiment with dead frogs. He kept a freshly

skinned frog hanging from a brass hook attached to an iron railing. He observed a

peculiar convulsive movement in the muscles of the frog each time the legs touched

the iron. He interpreted from this phenomenon that of electricity was within the body

of the frog itself is the source.

At that time, Alesandro Volta was the

professor of Physics at the University

of Padua in Italy. He refuted the idea

of Galvani. He had of the opinion that

the body of the frog was not the

source of electricity. The flow of

electricity was caused by the two

dissimilar metals in contact, namely,

brass and iron. The body of the frog

Flow ofelectricity

High potentialsmallno of

electron

largeno of

electron

low potential

Flow ofelectricity

Fig: 15.2: Direction of current

Fig: 15.3: Volta's pile

Copper

Zinc

Zinc

Cloth soaked with

diluted H2SO4

148 General Science

served only as a conductor of electricity. The convulsive movement in the muscles of

the frog was due to the flow of electricity whenever its legs touched the iron. In 1800,

he demonstrated the validity of his opinion. He placed several copper and zinc plates

of uniform size one over the other alternately to form a pile. Each pair of copper and

zinc plates was separated from the next pair by a thin piece of cloth moistened with

diluted sulphuric acid. The pile thus formed had a zinc plate at the top and a copper

plate at the bottom. When he connected the two plates by means of a conducting wire

to a bulb, it glowed indicating that an electric current was flowing through the wire.

This is how the first electric cell was invented.

Simple electric cell

The device used to maintain a continuous flow of electricity by transforming chemical

energy into electrical energy is known as an electric cell. The fundamental discovery

of Volta is that an electric current is generated by separating two dissimilar conductors

with acids. The construction of a simple electric cell is based on this principle. So, a

simple electric cell is also known as a simple voltaic cell.

Construction procedure

A glass vessel is required to make a voltaic cell. About three-fourths of the vessel is

filled with dilute sulphuric acid. A copper plate and a zinc plate are dipped partially in

the acid without keeping them in contact. The two plates do not touch each other and

stand vertically in the acid. The plates are provided with binding screws at the top.

The screws are used to make external connection by means of a wire.

Chemical reaction takes place

between the zinc plate and the acid.

As a result, the zinc plate becomes

negative charged and the copper plate

becomes positive charged. When the

plates are externally connected by a

conducting wire, electrons flow from

zinc plate to copper plate. It means,

according to the current convention,

electricity flows from the copper plate to the zinc plate.

A simple voltaic cell has some defects due to which flow of the current decreases or

stops, if it is run for sometimes. Also the zinc plate corrodes rapidly. So, this cell is not

used practically. Scientists have developed improved variety of cells by eliminating

defects in many ways. Leclanche cell is one of such improved variety of cells.

Fig 15.4 A simple Voltaic Cell

Sulphuric acidmixed in water

Zinc

Copper

Electricity 149

Lecianche cell

The cell consists of a strong solution of Ammonium Chloride contained in a glass

vessel. An amalgamated rod of zinc and a porous china pot are dipped in the solution.

A carbon rod is placed inside the porous pot and the pot is filled with a mixture of

broken carbon and manganese. Later, the opened of the pot is closed with pitch and a

small hole is drilled through the pitch so that gas produced inside can come out. The

carbon and zinc rods are provided with a binding screw.

In this cell, the carbon rod becomes positive charged and the zinc rod becomes

negative charged. When the rods are connected externally with a wire, electricity

flows from the carbon rod to the zinc rod.

Uses: This cell cannot maintain

continuous flow of electricity for a

longer period of time also. For this

reason, the cell is suitable for

intermittent use, such as in electric

bells, telephone, telegraph, etc. The

maximum use of this cell is found in

performing experiments in the laboratory.

Electric Circuit

Place a torch bulb on a torch battery. Will the bulb glow? No, it will not. Connect the

bulb and the battery with a wire as shown in the figure. The bulb will glow now. It

glows due to the flow of electricity through it. Electricity flows from the positively

charged end of the battery to the other end along the bulb and the wire. This path of

the flow of electricity is known as an electric circuit. The path requires to be closed or

complete in order to maintain a continuous flow of electricity. If the path remains

open or disconnected at any point, eleptricity does not flow along it. This path is then

Zinc rod Immersed

In mercuryBinding screw

carbon rod

glass vessel porous china vessel

Mixture of Manganese

dioxide and charcoal

solution of Ammonium chloride

Fig 15.5 Leclanche Cell

Fig 15.6 Electric Circuit

150 General Science

called an open circuit. A circuit can be made open or closed by connecting a key or

switch in the circuit. When you put the switch on, you close the circuit and when you

put it off, you open the circuit.

Thus, we see that there is a closed or complete path connecting the two ends of a

source in order to maintain flow of electricity from the positive to the negative end of

the source. This path is known as an electric circuit.

The working principle of torchlight

All of you have seen or perhaps used a torch light. Have you marked how it is made?

Torchlight is made of a hollow metal

cylinder. This cylinder works as a path for

the flow of electricity. The front of a torch

is so made that a bulb may be screwed to

fit in it. When batteries are inserted into

the torch, the lower end of the bulb

touches the positive end of the battery. A

spring holds the batteries tight to the end

of the bulb to avoid any gap in the circuit.

The spring, a metal conductor itself, is

attached to the lid at the bottom of the

torch. There is a switch on the body of the

torch. When the switch is put on, the

circuit becomes complete and the bulb

glows. When the switch is put off, the circuit becomes open and the. bulb goes off.

The torch bulb .is placed at the centre of a brightly polished metal hemisphere. A

circular glass plate covers the front end of the torch. Due to this arrangement, light

from the bulb is not scattered all around, but being reflected from the hemisphere is

directed forward. This increases the brightness of the light.

Use of electricity

Electricity is a kind of energy. So, it can be transformed into other forms of energy.

We transform electrical energy into light energy and heat energy to perform various

works of our everyday life.

Electric bulb: When you put on an electric bulb, you transform electrical energy into

light energy. If electricity is transmitted through a conducting wire, the wire becomes

hot. The thinner the wire is, the hotter it is. As the wire continues to be increasingly

hot, it continues to be increasingly red and finally it becomes white and bright. As a

result, the wire begins to emit light. This is what happens in an electric bulb. So, when

Fig: 15.7 The working principle of atorch light

Switch

Electricity 151

the bulb remains on, it becomes extremely hot. Don't touch a lighted bulb empty

handed. It may burn your hand.

There is a coil of a very thin wire inside an electric bulb. The coil is called filament.

Filament is made of tungsten wire. Tungsten has a very high melting point of about

34100 Celsius. So, it does not melt even when it is extremely hot. The bulb is made

completely a vacuum and filled up with a little of nitrogen or inert argon.

When you switch on an electric bulb, electricity flows through the filament of the

bulb. As a result, it becomes extremely hot and emits light. Thomas Edison, an

American scientist, invented the electric lamp in 1879 first.

Electric iron: The heat generation property of electricity is used in an electric iron

also. Inside the iron there is a wire of nichrome around a mica plate. The plate is

placed in between two mica sheets and covered tightly by iron sheets. Mica is a non-

conductor of electricity but a conductor of heat. Therefore, though electricity flows

through the wire, it does not come in contact with iron sheets. But when the wire gets

hot, it makes the iron hot too. The outer surface of the iron is triangular, heavy and

made of stainless steel. The bottom surface is well polished. The two ends of the

coiled wire are connected to a plug by long and thick wires. When the plug is turned

on, electricity flows through the coil and makes it hot. As a result, the external metal

cover becomes hot.

Electric heater: It consists of a circular disc made of china clay. The disc has a coil

of notches in it. A coil made of a thin and long nichrome wire is fitted in the notches.

The disc is kept in a metal frame with legs. The two ends of the coil are connected

with the power supply arrangement. When electricity flows through the coil, it

becomes red hot and radiates much heat. This heat is enough to cook food.

Fig: 15.8 Some example of the uses of electricity

Glass bulb

Thick wire

Coil of tungsten

Electric bulb

Electric iron

Electric heater

Steel cover

152 General Science

Exercise


1. Who invented the electric cell?

a. Luigi Galvani b. Alesandra Volta

c. Georges Leclanche d. Thomas Edison

2. In an electric cell, which of the following transformation of energy takes place?

a. Mechanical energy transforms into electrical energy

b. Chemical energy transforms into electrical energy

c. Heat energy transforms into electrical energy

d. Light energy transforms into electrical energy

3. The filament of an electric bulb is made of-

a. Steel b. Nickel

c. Tungsten d. Nichrome

4. In a Leclanche cell as a positive rod (electrode) we use

a. Carbon b. Copper

c. Zinc d. Iron

Which statement is correct?

a. Electricity always flows from high potential to low potential.

b. The electric pressure always remains less in case of positively charged body.

c. In Leclanche cell the mixture of manganese dioxide and charcoal is used.

d. The flow of electricity and the flow of electron remain in same direction.

According to the following figure, answer the questions 6 and 7.5. Electricity flows through the cell-

i. from high electric pressure to low electric pressure

ii. from low electric pressure to high electric pressure

iii. from a large number of electron to a small number of electron

Figure

acid + water

Electricity 153


a. i b. ii

c. i & ii d. ii, iii

6. When the two terminals of the electric cell is connected by a conductor

a. Electron flows from zinc plates to copper plates

b. Electron flows from copper plates to zinc plates

c. Electricity flows from copper plates to zinc plates

d. Electricity flows from zinc plates to copper plates

Creative Questions

1.

Figure 1

a. What is the use of the switch in the circuit?

b. Explain the flow of electricity through the circuit.

c. Explain the transformation of energy in the circuit

d. Give your opinion about the flow of electricity if we use Leclanche cell instead

of batteries.

2.

Figure: Electric Bulb

a. Write down the name of the gas inside the bulb.

b. Describe the function of the gas.

c. Explain the flow of electricity through the bulb connecting it to a battery.

d. Write down what will happen for the overflow of electricity through the bulb.

154 General Science

Chapter -16

Earth Crust and RockThe layers of the earth's interiorAlready you have learnt that the earth was very hot and in gaseous form at the time of

its birth. Gradually by radiating temperature, the earth first turned into molten form

and then due to further cooling it was hardened. This hard surface of the earth is

named as crust. The liquid part of under the crust is made of different elements.

Within the liquid parts of the inside of the earth, the heavier materials sank towards

the centre of the earth, the less heavier materials stayed above the heavy ones and the

lighter ones stayed near the surface below the crust in layers. This type of concentric

layer within the interior of the earth is called layers of the interior of the earth. The

geologists have. divided these layers into three classes. Such as-

(I) The Lithosphere

(2) The Mesosphere and

(3) The Barysphere

(1) The Lithosphere: The crust and its immediate lower portion together form the

lithosphere. The above thin layer of the lithosphere is called crust. The depth of this

lithosphere is 16 to 40 km. The lithosphere is made by the elements such as- Oxygen,

Silicon, Aluminium, Calcium, Sodium, Potassium etc. We can divide this lithosphere

into two parts viz- Sial and Sima. The sial layer which is formed by silicon and

aluminium is lighter than the layer of sima which is made by silicon and magnesium.

That is why this layer remain above of the layer sima. Above of this lithosphere is

surrounded by atmosphere.

(2). The MesosphereBelow of the lithosphere, there is a layer of about 2,885 km deep that is called the

Mesosphere. This layer is made of mainly silicon and magnesium. This zone is quite hot

Figure 16.1 : The layers of the earth's interior

Barysphere

Centorsphere

Lithosphere

Hydrosphere

Atmosphere

and remains great pressure from all sides. For this- reason, constituents of this layer is

highly concentrated like pitch that means an intermediate state of solid and liquid.

(3) The BarysphereThe layer which remains below of the Mesosphere up to a depth of 3,475 km to the

centre of the earth is called the Barysphere. The Barysphere is mainly made by heavy

elements such as- Iron and nickel.

The crust of the earth and its elementsThe hard surface that covers the earth is called crust of the earth. This crust is made of

various stones and mineral substances. The elements by which the crust is formed are

Oxygen, Silicon, Aluminium, Iron, Sodium, Calcium, Magnesium and Potassium. But

the average percentage of oxygen and silicon in rocks is 73.07% and all the other

elements together is 26.93%.

MineralsThe earth is made of different rocks. This rocks are composed of different minerals.

Two or more elements like- quartz, tale, topaz naturally forms minerals. You might

have noticed that the talcum powder that we use for our body is the powdered talc.

But small quantity of perfume and medicine needed to mix with this.

RockThe components by which the crust form is called rock. When two or more minerals

aggregate then rock is formed. Rock does not mean only the hard substances. In

higher class you will know that clay or sand is also a form of rock.

Classification of rocksOn the basis of their mole of formation rocks are classified into three classes. Such as-

(1) Igneous or primary rock: viz- Granite, Basalt, Dolerite etc.

(2) Sedimentary or stratified rock: viz Sand stone, Lime stone, Coal, Marine salt etc.

(3) Metamorphic rock: viz- Marble stone, slate, gneiss graphite etc.

(1) Igneous or primary rockCrores of years ago, when the hotted earth became cool and solid, the rock which was

first created on the surface of the earth is called primary rock. As this rock is formed

from the hot and molten matter, it is called igneous rock. As this igneous rock has no

layer so it is called unstratified rock. This rock does not possess any fossil, so, it is

called unfossilized rock also. When animals, trees remain in between layers of rocks

for millions of years then fossil is created. Trees or any other animal turn into ashes or

gases with the help of highly hotted molten matter. That is why this igneous rock

contains no fossil.

Classification of Igneous rockInitially the Igneous rock is divided into two parts. viz- extrusive and intrusive

156 General Science

igneous rock. When the hot and molten materials of the interior part of the earth come

out to the surface of the earth through craters or clefts and turn into rock by cooling

gradually is known as extrusive rocks. The crystals of this rock are finely grained and

smooth. For this reason, this type of rock is very hard. Basalt, Pumice stone, Breccias,

Tuff etc. are the examples of extrusive igneous rock.

Again, sometimes the molten materials of the interior of the earth solidify below the

surface and form rock, this is called intrusive rock. Granite and gabbros are the

examples of intrusive rock.

Characteristics of Igneous rocks

(i) Unstratified rock

(ii) Contains no fossil in the rock

(iii) Crystal forms with definite shape

(iv) Very hard rock.

(2) Sedimentary or stratified rockDue to heat, rainfall, wind, snow, glacier, river, ocean's flow and wave, the Igneous rocks

are pulverized and eroded to gravel, pebble and sand particle. These eroded materials

rowed by rivers flow and wind and deposited at the bottom of lake, sea and ocean in

layers. Due to the temperature of the bottom of the earth's interior, due to chemical

reaction of the deposition of skeletons and different types of plants and pressure from the

above water, the sediments of the lower layer solidify to form the rock.

As this rock is made by sedimentation, it is called sedimentary rock. This rock

contains fossils. As this rock is formed in layers, it is also called stratified rock. The

Himalayas and the Alps are the mountains formed by sedimentary rock.

Figure 16.2 : Sedimentary rock

Characteristics of sedimentary rocks(1) Stratified rock

(2) Fossil remains in the rock

(3) Mineral oil (viz- petrol) originates from this rock.

Earth Crust and Rock 157

(3) Metamorphic rockBecause of various regions Igneous rocks and sedimentary rocks remain in too much down

from the subterranean in many places. Due to tremendous heat pressure and chemical

reactions these rocks transformed into a new rock. For example- sand stone is changed into

quartzite, limestone to marble, coal to graphite and clay to slate. The Tajmohol is one of

the seven wonders of the world. This Tajmohol is made by marble rock.

Characteristics of Metamorphic rock

(1) It is a transformed form of rocks.

(2) This rock is very hard, so it is not easily eroded.

Economic importance and uses of rock

Rock is useful to man in various ways. Moreover, it is possible to do economic

development of a country by importing and exporting rock. The pulverized part of

rocks mix with different organic substances to make soil. On this soil people like and

grow crops.

(1) The quality and fertility, the capacity to retain water etc. depend on the rock below.

(2) The water of a region depends on the type of the rock of that region.

(3) Granite and basalt are used to construct road.

(4) Limestone, sand stone and granite are used as mortar for constructing buildings.

(5) Limestone is used to prepare lime and cement.

(6) Coal and mineral oil are used as fuel.

(7) Clay stone for example - china clay, fire clay, are used in pottery.

Exercise

Multiple Choice Questions:

1. Granite and basalt is used

a. to construct a road b. to make cement

c. to construct the walls of the house d. in every work

2. Which one is a metamorphic rock?

a. Lime stone b. Granite

c. Marble d. Basalt

3. Which mineral is formed by more than one element?

a. Silver b. Granitec. Diamond d. Quartz

158 General Science

4. Which type of rock is graphite?

a. Igneous b. Sedimentary

c. Metamorphic d. Clay

5. The other name of sedimentary rock is-

a. Stratified rock b. Primary rock

c. Metamorphic rock d. Instructive rock

6. In many regions , due to various causes, igneous and sedimentary rocks stay much

deep in the core of earth. Because of extreme heat, tremendous pressure and for

chemical reaction, these rocks change into a new type of rock. For example-

i. Sand stone changes to quartzite

ii. Lime stone changes to marble

iii. Clay changes to graphite

Which one is correct?a. i b. i, ii


Creative questions1. Mr. Rahim has shown three rocks marked 'a' 'b' and 'c'. Displaying the rocks he

said that the rock 'a' is used to construct road, 'b' is used to make cement and 'c' is

used to make slate for writing.

a. What is the name of the rock used as a writing slate?

b. Write down two main characteristics of the rock 'a'.

c. How is 'b' type of rock transformed into 'c' type of rock? Explain.

d. Discuss why there is no micro-organism in the rock 'a'.

2.

Figure

The elements which we get in the crust of earth are shown in graph.

a. Which elements do we get much in the crust of earth?

b. Arrange these elements according to the amount.

c. Find out the relative percentage of these elements which are much available.

d. Find out the location of the elements according to the layer of earth in the graph.

Earth Crust and Rock 159

Alu

min

ium

Cal

cium

Iron

Pat

assi

um

Mag

nes

ium

Sodiu

m

oxygen

Sil

icon

60

40

20

Chapter - 17

Sea and OceanBy this time we have learned something about sea, ocean and bay. In this chapter we

will know in detail about sea, ocean and bay. About 71 percent of the earth's surface is

covered by five oceans. These oceans are (1) Pacific Ocean, (2) Atlantic Ocean, (3)

Indian Ocean, (4) Northern Ocean and (5) Southern Ocean. The vast, extensive and

deep-water bodies on the earth's surface are called ocean. For example, the Pacific

Ocean is the largest and the deepest. The sea is a water body that is relatively smaller

than a ocean or is the part of an ocean that is nearer to landmass. For example,

Arabian Sea, Japan Sea, Red Sea and so on. The sea that is bounded by land on three

sides is called bay. For example, Bay of Bengal, Bay of Malaya and so on. But some

seas may be bound on four sides by land, for example, the Mediterranean Sea. Let us

now learn about the oceans.

Description of Ocean

(1) Pacific Ocean: The largest of the oceans is the Pacific Ocean. It has an area of

approximately 16 crore 60 lakh square kilometre. It covers about one-third of the earth's

surface. This Ocean is bounded on the east by the North and South Americas, on the

west by Asia and on the south by Antarctica. Its average depth is 4,270 metre. The

Ocean has many deep trenches. The greatest known trench is Mariana Trench that is

about 10,870 metre and lies off the coast of Mindanao of Philippines. The Pacific Ocean

contains many big and small groups of islands. Among them are Kuril Islands, Japan

Islands, Philippines Islands, Indonesia Islands and New Zealand Islands are notable.

Besides, there are several seas scattered along the western side of the Ocean. These are

Bering Sea, Okhotsk Sea, Japan Sea, Yellow Sea, East and South China Seas.

(2) Atlantic Ocean: The Atlantic Ocean has an area of about 8 crore 24 lakh square

kilometre. That is, about half the size of the Pacific Ocean. The Atlantic Ocean covers

about one-sixth of the earth's surface. On the east of the Ocean lie Europe and Africa

and on the west lies the North and South.Americas. The Atlantic Ocean is shaped like

the English letter 'S'. Its depth is relatively less than the Pacific Ocean. Its average depth

is 3,932 metre. The deepest trench in the Atlantic Ocean is the Puerto Rico Trench. In

the middle of the ocean bed there are several rocks or mountain ranges similar to the

Dolphin and Challenger plateau of North America. The islands in the Atlantic Ocean

include Iceland, Falkland, Georgia, Saint Helen, Brazil Islands, Coral Islands, Bermuda

and the Volcanic Island of Madeira are notable. The notable seas are the North Sea,

Baltic Sea, Caribbean Sea, Hudson Bay, Baffin Bay and Gulf of Mexico.

(3) Indian Ocean: The Indian Ocean is bounded on the north by Asia, on the south by

Antarctica, on the east by Australia and on the west by Africa continents. The area of

the Indian Ocean is about 7 crore 36 lakh square kilometre. The average depth of the

Indian Ocean is 3,962 metre. In the Indian Ocean, there is a trench named the Sunda

Trench, which lies off the southern coast of the Island of Java. The well-known

islands of Madagascar and Sri Lanka are in the Indian Ocean. Besides these, there are

Andaman and Nicobar Islands in the Ocean. The Lakkha and Maldives are the coral

islands in the Indian Ocean. The famous seas in the Indian Ocean are the Arabian Sea

and Red Sea and the bays are Bay of Bengal and Persian Gulf.

(4) Northern Ocean: The Northern Ocean is situated in the northern end of the

northern hemisphere. It is surrounded by landmass. The total surface area of the

Northern Ocean is about 1 crore 50 lakh square kilometre. It is about one-twelfth of

the area of Pacific Ocean and its average depth is about 824 metre. The notable

shallow seas in the Ocean include the Beaufort Sea and the Siberian Sea. Besides,

there are some islands and group. of islands. The major islands in the Ocean include

New Siberian Islands and Canadian Islands.

(5) Southern Ocean: The Southern Ocean surrounds the Antarctic continent. That is,

the Southern Ocean lies north of Antarctica. The total surface are of the Ocean is

about I crore 47 lakh square kilometre. It is covered by ice the year round and has an

average depth of about 149 metre. The ridge on its bed connects Australia with the

Antarctic continent.

Ocean Current

Human beings, animals, birds, insects and so on, do not remain stationary in one

place. They move from one place to another. But pushcart, boat, cycle, launch, car,

train, aeroplane and so on cannot move on their own, they have to be moved by others.

Similarly, wind flow and rotation of the earth make the water of the ocean flow from

one place to another. The regular flow of water in ocean and sea is called ocean

current. For example, the ocean currents in the Pacific and Atlantic Oceans.

Reason for Ocean Current

By now we have come to know that ocean current do not occur on their own. There

are few reasons that create ocean current. The major reasons for ocean current are

briefly mentioned below.

(1) Wind Flow: One of the major reasons for ocean current is wind flow. As the wind

flows continuously over the ocean, the water also flows along in the same direction.

The direction of wind changes with the changing seasons resulting in the change of

direction in the ocean current. For example, the seasonal wind in Bangladesh

Sea and Ocean 161

(2) Earth's Rotation: Ocean current is created due to the earth's rotation. The earth

rotates around its axis from west to east once in every 24 hours, this is called diurnal

rotation. Because of this diurnal rotation, the water in the ocean flows from west to

east creating a current. Earth's rotation is a major reason for ocean current.

(3) Variation in Temperature: The ocean current is produced due to the difference in

temperature at different parts of the ocean. Where heat from the sun falls perpendicular

to the surface water, there is more evaporation compared to where the heat from the

sun falls obliquely. This leads to an imbalance in the density and depth of water. In

order to maintain a balance, water flows from the region where the heat from the sun

strikes obliquely to the region where the heat from the sun strikes perpendicularly.

Thus ocean current are produced.

(4) Salinity: Saline water is heavier and has greater density. Fresh water is lighter and

has less density. So, the water of more salinity from warm region flows towards water

of less salinity in the temperate region. Besides, as water of more salinity is heavier it

sinks to the bottom, while the water of less salinity being lighter rises up. This causes

upward and downward currents in the ocean.

Influence of Ocean Current

Because of ocean currents: (a) Movement of ships is facilitated. If the direction of the

ship is the same as that of the current, the ship can move faster from one place to another.

(b) The ocean water does not turn into ice easily because of the warm ocean current. (c)

Rainfall occurs in the coastal region under the influence of warm ocean current. (d) The

meeting place of warm and cold currents becomes the habitat of most fish.

High and Low Tides

We all more or less have heard about high and low tides. But those among you, who

live near rivers not far from the sea, may have noted that sometimes the river water

increases and sometimes it decreases. This increase and decrease of water in the river

and ocean are called high and low tides respectively. The sun and moon attract the

water and landmass of the earth. Under the influence of this attraction the earth's

water swells up daily in specific places and elsewhere it recedes. This rising or

swelling of water is called high tide and the recession of water is called low tide.

Causes of High and Low Tides

All matters on earth attract each other continuously. The greater the size of matter, the

greater is the attraction. But as the distance between objects increases, the attraction

decreases. Like other objects, the sun and the moon also attract the earth. The sun is

many times larger than the moon but the distance of the sun from the earth is much

162 General Science

greater than that of the moon from the earth. Though the moon is small, it is the

nearest cosmic body to the earth. Moon is earth's only natural satellite. Therefore, the

attraction of the moon on the earth is relatively more. So, the diurnal variation of the

moon's attraction on different parts of the earth is the major reason for high and low tides.

Direct and Opposite Tides: The moon is continuously moving around the earth. As

the moon rotates, its attraction is greatest at the part of the earth that is facing the

moon. The effect of this attraction is more on the water than on the landmass, so the

water of the ocean is attracted to the point where the attraction is stronger.

Therefore, the water closer to the moon swells up and cause high tide. This tide is

called the direct tide.

At the same time, when the part of the earth facing the moon is experiencing direct

tide, the land under water on other side of the earth is attracted more towards the

moon. At this time, the gravitational force of the earth on the water decreases and

under the influence of repulsing force, water from all sides flow into the same place

causing high tide. The hfgh tide thus caused is called opposite tide.

Low Tide

When the direct tide is on going at one place of the earth, then opposite tide is on

going at the opposite end of the earth. At this time, the water decreases in the two

parts between the high tides. Due to decreased water, these two parts experience low

tide.

High and Low Tides Twice Daily

When any part of the earth is nearest to the moon, then that part and the part exactly

opposite experience high tide. At the same time, low tide occur at place on the earth

that is at right angles to the part with high tide (Fig. 17.1). Every day, every part of the

earth experience high tide twice.

Fig: 17.1 High low tides

Low tide

Low tide

Direct tide

a

d

M

M1

Water

Oposite tide

b

C

Moon

Sea and Ocean 163

Spring tide or flood tide: On new moon, the sun and the moon are in line on the

same side of the earth. Though the attraction of the sun is less than the moon but the

two attractions together is very strong. There is massive swelling of water in the high

tide due to this combined attraction. This high tide is called spring tide or flood tide.

Fig: 17.2 Spring tide or flood tide

On full moon, the sun and the moon are in line on the opposite sides of the earth. At

this time, the place where there is high tide due to moon's attraction, in the same place

due to sun's attraction there is high tide. As a result, there is massive swelling of water.

So, on full moon the high tide is called spring tide or flood tide.

Neap Tide or Ebb Tide: Eight days after new moon or full moon, that is, on the

eighth day of either fortnight of lunar month, the sun and moon are at right angle to

each other as they attract the earth. In this condition, the high tide due to attraction of

the moon is reduced by the attraction due to sun. As the attraction force of the moon is

stronger than that of the sun, so the part of earth facing the moon and the part opposite

experience high tide, while the part of earth facing the sun and the part opposite

experience low tide. For this reason, at these times the height or swelling of high tide

is at its minimum. So, the high tide on the eighth day of new moon or full moon is

called neap tide or ebb tide.

Fig: 17.3 Neap tide or ebb tide

E E

New moon

Moon MoonSun Sun

8 day after full moon 8 day after new moon

E ESun Sun

Moon

Moon

164 General Science

Intervening Time between High and Low Tides

When high tide occurs at a place at a certain time of the day, then on the next day the

tide does not occur there at the same time. There is a delay of 52 minutes. As the

moon requires 29 days to complete one revolution around the earth, so the moon

advances 1/29th of its orbit in one day. Therefore, during single rotation of earth, that

is in 24 hours the moon advances about 130 in its orbit. It takes about 4 minutes for the

moon to advance 130. In other words, it takes about 52 minutes to advance 130. So the

place that experienced direct tide today, will experience direct tide tomorrow not

exactly after 24 hours but after 24 hours 52 minutes of today's experience.

Influence of High and Low Tides: The high and low tides in the rivers or oceans are

beneficial to us. Among these the notable ones are:

(a) The big commercial ship can easily enter and leave the seaport due to high and

low tides

(b) The waste material in the river is carried out to sea during low tide, thus making

the water free from filth and dirt

(c) The high and low tides help to prevent siltation in the river bed

(d) High and low tides cause slight salinity of the river water, as a result the water

does not easily turn into ice

ExerciseMultiple Choice Questions1. Which one is the deepest trench of the world?

a. Mariana b. Puerto Rico trench

c. Sunda trench d. St. Helens

2. What one is Challenger?

a. Island b. Ridge

b. Sea d. Gulf

3. In which ocean is Madagascar situated?

a. Arctic b. Atlantic

b. Indian d. Pacific

4. What happens when the sunlight falls vertically on the water?

a. the water is less evaporated

b. the water is evaporated much

c. the temperature of water remains static

d. the water plants grow more

12

Sea and Ocean 165

5. During the new moon

a. The moon and the sun remain in one side of the earth

b. The moon and the sun remain in opposite side of the earth

c. The moon and the sun remain at right angle to each other

d. The earth remains in front of the moon

6. Aqib, at the time of travelling from Cox's Bazaar to St Martin noticed that the

water of the sea was flowing from one place to other and thus creating the ocean

current. The main cause of it is-

i. wind flows always on the sea.

ii. the rotation of the earth is from west to east.

iii. the difference of heat of the sunlight on the sea.


a. i b. i, iii

c. i, ii d. i, ii & iii

From the figures above, answer the questions 7 and 8 7

7. What type of tide is there in figure -1?

a. Ebb tide b. Neap tide

c. Direct tide d. Indirect tide

8. On the earth, in case of figure 2-

a. The attraction of the moon is more

b. The attraction of the sun is more

c. The attraction of the moon and the sun is more

d. The sun has no attraction

Figure-1

earth earth

moon

sun sun

Figure-2

166 General Science

Creative Questions

1. Sakib with his father went to visit the beach of Cox's bazaar. There he could see

the sea water rising gradually. After reaching a certain height, the sea water, he

could see, was falling down again in the same way. Sakib demanded his father

that he want to see this ups and downs of water of the sea for 24 hours.

a. What is the gradual rising of sea level called?

b. Explain the cause of rising up and falling down of sea water.

c. How many times Sakib could see this up-down of water if his desire is fulfilled?

d. Explain the cause of rising up and falling down of this sea level.

2. On 23 March 2008, Apu came to know from the local coast guard that the time of

tide is at 06.32 am next day. Apu went to the sea shore on the next day and saw

the tide at that time. But on 25 March 2008, Apu could not see the tide at the

same time.

a What is tide?

b. Why was the tied not seen on 25 March 2008 at the same time?

c. Find out the time of tide on 25 March 2008?

d. Explain the mathematical relation between the time of tide and the moon and

the earth.

Sea and Ocean 167

Chapter - 18

Weather and ClimateSuppose in a morning you got up from bed and saw that it had been raining heavily.

After some time the sky became clear. The sun was shining and the temperature went

high. Just after that you saw strong wind was blowing. The wind was cold. At

afternoon you could see accumulation of dark clouds on north sky. It might rain at any

moment. We have just heard about some situations of the day's weather. Sum total of a

day's situation or a few day's situations are called weather. It means the total amount

of temperature, pressure, wind, humidity and rainfall of a particular day or few days

unified are called weather. There is one meteorological office in each divisional city in

our country. We get weather information of different places of the country from these

offices. In the whole country or in some areas of the country, weather condition does

not remain the same in every single day of the year. It may be very hot in one day and

very cold in another day. One day, there may be raining from dawn to dusk.

On the other day when there may be no rain at all. The average weather condition of

an area for 30 to 40 years is called climate. From these descriptions we could

understand that weather is the total condition of wind, humidity, temperature and

rainfall of a single day or few days of an area. Where as climate is the average weather

condition of a country or an area for few years.

Differences between weather and climate

Climate

1. Average conditions of few year's

weather of an area is the climate.

2. All the composite parts or elements

of weather are also same in case of

climate.

3. Climate is the atmospheric conditions

of a country or an area for a long

period.

4. Climate of an area does not change

rapidly.

5. About 30 to 40 years average

weather report is to be consulted to

calculate the climate of an area.

Weather

1. Average condition of atmosphere of an area

for a period of few days is called weather.

2. Temperature, pressure, humidity, blow of

wind and rainfall are the elements of

weather.

3. Weather is the condition of the atmosphere

for a short period.

4. Weather condition changes rapidly.

5. Average weather condition can be calculated

by consulting few day's weather report.

Composite parts or elements of weather and climate

We have learnt earlier that each and every substance is made of one or more things.

Similarly, weather and climate are also made of few composite parts or elements.

Composite parts or elements of weather and climate are the same. Weather and

climate of a particular area have the following composite elements.

1. Air temperature,

2. Air pressure,

3. Wind movement,

4. Humidity of air.

These elements are the elements of both weather and climate. With the variation of

these composite elements; weather and climate of an area also varies. In short these

composite elements are discussed below.

1. Air temperature : Main source of temperature is sun. The air may be hot or cold

depending on the amount of heat absorption by air. Weather and climate of an area

depend on high and low temperature of air. For example, when temperature of an area

is high the weather and climate of that area will be hot.

2. Air pressure : Air always exerts pressure all around. Like other substances, it has

weight. Air pressure is formed due to this weight. This pressure is called air pressure.

Direction of wind depends on air pressure. For example, air from high pressure zone

blows to the direction of low pressure zone. Cold wind coming from the sea causes a

fall in temperature of the shore areas.

3. Wind movement: Blowing of air occurs due to variation in temperature and

pressure of the air. Air blows continuously from one place to another. The blowing air

is called wind.

4. Humidity of air: Presence of water vapour in the air is called humidity. Due to

presence of humidity air temperature can not go up. But in this condition there is a

probability of rain. Temperature of air can not rise up due to rain. But in desert or

rainless areas air temperature goes up.

Climatic factors:

When one or more things controls the functioning of some thing, it is called the factor

or factors. For example, good food is necessary for good health. So, good food is the

factor of good health. Climate is different in different parts of the world because

climate is controlled by few natural factors. These factors are 1) latitude 2) Altitude 3)

Distance from sea 4) wind movement 5) Ocean currents 7) Location of mountains 8)

Location of forests and 9) Land relief etc.

Weather and Climate 169

In higher class you will learn how these factors affect or regulate the climate of an area.

Classification of climate

Intensity of cold and acuteness of summer temperature are the basis of classification

of world climates. These climates are classified into two types; such as- 1) Extreme

climate and 2) Moderate climate.

1) Extreme climate: Extreme cold and intensive heat are the characteristics of

extreme climates. These climates are seen in the interior region of a continent. That is

why, this type of climate is termed as continental climate.

2) Moderate climate: In this type of climate, there is no extremeness of winter

and summer. This climate prevails in the areas adjacent to the sea. It is called

maritime climate.

Causes of change in temperature

Atmospheric temperature is not same in all places of earth surface. In some places the

temperature is high. For example, temperature of jakobabad of Pakistan is high, while

it is low at Darjiling of India. Defference in temperature depends on some factors.

These factors are I) Oblique sun rays 2) Latitude 3) Day length 4) Altitude 5)

Humidity 6) Wind movement 7) Ocean current and 8) Depth of atmosphere etc.

Causes of change in air pressure

The normal air pressure on sea surface is 15 pound per square inch. That means 1

kilogram per square centimeter. Air pressure does not remain same all the time. In a

particular place air pressure is different at different times of the year. It is also

observed that the air pressure changes in morning , noon and evening. Changes in air

pressure occurs due to the following factors.

1. Variation of temperature: Volume of air is increased and density of air is reduced

when heat is applied. The hot air is lighter so its pressure is low. On the other hand,

cold air contains less water vapour; so it exerts more pressure . Therefore. when the

air is hot, the air pressure is low.

2. Altitude : Density and weight gradually decrease with increased heights from the

sea level. Air pressure in high altitude is low. So higher the altitude from sea level,

lower is the air pressure.

3. Difference in water vapour contents: Air with water vapour is wet and warm,

which is lighter compared to dry air. That's why pressure of air with water vapour is

low. The dry air is heavy so its pressure is high. Warm air is more capable of holding

water vapours. It is observed that the air pressure in rainy season is low.

170 General Science

Change in air pressure has close relationship with change of weather. When air

pressure is high, the atmosphere is clear and normal. But when air pressure falls, a

low pressure is developed. Then there is a possibility of rain and storm.

Concept of determination of average temperature and pressure of air

We have learnt from the first chapter of this book about temperature and thermometer.

Again in the third chapter we have learnt about barometer and its use. We can

determine daily, monthly and yearly average temperature of air by using maximum-

minimum thermometer. Barometer is used to measure air pressure. By observing

ascending and descending mercury level we get the forecast about rain and storm.

Cause of blowing air

Air blows constantly. To maintain the equilibrium in air pressure, blowing of air

occurs. Besides this, there are other factors which make the air blow. Let us discuss

these factors in short.-

1. Variation in air pressure is the main cause of blowing of air.

2. Hot air when full of water vapour becomes light and goes up and that space

becomes partially vacuum. For maintaining balance, air from high pressure zone blow

towards low pressure zone.

3. Heavy air from high pressure zone blows at low altitude towards low pressure zone.

Where as the light air from low pressure zone blows at high altitude towards high

pressure zone. As a result, a circular path of blowing of wind is formed.

4. The earth rotates from the west to the east; so in north hemisphere air current turns on

the right. While in the south hemisphere it turns on the left. It is known as Ferriel's law.

Types of clouds

Usually we term it cloud when we see white cottony body floating in the air. Actually

cloud is the collection of water drops formed on dust particles floating in the air.

The clouds are found in different altitudes in the sky. Depending on altitude clouds are

classified into (1) Cirrus cloud (high level cloud ), (2) Altocumulus cloud (semi

high level cloud) and (3) Stratocumulus cloud (low level cloud).

Different climatic regions of the world

The climate of all places of the world is not the same. Some where rainfall is high and

there are places where there is no rainfall at all. In some places day temperature is

high but the night is cold. Some region is rich in vegetation while there are regions


where there is no vegetation. Some areas are good for cultivation but in some areas

cultivation is not possible. It is because, the climatic factors are not similar every

where. Variations of north-south climates are more than the variations in eastwest

climates. Climatic changes are more on the land than in water. In north hemisphere

land mass is greater so climatic changes are greater than south hemisphere. On the

basis of different climatic factors, the earth is divided into few climatic regions; like

Tropical, Temperate and monsoon, the Mediterranean, Arctic and antarctic regions.

Effect of climate on plants and agriculture

Growth of plants depends on climate (rainfall and temperature). It means rainfall

determines nature of vegetation of an area. For example, forest is formed where there

is sufficient rainfall. Where rainfall is scanty, grass land prevails there. Where there is

no rain at all, the thorny bushes may grow there. In low temperature slow growing

small plants and in high temperature rapidly growing plants grow.

In different climatic regions due to climatic effect different varieties of plants grow. In

areas with heavy rainfall (170 to 255 cm) deep forests of evergreen trees prevail.

These areas are located near equatorial region. Plants like Teak, Mahogany, Rubber

etc. grow in this type of forests. On the other hand, when rainfall ranges from 125 to

200 centimeters in heavy monsoon rain prone area; plants like Mahogany, Sal, Cane,

Banyan, Mango, Black berry, Jackfruit etc. grow well. Bangladesh is a country under

the influence of monsoon wind. In Mediterranean region sufficient rain fall occurs in

winter. Plants like Seder, Pine, Oak, Cork, Tulip etc. grow in this region. Climate has

a number of influences on agriculture. Climate determines the type of crops of any

climatic zone. Areas near the equator experience heavy rainfall, which is not good for

cultivation. Rubber, Coffee, Sugarcane, Paddy etc. are the main agricultural products

of these areas. It was mentioned earlier that the Mediterranean region experience

heavy rain. This heavy rain helps this area to cultivate citrus fruits. Besides Wheat,

Maize, Cotton etc. are produced here. Monsoon rain zone is very congenial for

cultivation. Medium rainfall and moderate temperature help cultivation of Paddy,

Wheat, Millets, Sugar cane, Oil seeds, Tobacco, Jute etc. In desert areas rainfall is

negligible and the temperature fluctuates. That is why, big trees do not grow and

cultivation is not possible.

From the above discussion, it can be said that differences in plants and agricultural

products is due to variation in climatic conditions in different regions. Besides,

human. nature, life style, clothing, complexion, food habit and culture vary due to

difference in weather and climate.

172 General Science

Exercise


1. How many controlling factors are there in climate?

a. 9 b. 8

c. 7 d. 6

2. In one square centimeter the normal pressure of air is about-

a. 1.5 kg b. 1 kg

c. 0.800 kg d. 0.750 kg .

3. With the help of maximum and minimum thermometer-

a. The daily temperature of the air is recorded

b. The daily and monthly average temperature of the air is recorded

c. The daily, monthly and the yearly average temperature of the air is recorded

d. The temperature of air cannot be recorded.

Answer the questions 4 and 5 from the following passage.

Malaysia is a country surrounded by sea in its three sides. Throughout the year the

average temperature of the country is 250 to 300. Heavy rainfall occurs in the country.

The amount of rainfall is 150 to 200 centimetre.

4. The country grows more-

a. shrubs with spike b. bushes

c. trees d. creeper

5. In the climate of Malaysia there is-

i. extreme hot

ii. extreme cold

iii. always the temperature remains same.

Which one is correct?a. i b. ii

c. iii d. i, ii , iii


Creative Questions

1.

Figure

Month of March- April (Chaitra-Baishakh)

Average temperature 350c.

a. Of which element is the flow of wind?

b. Explain the cause of opposite direction of flow of wind in two different situations

in the figure.

c. Explain the condition of weather in the month of March and April (Chaitra-

Baishakh).

d. Describe the effect of flow of wind in our daily life.

2.

Sl No Name of Type of grown plants/crops

the country

1 'a' Shrubs with spike, bushes, dates etc.

2 'b' Orange, lemon, sugar cane, paddy, coffee etc.

a. What is climate?

b. What will be the climate of the country 'a' like?

c. Explain why the fruits such as orange and lemon grow more in the country 'b'?

d. Compare the climate of the country 'a' to that of 'b', according to the information.

Sun

At day

At night

wind movement

surface surface

sea sea

wind movement

Moon

174 General Science

Chapter -19

Common Laboratory Processes

Science is practical oriented subject. When you learn science by doing yourself, it is a

joyful learning. You become inquisitive to science and can learn and receive actual

messages of science more clearly. When you learn science by doing, the knowledge

you get will be meaningful and sustainable. You will get scope for observation,

experimentation and drawing conclusion. But this process needs laboratory,

equipments and necessary chemicals. But in place of a laboratory, simple experiment

can be done in the class room also. For completing an experiment we follow some

common processes. These are called common laboratory processes. In this chapter

you will learn about few laboratory processes for separating solutes and solvents from

solutions. These processes are sedimentation, filtration, evaporation, condensation,

crystallization etc.

Sedimentation

Separation of solutes from solution by sedimentation process.

Experiment-19.1: Take some water in a glass beaker and add one or two spoonful of dust

in it and stir well and observe the mixture.

What have you seen? Water is

turbid. Can you say why the water

looks turbid? It looks turbid because

dust is insoluble in water. Now

leave the beaker with the mixture on

the table for some time. Then

observe the mixture. What have you

seen ? All mud have accumulated at the

bottom and water at the upper level looks clean and clear. Now make the beaker

slanting and slowly pour clear water from the top in a separate container leaving mud at

the bottom of the beaker. Heavy solutes mixed up with liquid, can be separated by this

process. The process is known as sedimentation process. During sedimentation process,

the pouring of clean and clear water from above the sediment is called decantation.

Those heavy insoluble substances settled at the bottom are called sediment. In our

country specially in villages, sedimentation process is applied for various purposes.

In rainy season river water becomes turbid because at this time insoluble mud is

Fix: 19.1: Separation of substancesby Sedimentation

mixed up with water. People who reside near the river, use river water for various

purposes. The village women collect turbid water in pitchers from the river. They let

the mud settle at the bottom of the pitcher and water from the top is collected by

decantation process . They are using sedimentation process in their every day life

without knowing its name.

Filtration

You have learnt about decantation. It is a time consuming process yet the heavy

materials are not completely separated. It is because when the insoluble substances

are very light, they never settle at the bottom, rather they remain floating. So, they

cannot be separated completely by decantation process. In this situation filtration

process is the best. In filtration process unnecessary delay can be avoided for

separation of insoluble substances. On the other hand, fine insoluble particles can also

be separated by this process. In filtration process filter paper or sieve is used to

separate these fine insoluble solid particles. So, process of separation of fine solid

insoluble substances from a liquid medium by sieving with filter paper or fine sieve is

called filtration process.

Separation of solutes from solution by filtration process.

Experiment 19.2: Take some water in a beaker and mix with sand. Take a filter paper

and fold it twice. Follow the diagram and place the paper on the funnel. Now apply

some water on the paper so that the filter paper adheres to the funnel and place the

funnel in the ring fixed to a stand. Place a beaker under the funnel in such a way that

the tube of the funnel touches the wall of the beaker.

Place a thin glass rod at the notch (lip) of the beaker. Pour the turbid mixture from the

beaker down the glass rod slowly on the filter paper. You will see clear water flowing

down the funnel tube to the beaker

placed under the funnel. Sand is

deposited on filter paper as residue.

The process of separating insoluble

fine solutes by means of filter paper

or fine sieve is called filtration

process or sieving. The solute

remains on the filter paper or sieve,

is called residue; while the clear

water collected In the beaker is

called filtrate. Fix: 19.2: Filtration process

176 General Science

You have learnt about two separation processes of heavy solid particles from a

solution. Could you point out the differences between these two processes ? Let us

know the differences of decantation and filtration processes.

Differences between Decantation and Filtration

Evaporation

You have marked that rivers, canals, ponds, beels etc. become dry in summer season.

When you keep water in an open container; the quantity of water decreases gradually and

finally the container becomes empty. Now the question is why in summer, rivers, ponds,

canals, and beels become dry? Why do wet clothes become dry when it is spreaded in the

sun? Why does water filled open container become empty after few days?

Try to answer these questions. In every above mentioned cases water was evaporated

in the form of water vapour by heat of the sun. The result is decrease in water level or

empty container. When liquid substances are converted into its vapour by applying

heat or in natural condition; the process is called evaporation. In chapter seven you

knew about the process of extraction of salt from sea water. By applying natural

evaporation process salt is extracted. Solutes can be separated from the solution by

evaporation process.

Separation of solutes from solution by evaporation process.

Experiment-19.3: Take some water in a beaker. Add some salt and stir gently to

prepare salt solution. Now let us separate salt from the solution.

Put a wire net on a tripod stand and place the beaker containing salt solution. Now

apply heat below the wire net with a spirit lamp. Constantly stir the solution with a

stirrer during heating. Now you look at the solution. What have you seen? The water

Filtration

1. Filtration process is followed for

separating heavy insoluble substances

by sieving through filter paper.

2. Following this process minute floating

insoluble substances can be easily

separated.

3. By filtration floating insoluble solid

substances can be easily separated

Decantation

1. Decantation process is followed for

separating heavy insoluble substance

by sedimentation.

2. It is impossible or very difficult to

separate very minute floating insoluble

substances in this process.

3. By this process floating solid insoluble

substances can not be separated.

Common Laboratory Processes 177

has evaporated and the salt is left

behind the bottom of the beaker.

In evaporation process solutes may be

recovered from the solution, while

the solvent is lost as vapours. In our

country molasses, patali and sugar are

produced by evaporation process

from extracted juice of Sugar cane or

Date palm.

Condensation

The process of converting vapours of water or other substances into liquid form by

deep cooling is called condensation

process. Water can be extracted

through condensation process from

water vapour. In extreme coolness

water is converted into ice. In

summer, water from rivers, ponds,

canals and beels evaporate and float

in the air. When it becomes cool and

condensed and form cloud and

finally it falls on the earth surface as

rain. Insoluble heavy solutes can be

separated from the solution by evaporation or by filtration; but the solvent is lost as

vapour. But when we use evaporation and condensation together, we can separate and

recover both insoluble solutes and solvent. This combination of evaporation and

condensation is called distillation process. The process in which solvent is converted

to vapour and again vapour to liquid form, is called distillation process.

So, distillation process = evaporation + condensation

Do the distillation experiment with the help of your class teacher and separate soluble

solute and solvent from a solution.

Crystallization

When concentrated aquatic solution of any solid substance is prepared in high

temperature and allowed to cool, the solid substance falls at the bottom as crystallized

sediment. These granular substances are called crystals. The process of preparation of

crystals is called crystallization. Smooth and geometrically shaped crystals of salt,

Fix: 19.3: Evaporation process

Fix: 19.4: Distillation process

178 General Science

sugar, copper sulfate, alum, sugar candy (Misri) and hira kosh are very beautiful to

look at. Preparation of salt from sea water and sugar candy from sugar is done by this

process. In chapter seven you have learnt how to prepare sugar candy from water

solution of sugar. You can prepare sugar candy (Misri) at your home by this

crystallization process.

Separation of solutes from solution by crystallization

Experiment 19.4 : Prepare separate

saturated solutions of sugar, copper sulfate,

alum and common salt. Take a solution in a

beaker and apply heat. When the solution

dries up to half of the original volume, stop

applying heat. Leave the beaker with

concentrated solution for some time. When

the beaker is cold separate crystals of solid

substances.

Total amount of solid can be separated from the solution by evaporation process. But in

crystallization process some solid substances remain in the solution. A portion of solid

substances become separated and remain at the bottom of the solution as crystals.

Separation of solutes from a mixture of salt and sand

Experiment-19.5: Make mixture of salt and sand on a piece of paper. Take the

mixture in a beaker and mix them properly by means of a glass stirrer. You observed

earlier that, salt is soluble in water but not the sand. Now add water and stir with

stirrer, salt will dissolve in water but sand after floating for sometime will settle at the

bottom of the beaker. When you filter the solution, salt with water will pass through

the filter paper but the solid sands will remain on the filter paper as insoluble residue.

In this process sand will get separated from the solution. When salt solution is heated

by means of a spirit lamp or a Bunsen burner; water will be evaporated leaving salt at

the bottom of the beaker. In this process components from a mixture of salt and sand

can be separated. In this experiment,

a) at first water soluble salt was dissolved.

b) insoluble solid sand was separated by filtration process.

c) from filtrate saline water, solid recovered by evaporation.

Fix: 19.5: Separation of substancesfrom a mixture of sand and salt


Exercise


1. Why does the river water look turbid in the rainy season?

a. Insoluble floating substances remain mixed with water

b. Insoluble dust remains in water

c. Soluble substances remain in water

d. River water always remain turbid

2. What is the process by which solid substance insoluble in liquid can quickly and

completely be separated?

a. Condensation b. Vaporization

c. Filtration d. Crystallization

3. What is the name of the insoluble solid that sticks on the filter paper during filtration?

a. Filtrate b. Sediment

c. Crystal d. Residue

4. How can we recover salt and water from a solution of them?

a. Filtration b. Vaporization

c. Condensation d. Distillation

5. Which processes are applied to separate salt and sand from a mixture of them?

a. Solution and filtration b Filtration and distillation

c. Filtration and vaporization d. Filtration and crystallization

6. In which of the three events does evaporation take place?

i. Drying the cloth in sun light

ii. Preparing salt from the sea water

iii. Preparing sugar candy from the solution of sugar


a. i b. i, ii


Answer the questions 7, 8 and 9 from the following information:

In a beaker, a mixture of blue-vitriol, naphthalene, cobalt and sand is taken. By simple

laboratory processes such as distillation, sublimation, magnetization, filtration,

vaporization etc. the components can be separated.

180 General Science

7. By which process can naphthalene be separated?

a. distillation b. sublimation

c. filtration d. magnetization

8. Which one is to be separated first and by which process?

a. cobalt, sublimation b. blue-vitriol, filtration

c. naphthalene, sublimation d. cobalt, magnetization

9. What will you do to separate the mixture of blue-vitriol and sand?

a. filtration b. Filtration and vaporization

c. vaporization d. sublimation

Creative Questions

1. Sugar and salt are bought from the market. It is seen that dust and sand are mixed

with both of them. By using simple laboratory processes such as decantation,

filtration, vaporization, crystallization etc. pure and clear salt and sugar can be

separated.

a. What will you do first to get clear salt and sugar individually?

b. What will happen when decantation takes place?

c. Describe the process of getting clear solution of salt and sugar.

d. Which process will be better to get uniform crystal between vaporization and

crystallization? Put your argument.

2.

Figure

a. By which process we can get both salt and water from beaker -1?

b. How will we get ammonium chloride (NH4CI) crystal from the beaker-2?

c. Find out the process to extract the components of beaker -3?

d. Of the three specimens which one is called solution? Explain.

No-1 No-2 No-3


salt + waterammonium

chloride + water nickel + sand+water

Chapter - 20

Production of Essential Goods from Discarded Raw Material

You study subjects like BangIa, English, Mathematics, Social Studies, Religious

studies etc. in your class. Did you mark the basic differences between studies of

science and other subjects? Science is a practical oriented subject. Here, learning is by

doing. When you study science by doing, you are getting the following benefits:

1. You will get practical working experience.

2. You will gain practical skills.

3. You will develop inquisitiveness about nature.

4. You will learn to think logically.

5. You will be able to solve many day to days problems by applying scientific knowledge.

6. You will be able to increase standard of your life by applying scientific knowledge.

Science is a practical oriented subject, when you learn science by doing yourself the

learning becomes joyable. And you will not forget the learning for long. You can make

small but essential things for your house and family. Every day we discard and

throwaway many things after its use. Some of these discarded things may be

converted to useful goods when they are properly processed. In this chapter you will

learn how to produce useful goods out of these discarded materials lying around you.

What is waste raw materials?

We throw away things when they become old and useless. Thrown away useless things

are called discarded or waste materials. We use papers, polythene clothes or tin, metallic

foils etc. as wrapping materials. After using the materials, the packing materials are

discarded as wastes. There are few materials which have no other alternative but to throw

away after use. These materials are metallic can, broken glass, plastic materials,

vegetables and fruit peels, fish scales, bones etc. We also throw old books, note books,

paper, ball pen and clothes after use. If these things are thrown here and there they will

make the surroundings dirty, polluted and spread bad ordure. It creates imbalance in the

environment. However, there are few discarded waste materials which may be reused in

production of useful goods. It is economically profitable in one hand and on the other

hand, the environment remains free from pollution. The wastes which can not be

recycled, must be burried under the soil. It helps keeping atmosphere clean.

Sources and reuse of few discarded waste materials.

You can make useful things from some of the waste materials easily and cheaply by

applying your knowledge and skills in science education. Things used to produce new

products are called raw materials. From jute gunny bag, carpet, thread, rope etc. are

produced. Here jute is the raw material. Sugar molasses and spirit are produced from

sugar cane. Similarly yarn and clothes are produced from cotton; chemical fertilizer

from natural gas; biogas from cow dung; pulp and paper from wood and sugar cane

trash. Write in the following tabular form, the names of raw materials and

corresponding products in above cases.

Raw materials Produced goods

1. Bamboo, wood and sugar 1. Pulp and paper

cane trash.

2. 2.

3. 3.

4. 4.

5. 5.

6. 6.

You have learnt what is raw materials. Now let us prepare few utility goods using

discarded wastes as raw materials.

Preparation of few utility goods from discarded waste materials

Principles of selecting waste raw materials:

Sources of waste raw materials should be available from near surrounding

environment. We throw many things during or after its use. Raw materials should be

Source Discarded waste materials ReuseMetals Old and abandoned parts of By recycling these can

vehicle engines, metallic can etc. be made useable.

Glass and Broken glasses, used or unused Recycling in a factory

Plastics glasses, plastics etc. to produce new

products

Papers and Used papers clothes etc. Processing for the

clothes preparation of pulp.

Organic Peels of vegetables, onion, Production of organic

matters garlic, fruits etc. Waste fertilizer or manure.

materials of fish and meat,

excrement of cattle.

Production of Essential Goods from Discarded Raw Material 183

selected from these unusable materials. During selection of raw materials, the

following things should be taken into account.

a) Availability of raw materials from activities of every day life should be ensured.

b) Raw materials should be available free of cost.

c) Selecting such raw materials from which cost of production will be low.

d) Producing materials with low production cost from cost free raw materials and

must ensure the quality of the product.

e) Production should be usable for different purposes of the family. Raw materials

should be selected basing on these principles.

Products from waste or discarded raw materials are used in educational purposes and

to meet house hold needs. This may be taken as a profession by some people. How

waste raw materials can be utilized is described below with examples.

Paper

The material we need most for education is paper. Every day you read beautiful

books, magazines and newspapers. We write about knowledge, science, civilization

and literature on paper. Paper is the flame of civilization and main constituent part of

human civilization . After use we throw away these papers. These waste papers can be

utilized in various ways.

a) In books, periodicals, journals and news papers valuable pictures, advices and

writings are published. These writings, pictures, advices and messages may be used in

future. So, before throwing these papers you can preserve paper cuttings of valuable

writings, pictures and messages in an album.

b) When preparing note books, papers are cut to shapes and sizes. These cut pieces

may be utilized as paper tag and slip paper of shops. These cut pieces may also be

utilized to make paper bags and packets.

c) Paper pulps may be prepared from waste paper by boiling in water. Pitch board of

any size can be made by pouring pulps on a bamboo sieve and applying pressure on it;

Coloured pitch board may be prepared by applying wax and colour on pulps. Many

people earn their livelihood by picking waste papers, coloured cartoons and packing

boxes used for sweets. These waste papers can be used to make packing materials for

sending goods abroad .

Old batteries

You have seen batteries used in a torch light. When used up, we throw these batteries

away. Carefully take out the carbon rods from the batteries before throwing them. Be

careful to retain the copper caps on the rod. You can use these carbon rods as easily

available learning materials when you study about water analyzer, electric cells etc.

184 General Science

Discarded glass and plasticsGlass and plates occasionally break in every day use in a family . We can use these

broken glasses and plates for different purposes . When water glass is broken it may

be shaped by means of a glass cutter. It may be used as a pen holder. Pieces of glass

may be used on brick made boundary walls of houses. It can be used with mosaic on

the walls also.

Ball pen

After use when the ink of a ball pen is exhausted we throw it. Collect 10/12 numbers

of empty ball pens and a discarded

electric bulb. Now with the help of a nail

and hammer break the pitch coating of the

bulb. Take care so that the brass rim is

intact, Now push the nail deeper and by

revolving movement; break the inner

glass structure. Then bring out these glass

pieces and thin wires.

Now insert small pieces of ball pen into this empty glass bulb. Follow fig. 20.2 and

accordingly apply heat to the bulb by

means of a spirit lamp. After some time

the pieces of ball pen will start to melt.

Now add more pieces of ball pen if you

feel necessary. You will find that all the

pieces of ball pen have melted. When the

bulb is more than half filled with this

melted plastic; stop heating and allow the

bulb to cool down gradually. When the

bulb is perfectly cold give the bulb a mild

beating with the hammer. The bulb will

break into pieces and a beautiful semi spherical plastic paper weight will come out. In

this way, discarded materials can be used to make useable thing like a paper weight.

Waste organic mattersWe can utilize organic waste materials like peels of vegetables, onion, garlic, fruits,

vegetables etc. trashes of coconut and sugar cane, fish scales, animal viscera, bones,

egg shells, excrements of cattle and poultry and left over portions of food. Vegetable

peels and peels of fruits and vegetables are buried to make valuable organic fertilizer.

Dye can be prepared from peels of onion and Pome granate (Dalim), saw dust of

Jackfruit timber by boiling in water. Trashes of coconut and green coconut may be

Fig: 20.1: Removal of pitch from a discarded electric bulb

Fig: 20.2: Preparation of paper weight from discarded ball pen


used after drying in the sun for making ropes, mat, sofa and mattresses etc. You can

make different artistic items by using the shell of the coconut. Sugar cane trash can be

used as fuel. Paper pulps can also be prepared from this trash. Paper is being produced

from sugar cane trash in North Bengal Paper Mills located at Pakshi.

Egg shell is very rich in calcium content. Fish food may be produced by making

powders of egg shells. Fish scale is rich in phosphorus, while fish viscera contains

nitrogen. These wastes may be used as organic fertilizer in vegetables garden.

Dung of cattle specially of cow can be used for producing biogas. Excrement of

poultry is a valuable fish food. It is also used as organic fertilizer. Too much use of

chemical fertilizer is bad. It is one of the agents of water pollution. Use of too much

chemical fertilizer and insecticides are one of the causes of air and water pollution. It

disturbs natural balance. But when organic fertilizer or manures are used, there is no

bad effects; rather it helps to increase the yield of crops.

In our every day life we discard many things. With a bit of intelligent thinking we can

use these things for various purposes. Use of these things bring economic benefit to

us. On the other hand, the environment will also remain clean. To keep our

environment clean means make this beautiful earth more beautiful. Before discarding

a thing at your home or school you should think how can you use that. Then we will

have no want though we have shortage of wealth.

Exercise


1. In which paper mill, paper is made from sugar cane trash?

a. Khulna Newsprint Mill b. North Bengal Paper Mill

c. Chandraghona paper mill d. Patenga Paper Mill

2. From which waste raw material nitrogen is obtained?

a. egg shell b. fish scales

c. fish intestines d. waste bones

3. Which principle is followed to select raw materials for production of any goods from

waste raw materials?

a. raw material should be beautiful and costly

b. goods made of raw materials should be a standard one

c. goods should be made with low costing from raw materials of no cost

d. raw materials are to be purchased from the market

186 General Science

4. Bamboo, wood and sugar cane trash are usually used in-

i. producing thread and cloth

ii. Producing pulp and paper

iii. as construction materials


a. i b. ii

c. i and ii d. i, ii and iii

Creative Questions

1.

Figure

(Preparing necessary goods from waste raw materials)

a. What are the substances in figure called together?

b. Why are these harmful for environment?

c. How can .we make a useable thing by using used ball pen and bulb?

d. The re-use of the things of figure helps to keep environment clean. Explain.

2. Rafeza Begum keeps broken glass, waste vegetables, plastic bottle of water and

broken jar etc. in the yard of her house. Neighbours feel uneasy to walk for the

bad smell created from that.

a. What is waste material?

b. Identify the waste organic materials from the materials mentioned above.

c. How can Rafeza Begum make a pen pot using her waste materials?

d. Describe what Rafeza Begum should do to prevent the bad smell.

used bulb used pen broken glassused battery


Chapter - 21

TissueGeneral Characteristics and Functions of Cell and Tissue

CellEvery animal is different from one another. In spite of differences in size and look,

some of the basic functions are same in all animal cells; such as digestion, respiration,

movement, excretion, response to stimuli, growth, reproduction and death. Besides,

every animal has its own characteristic design; which is inherited from their parents.

To perform these functions different organs and structures are there. For example,

digestive system, lungs, heart, blood vessels, bone, muscles, kidney, brain, nerve,

reproductive system etc. These organs and structures are developed from different

types of cells. So, cell is called the structural unit of living body of organisms.

To build a brick house, bricks, cements and sands are needed. A building is

constructed by setting bricks one after another. In case of animals, body is built by a

large number of cells together. But all cells are not of same size. To perform different

functions, size and nature of cells differ from each other. For example, muscle cells

are elastic and flexible. But the cells forming bones are strong and nonflexible.

You should have more knowledge about a biological cell. Cells of different animals

are same but characteristic designs are different and unique for every species of

animal. Genes are arranged in a linear fashion on the chromosomes in a cells. These

genes bear the characteristics of the species.

In a normal cell a piece of protoplasm is surrounded by a cell membrane. It contains

cytoplasm and nucleus. Organelles like mitochondria, golgi complex, ribosome,

endoplasmic reticulum etc. are present in the cytoplasm. Chromosomes are borne inside

the nucleus. Genes are arranged in a row on the chromosome and bear the characteristics

of the species. For these reasons, children inherit characters of their parents.

From above discussion now you can realize the importance of cells in a living body.

Considering these characters, the modem definition of cell may be given as follows :

1. Structural and functional unit of organism is the cell.

2. Cytoplasm bounded by a membrane and controlled by nucleus, is called a cell.

3. A piece of protoplasm with all the characteristics for expression of life, is called a cell.

Tissue

When a group of cells in multicellular organisms perform a definite function: it is

called a tissue. Ultimate goal of these cells are same but they may vary in size and

shape. It depends on the nature of function of the cell and tissue. In short a tissue may

be defined as "A group of cells having same origin with similar or different size and

shape and perform a particular function, is called a tissue." Animal body is build up with

different types of tissues, such as a) Epithelial tissue b) Muscular tissue c) Connective

tissue d) Nerve tissue e) Reproductive tissue.

a) Epithelial tissue;The tissue that covers the exposed

parts of the body and remain as layers

on interior walls of different cavities

and canals are called epithelial tissue.

Outer covering of our skin and

internal lining of our mouth etc. are

composed of epithelial tissue.

Different glands are also composed of

epithelial tissue.

Characteristics of epithelial tissue

1. Cells of epithelial tissue are arranged in one or two layers.

2. Cells are arranged closely on basement membranes.

3. In this type of tissue there is no intercellular matrix.

Functions of epithelial tissue

1. Main function of this tissue is protection and covering. It protects internal and external

organs from injury.

2. Some cells of this tissue participate in secretion.

3. Taste, such as cover cells of tongue.

4. Excretion, such as cover cells of renal tubules.

b. Muscular tissue

Animal tissues having the ability of contraction and expansion are called muscular

tissues. Based on position, structure and function, they are of three types.

1. Voluntary or striated muscles

When contraction and expansions of

muscles depend on the will of the

organism; it is called voluntary or

striated muscles. They are connected

with bone, such as biceps muscle,

triceps muscle etc. the jaw moves

because of contraction and expansion

of these voluntary muscles.

Nucleus

Cell

Membrane

Fig:21.1: Epithelial tissue

Transverse strips

Muscle cellNucleus

Fig:21.2: Voluntary muscle

Tissue 189

Characteristics of voluntary or striated muscles

1. Voluntary muscles are elastic and have capacity of expansion.

2. The cells of these muscles have transverse black and white stripes (striated).

3. Cells of these muscles have more than one nucleus.

4. It becomes compressed when excited.

5. The cells of these muscles are covered with a layer, called circolemma.

Functions of voluntary muscles

1. It helps in movement of different organs and organelles.

2. It helps in movement of the animal.

Involuntary or smooth muscles

When contraction and expansion of muscles are not dependent on the will of the

organism is called involuntary or smooth muscles; such as muscles of stomach,

intestine, kidney etc.

Characterisics of involuntary muscles

1. Involuntary muscles do not have

transverse black and white stripes.

2. Each cell with only one nucleus.

3. There is no circolemma.

4. Each cell is long and spindle shaped.

5. These muscles show rhythmic move-

ment when contract and expand.

Function of involuntary muscles

1. Capacity of contraction is mild and

rhythmic.

2. Food move through digestive canal

due to contraction of intestinal muscles.

Cardiac muscles

It is a special type of involuntary muscle. The heart is built with this type of muscles.

Characterstics of cardiac muscles

1. Cardiac muscles are lightly striped.

2. These muscles are branched.

3. Muscles with thin circolemma

Muscle cell

(Muscie fibre)

Nucleus

Fig:21.2: Involuntary muscle

Fig:21.2: Cardiac muscle

Nucleus

Branch

Interealtate

190 General Science

4. Fibres of cardiac muscles are small and cylindrical.

5. Interconnected with each other by lateral branches.

Functions of cardiac muscles

1. Contraction and expansion of cardiac muscles cause circulation of blood through

the heart to the rest of the body.

2. To make the heart contract and expand in a rhythm.

Connective Tissue

This tissue connects different tissues and organs, such as blood, bone, cartilage, fat

cells etc.

Characteristics of connective tissues

1. This muscle cells originate from mesoderm of the embryo.

2. Cells of these muscles are not arranged in a definite layer.

3. These cells secrete large amount of noncellular matrix and stored in

intercellular spaces.

4. In this muscle, number of cells is less but amount of matrix is high.

5. Cells of this muscle is modified variously to perform different functions.

Functions of connective tissues

1. The matrix substance of bones is calcium. Bones make the structure, bear weight and

give rigidity of the body.

2. Tendon connects muscles with bones.

3. Fibrous connective tissue help in expansion and contraction of lungs and blood vessels.

4. Loose connective tissue work as filler in the gaps in between two organs.

5. Fat cells store fats.

6. Cartilages are strong but elastic.

7. Blood transports different substances (oxygen, food, nitrogenous wastes) from one

place to another. Beside, this blood resist microbes causing diseases. The matrix of

blood is a fluid.

Nerve tissue

Tissue that can respond to stimuli and produce sensation is called nerve tissue. The

unit of nerve tissue is nerve cell or neuron. The brain is made of neuron. Each neuron

is composed of three parts; such as-

a) Cell body

b) Dendron

c) Axon

Tissue 191

a) Cell body: It is the main portion of nerve cell. Its shape varies, such as elliptical,

star-like, round etc. Each cell contains one nucleus.

b) Dendron: The branches arise from all arround the cell body are called dendrons.

Branches from dendrone are called dendrites.

c) Axon: A long, unbranched and thread like structure arising from the cell body is

called axon. Its length may be from 1 mm to few meters. Axon means axis. Axon is

covered with a structure neurilemma.

The axon of neurone is connected with a branch of dendron. By interconnecting a

large number of neurons a nerve is formed. The connecting point of axon and dendron

is called synapse. Stimulus coming through this synapse to dendrone from adjacent

nerve cell.

Function of nerve cells

1) When responding to stimuli it sends impulses from different sense organs and

sensitive parts to different parts of the body.

2) Relevant parts of the body respond to this stimuli. For example, when mosquito

bites, the impulse is sent to the brain. The brain inform the hand and the hand try to

kill the mosquito.

3) It retains these impulses and the facts in the memory.

4) It co-ordinates among different physiologic activities of the body.

Exercise

Multiple Choice Questions.

1. Which type of muscle at biceps?

a. Epithelial b. Voluntary

c. Involuntary d. Cardiac

2. Which muscle cells contain more than one nucleus?

a. Voluntary b. Involuntary

c. Nerve d. Cardiac

3. Muscle of which organ is connected by the lateral branches?

a. Lung b. Liver

c. Heart d. Stomach

192 General Science

A

4. Whose covering is the neurilema?

a. Cell body b. Axon

c. Dendron d. Dendrite

5. Which one of the following diagrams contains muscular tissue?

a. Fig: A b. Fig: B

c. Fig: C d. Fig: A and Fig: B

6. The characteristics of connective tissue is-

i. It is originated from the embryonic mesoderm

ii. The cells of this tissue are regular, not arranged in definite layer.

iii. The quantity of matrix is more.


a. i b. ii


Answer the questions 7 and 8 from the following diagram-

Fig:

7. The part A of the diagram is

a. Intercalated b. Sarcolemma

c. Basement membrane d. Matrix

Fig: A Fig: B Fig: C

Tissue 193

8. One of the main functions of the tissue of diagram above is to-

i. help animal in locomotion

ii. help in excretion

iii. hill up the gaps between different organs


a. i b. ii


Creative Questions

1. While teaching the students about tissue in the class ,the Science Teacher said that

one kind of tissue is mainly responsible for the pain in skin from mosquito biting.

The cells of this tissue are made of three components. The three components,

combining one by one, carry sensation to the brain.

a. What is tissue?

b. Mention the name of three parts of the above cell and write which part receives

stimuli.

c. Explain what will be the problem if these cells of your skin do not work.

d. Out of the three parts of the above cell analyse that part which one is most

important to you.

2.

Fig: D

a. What is the name of the matrix of tissue of the fig: A?

b. Describe two main characteristics of the tissue of fig: B.

c. Explain similarities and dissimilarities of the tissues of fig: A and Fig: D

d. Fig: C is constructed by a special type of tissue. Discuss.

Fig: A Fig: B Fig: C

194 General Science

Chapter - 22

Human Body: Digestive and Excretory Systems

Different Systems of Human Body

All living things, either plants or animals, have a definite body structure. Body

structure of living things is initiated with cells. Animal body is formed by animal cells

and plant body by plant cells. Cells originating from the same source, with similar

nature and suitable for the same function is collectively called tissue. Again, when

some tissues collectively perform same function it is called an organ. Similarly,

several organs together form a body system. In other words, several organs

collectively performing same function is called body system.

To carry out the total biological functions of our body there are several body systems.

For example: (1) skeletal system (2) digestive system (3) respiratory system (4) blood

circulatory system (5) excretory system (6) nervous system (7) reproductive system

and (8) co-ordination and conduction system.

Digestive System

You all have seen running train. There is an engine in front of the train. This engine

pulls the train. The energy that the engine requires to pull the train is derived from the

burning of coal or diesel. Not only train, in fact all motorised vehicles like aeroplane,.

car and so on require petrol or diesel to run. Energy is stored in petrol, diesel or coal.

By the process of photosynthesis, plants convert the solar energy into chemical energy

and store it. The potential energy contained in petrol or coal is also indirectly derived

from solar energy through photosynthesis. This energy is used by engine.

Our body is made of innumerable cells. Just as engines require energy to work, so also

our body requires energy to keep alive and active. This energy is derived from food.

Food too contains energy derived from solar energy. The energy required for body's

different functions is derived from the potential energy stored in food. So, we have to

take food to meet the requirements for repair of wear and tear, growth, energy

production, protection from disease and so on. The body cells cannot directly take up

complex food. So, the complex food has to be broken down into simple form suitable

for cellular uptake.

The human body process by which food is converted into simple form suitable for

cellular absorption is called digestion. The system responsible for digestion of food is

called digestive system. The alimentary canal and digestive glands constitute the

digestive system.

Alimentary Canal

Our alimentary canal starts from the

buccal cavity and extends up to the

anus. This canal is formed of the

following parts. These are (1) mouth

aperture (2) buccal cavity (3) pharynx,

(4) oesophagus (5) stomach (6) small

intestine (7) large intestine and (8) anus.

(1) Mouth Aperture: It is the

beginning of the alimentary canal. It

is an opening used for taking in food

and is lined by the upper and lower

lips. The closing and opening of the

lips regulate intake of food.

(2) Buccal Cavity: The mouth is followed by the buccal cavity. The buccal cavity is bounded anteriorly by the two mandibles, which contain the teeth. It is bounded above by the palate and on the floor lies the tongue. Three pairs of salivary glands are situated on either side of the buccal cavity.

Teeth help in grinding of food into small pieces. While the food is in the mouth the tongue helps to recognise the taste and help in mastication. Human beings have thirty-two teeth, sixteen in each jaw. These teeth are of four kinds- biting tooth or incisor, tearing tooth or canine, anterior grinding tooth or premolar and grinding tooth or molar. Each kind of teeth has different function. Incisors break food into small pieces. The canines are used to cut and tear flesh. The premolar and molar teeth grind the food and make it soft. The grinding of food can be accomplihed by premolars alone. The absence of molar teeth does not make a difference in grinding. The molar teeth erupt much later than the other teeth. Third molar in each jaw is often called wisdom teeth.

(3) Pharynx: Pharynx is situation next to buccal cavity. Through the pharynx food passes into the oesophagus. There is no enzyme in this part of the alimentary canal and so no digestion takes place.

(4) Oesophagus: It extends from the pharynx above to the stomach below. It carries food from the pharynx to the stomach.

(5) Stomach: Stomach is situated between oesophagus and small intestine. The peristaltic movement of the pharynx and oesophagus conveys the slippery food to the stomach. The stomach is shaped like a sac. Its walls are thick. There is a muscular sphincter at each of the upper and lower ends of the stomach.

(6) Small Intestine: The small intestine commences from the stomach and is the

buccal cavity

Gall bladder

Oesophagus

Diaphragm

Stomach

Pancreas

Transverse colon

Descending colon

Ileum

Anus

Fig: 22.1 Human digestive system

Liver

Duodenum

Ascending colon

Caecum

Appendix

196 General Science

longest part of the alimentary canal. The small intestine is divided into three sections: (a) duodenum (b) jejunum and (c) ileum.

(a) Duodenum: The duodenum is the first part of the small intestine, that is, ihe part just after the stomach. It looks U-shaped. Bile from gall bladder and pancreatic juice from pancreas passes through a duct into the duodenum to mix with the food.

(b) Jejunum: This part lies between duodenum and ileum.

(c) Ileum: This is the terminal portion of small intestine. The absorbing organs in this part of the intestine look like fingers and are called villi. The villi help in the absorption of digested food.

(7) Large Intestine: This is the last part of the alimentary canal. It extends from ileum of the small intestine to anus. In length, it is shorter than small intestine but has a larger luminal diameter than the small intestine. The large intestine has three parts: (a) caecum (b) colon and ( c) rectum.

(a) Caecum: It is the first part of the large intestine, to which is attached a tubular sac called appendix.

(b) Colon: The colon extends from caecum to rectum. Colon has three sections-ascending colon, transverse colon and descending colon.

(c) Rectum: The rectum is the terminal portion of the large intestine. It resembles somewhat a sac. The waste products of digested food are stored here as stool.

(8) Anus: The terminal end of alimentary canal is the anus. Through this terminal opening, the alimentary canal is connected with the exterior. Faeces are passed out of

the body through anus.

Digestive Glands and their FunctionThe glands outside the alimentary canal that produce juice, which help in the digestion of food are called digestive glands. Salivary glands,1iver and pancreas are the digestive glands. Salivary glands secrete saliva that contains water, sodium bicarbonate and an enzyme salivary amylase. The water moistens the food, the bicarbonate neutralises the acidity of food and the ptyalin digests carbohydrates.

Bile secreted from the liver is stored in the gall bladder. Bile helps in the digestion of fats in the food. The enzymes trypsin, chymotrypsin, lipase and amylase are secreted by the pancreas. Trypsin and chymotrypsin digest protein, lipase digests fat and

amylase digests carbohydrates in food.

Fig: 22.2 Digestive glands

Pancreas

Duodenum

Gall bladder

Bile duct

StomachLiver

Human Body: Digestive and Excretory Systems 197

Digestive ProcessWhat is digestion?The form in which we eat food cannot be accepted by the body. Under the influence

of enzymes in the different parts of the alimentary canal, the food is broken down into

smaller, simple absorbable compounds. Protein type of food is broken down into

amino acids, carbohydrates into simple sugar (monosaccharides) and fat type of food

into fatty acids and glycerol, the forms in which food is absorbed. The process

through which these changes occur is the digestive process or digestion. Thus,

digestion may be defined, as a process whereby hard and complex food under the

influence of enzyme is broken down into simple absorbable form.

Digestion in the buccal cavityAfter the food is chewed in the buccal cavity the food is broken down into small

pieces. During this time saliva is mixed with the food. This lubricates the food and

helps in deglutition.

Saliva contains the enzyme ptyalin that partially breaks down carbohydrate into sugar

maltose. (Enzymes are protein like substance secreted by living cells that accelerate

the organic chemical reaction and remains unchanged at the end of the reaction.) After

this, the food passes from the buccal cavity into the stomach through the oesophagus.

Digestion in the stomachWhen the food reaches the stomach, the wall of the stomach secretes gastric juice.

The gastric juice contains the enzyme pepsin that digests the food proteins into

peptones. Besides, stomach secretes hydrochloric acid that destroys the disease

causing microbes in food and creates an acidic environment that accelerates the action

of pepsin on food. The stomach wall secretes plenty of mucin, which coats the inner

lining of stomach, protecting it from the harmful effects of the hydrochloric acid or

enzyme. By the light peristaltic movement of the muscles of stomach the food mixes

with the gastric juice and digestion begins. The food is formed into a pulp and passed

from the stomach to duodenum.

Digestion in the duodenumIn the duodenum, the food is mixed with bile and pancreatic juice. Through a

common bile duct the bile and pancreatic juice enter the duodenum.

Bile is greenish-yellow with a bitter taste. The function of bile is to break fat part of food

into small droplets. The carbonate in the bile helps to neutralise the acid from stomach.

The pancreatic juice contains four enzymes-trypsin, chymotrypsin, amylase and

lipase. The function of chymotrypsin and trypsin is to digest protein type of food, that

is, they convert the peptones formed in stomach into polypeptide and some amino

acids. Amylase converts the carbohydrate type of food into simple sugar or glucose.

Lipase breaks down fat type of food into fatty acids and glycerol.

198 General Science

Intestinal enzymes and their actions are given below:

Digestion of all food is completed within the duodenum. The digested food is absorbed in the subsequent parts of small intestine. There are many finger-like projections in the wall of small intestine. These are called villi (singular: villus). The villi contain many blood capillaries. They absorb the food, which is transported by veins to the liver. The liver sifts and sorts the different types of food and distributes it to the different parts of the body. These food products are used for repair of wear and tear, growth, heat production, reproduction and so on in the body. The liver also arranges to store the food that is not immediately required by the body.

Digestion in large intestineThe chief function of the large intestine is to absorb water from the food. This function is very essential, as water forms 90 percent of the constituents of each cell. In order to keep body loss of water to the minimum, the large intestine absorbs the water from the food.

The last part of large intestine, that is rectum, stores the undigested food material as stool pending defaecation.

Excretory SystemMany waste products are generated as a result of metabolism in the body. These waste products are harmful for the body. So, these have to be eliminated from the body. The sum total of chemical reactions that are on going in every cell of the body is metabolism.

Due to the metabolism of protein type of food, nitrogenous waste products like urea, uric acid are produced in the body. Again, carbon dioxide is produced due to metabolism of fat and carbohydrate type of food. In true sense, the elimination from the body of nitrogen-containing compounds generated from the metabolism of protein type of food is called excretion.

The organs that take part in excretion collectively form the excretory system.

KidneyKidney is the major organ of excretion. It resembles a bean. Kidneys are two in number. The kidneys are situated at the back of the body, one on either side of the vertebral column. Outer part of each kidney is called the cortex and the inner part is

ActionBreaks down protein into amino acids

Breaks down fat type of food into fatty acids and glycerol

Breaks down simple sugar into glucose

EnzymeErepsin (collective name for the enzymes acting in protein digestion Lipase

SaltaseSucrase Lactase


the medulla. The hollow part beneath the medulla is called the pelvis of kidney. A long narrow tube extends downwards from the pelvis called ureter. The ureters transport urine from the kidneys to the urinary bladder.

The waste product-carrying artery enters the kidney and breaks up into many branches. These branches end in a twisted coil of capillaries called glomerulus. Surrounding the glomerulus is a capsule. A tube is attached to the capsule. Glomerulus, glomerular capsule and the tube together are called nephron. The nephron is the functional unit of kidney. Each kidneycontains about 10 lakh nephrons. Behind the nephron is a convoluted tube that is surrounded by capillary network. The combined length of these convoluted tubes is about 200 miles. The blood is cleansed, as the waste products from the blood are filtered out in the nephron. The filtrate is passed into the nephron tube. During the first filtration, many essential elements are filtered out of blood along with the waste products. But as the filtrate passes through the nephron tube, these essential elements are again filtered and reabsorbed into the blood. After this second filtration, only the waste products remain in the tube dissolved in water. This forms the urine. Urea and

uric acid are the major nitrogenous waste products in human body.

Exercise

Multiple Choice Questions (MQ)

1. Which enzyme plays a role in digestion of protein?

a. Ptyalin b. Mucin

c. Pepsin d. Lipase

2. By the influence of which enzyme starch is converted to glucose?

a. Pepsin b. Trypsin

c. Lipase d. Amylase

3. Where are villi located?

a. Oesophagus b. Stomach

c. Small intestine d. Kidney

Fig: 22.3 Kidney

Ureter

Medulla

Cortex

Pelvis

200 General Science

4. Which one is the main excretory organ of human body?

a. Liver b. Lung

c. Skin d. Kidney

5. How many kidneys are there in human body?

a. One b. Two

c. Three d. Four

6. Liver is the human body's

i. largest digestive gland

ii. storehouse of gal

iii. producer of gal


a. i ii b. ii, iii


7. Enzyme is-

i. one type of protein

ii. if wish we can take it from outside

iii. secrets through the vessel


a. i b. ii


Answer the question number 8 and 9 as per the following diagram-

Fig:

8. The name of the label A of diagram is-

a. Cortex b. Medula

c. Pelvis d. Ureter

A


9. Organ of the diagram expels-

i only nitrogenous waste materials

ii. waste of blood through urine

iii. waste of blood by filtration


a. i ii b. i iii

c. ii, iii d. i, ii & iii

Creative Questions

1.

Fig:

a. What is the name of the organ labelled A?

b. What is the relation between the organs labelled A and B?

c. Explain what will be the problem if the organ labelled B does not work properly?

d. Discuss, the organ labelled A is a digestive gland and also the storehouse of food.

2.

Fig:

a. What is called the enzyme secreted from 'A'?

b. How does enzyme work?

c. Explain how does the organ B play the main role, though it does not secret

enzyme.

d. 'A' can be compared with a chemical industry. Discuss.

A

B

A

B

202 General Science

Chapter - 23

Population Growth and Environmental Pollution

Bangladesh is a very densely populated country. The country's population is

continuously increasing. The rapid increase in the population is giving rise to many

problems. In around 1941, the population of Bangladesh was more than 4 crore. In

around 1981, the population became 9 crore. According to the population census of

1991, the population of Bangladesh was about 11.15 crore, the population growth rate

was 2.17 and the density of population was 755 per square kilometre. According to

the population census of 2001, the population of Bangladesh is about 12.93 crore, the

population growth rate is 1.48 and the density of population 876 per square kilometre.

*With the increasing population, there is increased need of food, housing, health care,

education, transport and so on; as a result the standard of living is decreasing.

In a country or region, how the increased population pollutes the environment and what problems it creates are discussed here:

Environment PollutionOne of the bad sides of rapid population growth is environment pollution. For various

reasons the environment becomes harmful for survival and unfavourable for

habitation. Such an environment is called polluted environment. And the process by

which environment is spoiled is called environment pollution. The three major

components of environment are air, water and soil. The reasons and effect of pollution

of these components are discussed here.

Air Pollution

We all known that polluted air spread different types of disease causing microbes.

This air is polluted in various ways. Trash from household, excreta, rotten shallow

water body (doba) and stack of rubbish, disease causing microbe released by sneeze

and cough of patient, all pollute the air. Besides, air is also polluted by the industrial

wastes and the smoke of half burned fuel that is released into the air by the motorised

vehicles and industries and so on. The reasons for air pollution are related to the

increase in population in various ways.

When there is rapid increase in population of a region, it brings in its wake the need

for increased food, housing, transport, employment and other necessities. Wherever in

urban or rural area, too many houses if built in a limited area, increases the population

density of that area. Currently, many unplanned slums have developed in the urban

areas. In these places it is not possible to set up a well organised system for the

*Initial census report, August 2001, Bangladesh Bureau of Statistics

removal of waste and excreta. As a result, the waste and excreta collect there, rot and

spread foul smell and different types of poisonous gases are released. These gases

pollute the air and produce many disease producing organisms.

To provide livelihood for the increasing population and for the development of the

country, new industries are set up. Electricity generation plants have to be set up to

run the industries and also to provide comfort for the people. For transportation, the

number of motorised vehicles has to be increased too. Carbon dioxide and sulphur

dioxide gases are released with the smoke from industries and vehicles. These

poisonous gases also pollute the air. These gases are very harmful for health.

Fig: 23.1 Stack of rubbish and dirt pollute the air in city

Some vehicles release a lot of smoke. This black smoke contains half burned carbon

and other poisonous gases. Black smoke is produced from vehicles that are not

maintained properly. Smoke is also produced from the burning of fuel in brick-fields,

cooking oven and by burning of old tyre and dried herbs and shrubs. So far so, the

smoke from tobacco, biri and cigarette also pollute the air.

You know that oxygen is needed for our survival. It is possible to get more oxygen

from clean air. In order to keep the atmosphere clean and free from pollutants, besides

other measures, it is necessary to keep the population limited. If it is possible to

control the population growth, it is possible to limit the causes of air pollution.

Water Pollution

Water has multitude of use in the daily life. We use water for drinking, washing and

mopping. Plenty of water is needed in the factories to produce different items. Where

do we get this water? We get water from different sources as rain, spring, river, canals,

lake, beel, doba, pond and so on. In the rural areas, people collect water from such

water sources. For drinking water, in some villages there are dug-well, draw-well and

204 General Science

tube-well. But relative to the population, the source of pure drinking water is not

sufficient. In the urban areas, the main source of water is tap. The city authority

collects water from rivers or from underground water. The water so collected is

purified and supplied through pipes. In urban or rural areas as the population increases

the demand for water also increases. So in order to remove scarcity of water the

population in the area has to be kept limited.

Increased population requires increased amount of food. Irrigation and chemical

fertilizer have to be used to increase the production of food. To control insects in the

crop field different insecticides are used. A part of this insecticide gets mixed with the

rainwater and is washed into rivers and canals, beel, doba and ponds where they

pollute the water. As a result, fish and other aquatic animals die. Again, when the

content of fertiliser increases in the pond and doba, there is increased growth of algae

and other aquatic plants. When these plants die and rot in the pond water, then the

oxygen in the water is exhausted. As a result, fish die and the water becomes foul

smelling and polluted. Water is also polluted when bamboo, cane and jute are soaked

in it and when cow, buffalo, goat are bathed in the water.

Fig: 23.2 Fish are dying due to insecticide drained with rainwater into water bodies

When food scrap and rubbish, carcass of animal, perishable household trash and so on

rot then various types of disease causing microbes are formed. These rubbish and

disease causing microbes are transported by rain water, wind and other animals. In the

end, in some way or other these may pollute the river, canal, doba, pond, beel, draw-

well and tap water. Drinking of such contaminated water by human cause cholera,

diarrhoea, dysentery, typhoid and other diseases.

When population increases, the waste production also increases. In the absence of

proper disposal system, the waste accumulates, rots and pollutes water. Especially in

urban areas, water is polluted if there is any interruption in the sewerage system. Under

such circumstance, it becomes difficult to supply pure drinking water in urban areas.

Population Growth and Environment at Environmental Pollution 205

Disposal of excreta near water bodies, wells and tube-wells may pollute water. The

dirty water from outside may seep into the wells and pollute the water. Excreta of

animals and garbage may fall in the well and pollute the water too.

It has been mentioned earlier that industries have to be set up to provide employment

for the increasing population and for development. The wastes form these industries

pollute pond, rivers, canals and other water bodies.

Fig: 23.3 Industrial waste is polluting water

In the country, different industries like textile, jute, paper, chemical, fertilizer, leather

and tannery are usually situated beside the rivers. Wastes from these industries pollute

the river water. This polluted water may be the cause of our death. So, it is extremely

essential for us to keep the water pure. To a great extent, human beings are responsible

for pollution of water.

If population increases, the chances of water pollution increases. So, if the population

can be kept within limits, it may be possible to keep the water pure and free of filth.

Soil Pollution

Usually, all the reasons that pollute water may also pollute the soil. The nature of soil

is such that with the help of bacteria it can decompose the wastes and mix it with soil.

But when the production of rubbish and waste exceed the soil's capacity to handle it,

the organic waste and acid released make the soil acidic. These days, polythene bags

and plastic goods are being increasingly used in Bangladesh. These are made of

chemical products that do not rot in the soil. As a result, pollution of soil may lead to

loss of its fertility. Excessive use of fertilizer and insecticide are the major reasons for

soil pollution.

We are especially dependent on plants and crops. If soil is polluted, the trees do not

grow well, crops production is hampered. So, it is earnestly necessary for us to keep

the soil free from pollution.

206 General Science

Air, water and soil of the environment are essential for our survival. It is the

responsibility of every one of us to appropriately preserve and keep pollution free

these valuable resources. The following steps must be taken to reduce pollution of

environment.

(1) Ignorance about pollution and lack of responsibility are the main reasons behind

environment pollution. So, the people need to be educated and made aware of the

importance of keeping the population limited.

(2) Open-air defecation here and there should be avoided and sanitary latrine used.

(3) Household waste and food scraps should not be strewn but collected and sanitary

disposal of the same arranged by burying.

(4) The industries should be planned in such a way that environment pollution is

minimum. Before the smoke from industries is released in air, it has to be made

pollution free by chemical process using modern technology. Laws concerning air

pollution have to be appropriately implemented.

(5) Instead of polythene, the use of jute products have to be increased.

(6) The engines of road and water vehicles should be kept in good working condition

such that they do not pollute the air and water by toxic gas and oil.

(7) Chemical fertilizer and insecticide are to be used appropriately and adequately.

(8) Smoking has to be stopped and people made aware against smoking.

(9) Above all, people need to be made aware about keeping the population limited.

Dense Population and Unhygienic Environment

In our country there are no big cities like those in developed countries. Except for

Dhaka, Chittagong, Rajshahi, Khulna and Narayanganj, the other cities are not so big

yet. Because of the increase in population, these days there is scarcity of opportunity

of livelihood and living in rural areas. Many people are converging in these cities in

search of work. As a result, the problem of accommodation in cities is getting acute

by the day. Because of scarcity of accommodation, people are making houses here

and there in unplanned way. As a result, dirty slums are growing in every nook and

corner of the city. People live close together in these slums. The necessary air and

light can not enter these houses. The water supply and sewerage system is almost non-

existent is these slums. So, people dispose waste and excreta here and there. Because

of the mismanaged waste disposal system, the rubbish and excreta ctfllect in places

and rot. Monsoon water, wind and animals carry this rotten rubbish in various ways

such that they mix with water to pollute it. Due to use of this polluted water, the slum

people are afflicted with various water-borne diseases.


Fig: 23.4 A dirty slum in city

The rapid increase of population in the urban areas on one hand is creating unhygienic

environment and on the other hand is giving rise to various social problems.

Especially, providing employment is a great problem. The curse of unemployment

makes the youth disappointed and unsettled.

Only healthy, sound and educated .population can contribute significantly to the

development of a country. Balanced diet and pure water supply, hygienic excreta

disposal system, good management of education and health and recreation

opportunity all contribute to people being healthy and able workforce. Due to excess

population compared to scarce resource, it is not being possible to adopt effective

measures to provide these opportunities in our country. Everybody has to be aware of

the fact that population should not grow in unplanned way.

Lots of problems arise if in a country the population increases without corresponding

increase in resource. In order to understand these problems of population and to take

corrective measures, we must know well about the population in different ways. We

need to gain knowledge so as to know "what is demography"? what are the reasons for

increasing population and so on. Some common topics of demography are given

below in simple language.

Concept on Demography

The population increase in any country depends on three factors-birth rate, death rate

and migration. To clearly understand the information and problems related to

population, we need to have an understanding of demography. Scope of demography is

the review and analyses of population structure, density, birth rate, death rate and so on.

208 General Science

Reason for Increase in Population

Birth rate and death rate: You may have noted that in any area, on one hand birth of

babies occur and on the other hand people of different ages die. The birth of a baby

implies increase in population and on the other hand death of a person implies

population decrease.

If in a place in one year the number of babies born is the same as the number of death

of people, then there will be no increase in population in that place. But if the number

of death is less than the number of birth, then the population of the place will increase.

This increase or decrease of population depends on two other factors too. These are

immigration and emigration.

The population of a country decreases due to emigration. Again, due to people coming

into the country the population increases. In our country, the usual reason of

population increase is the difference between birth rate and death rate. With the help

of demographic formulae how crude birth rate, crude death rate, natural growth rate,

dependency ratio and so on can be calculated are described below.

Crude birth rate: The crude birth rate of a country is the number of live births per

1000 population of the country in that year.

Formula:

Example: Let us consider that in one year in a village 96 babies were born alive. The

mid-year population in that village was 2000. What is the crude birth rate for that village?

Then according to the formula,

Crude death rate: The crude death rate of a country is the number of death of all

ages per 1000 population of the country in that year

Formula:

Crude birth rate = X 1000 Number of live births during a year

Total mid-year population

Crude birth rate = X 1000 Number of death of all ages during a year


Crude birth rate X 1000=96

2000

=962

= 48 persons (per thousand)


Example: Let us consider that in a village the number of death in a year is 48 and the

mid-year population was 2000. Then what is the crude death rate for the village?

According to the formula,

Natural Growth Rate of Population

When crude death rate is subtracted from crude birth rate, the net residue will give the

natural growth rate of the population. This increase is usually expressed as the

population's annual growth in percentage. So, in order to express this in percentage,

the rate has to be divided by 10. (That is, it is expressed per 100 or in percentage

when 1000 is divided by 10.)

*The current natural growth rate in Bangladesh is 1.48.

Formula:

Considering the above two examples, the crude birth rate is 48 person and the crude

death rate is 24 person. Therefore, the natural growth rate for the village is

Dependency ratio

Generally, children less than 15 years cannot earn. Children of this age are considered

dependent on others In the same way, persons above 64 years are considered

dependent, for at this age people are usually not capable of earning or their ability to

work decreases. People between 15-64 are considered as productive workforce.

Children under 15 and adults above 64 years are considered dependent on the

workforce. The following formula can be used to calculate the dependency ratio of a

country or region.

Formula

Dependency ratio = X 1000Children below 15 years + adults above 64 years

Population between 15 - 65 years

Natural growth rate =Crude birth rate - Crude death rate

10

Crude death rate X 1000=48

2000

=482

= 48 persons (per thousand)

48 - 24

10

24

10= 2.4==

210 General Science

Example: In a country, children below 15 years are 43 percent of the total population

and adults above 64 years are 3 percent. What is the dependency ratio of the country?

Solution: Population of productive workforce = 100 - (43 + 3) = 100 - 46 = 54

According to the formula,

This is expressed in percentage to avoid the fraction. This means that 85 persons are

dependent on 100 persons of productive workforce

Life Expectancy

After a child is born, we cannot say how long the child will live or what will be the

child's life span. But we can make an expectation of the child's life span. If the

number of deaths in a year in a place is known and if their life span is known, then the

average number of years survive by these people can be calculated. This average

number of years survived is the life expectancy for the people at that time in that

country. This life expectancy fluctuates. Life expectancy increases with improved

standard of living. And keeping the population within limits is the prerequisite for

improved standard of living in a country. The average life expectancy of male and

female in Bangladesh is 60.8 years:

Summary

Environmental pollution: We live in an environment. Release of any unwanted or

harmful thing into the environment pollutes it. The process by which we harm the

environment is environment pollution.

Air pollution: Air pollution occurs by black smoke from industries and vehicles,

waste products, trash from household, excreta, sewage, rotten shallow water body

(doba), cough and sneeze from patient and so on. In order to meet the demands of the

increasing population, air pollution is increasing.

Water pollution: Because of increasing population increased amount of food

production is needed. So land has to be irrigated and fertilizer and insecticides and so

on have to be used. The use of these is the important reason for water pollution.

Throwing of dirt, rubbish, carcass, and so on in water also causes water pollution.

Soil pollution: Soil pollution occurs due to the same reasons as water pollution.

Throwing of excess rubbish, waste product, polythene and plastic products and use of

excess fertiliser and insecticide are the important causes of soil pollution.

Dependency ratio X 1000=43+3

54

=4654

= 85 persons (approximately)

* Initial Census Report, August 2001, BBS

* Bangladesh at a Glance, May 2001, BBS


Unhygienic environment: Because of the rapid increase in population, people make

houses here and there in unplanned way and dirty slums crop up. People live close

together in these places. These slums do not get the necessary air and light; water

supply and sewerage systems are absent. Such an environment is called unhygienic

environment.

Demography: The subject that deals with population structure, density, birth rate,

death rate and so on is commonly called demography.

Reason for increase and decrease in population: The population increase and

decrease in any country or region depends on three factorsbirth rate, death rate

and migration.

Crude birth rate: The crude birth rate of a country is the number of live births per

1000 population of the country in that year.

Crude death rate: The crude death rate of a country is the number of death of all

ages per 1000 population of the country in that year.

Natural growth rate: When crude death rate is subtracted from crude birth rate, the

net residue will give the natural growth rate of the population.

Life expectancy: If the number of deaths in a year in a place is known and if their life

span is known, then the average number of years survived by these people can be

calculated. This average number of years survived is the life expectancy for the people

at that time in that country .

Exercise


1. Which transport does not pollute the air?

a. Motor cars b. Bicycle

c. Ships d. Aeroplane

2. What is the main cause of population growth in our country?

a. Gap in birth and death rates b. Emigration

c. Only increase in birth rate d. Immigration only

3. The main cause of water pollution is-

i. Chemical fertilizer

ii. Organic fertilizer

iii. Insecticide

212 General Science


a. i b. ii

c. i, ii d. i, ii & iii

4.


a. Crude death rate b. Crude birth rate

c. Normal growth rate d. Life expectancy

Creative Questions

1. Reaching Sadarghat, Dhaka for the first time by launch, Zarina noticed that the

water of the river Buriganga is fetid and black. Then she started for the house of

her relative at Azimpur by rickshaw. After a certain distance she saw that the

rickshaw was not moving. There was a big jam of vehicle in the front. A number

of buses and trucks were blowing horns loudly and emitting black smoke. Zarina

felt one type of irritation in her eyes and nose. She also noticed that the dust bin

was overloaded with rubbish by the road. Dogs and crows were pulling away the

rubbish. Flies were flying and bad smell was spreading.

a. What are the causes of water becoming black and fetid?

b. Why did Zarina feel irritation in her eyes and nose?

c. Give your opinion about the environment of Dhaka as per the scenario noticed

by the Zarina.

d. Describe the relation between the overpopulation of Dhaka and the problem

mentioned above.

2. Rahima and Kamal, coming from a village, live by the side of the rail line at

Kamlapur in Dhaka. Their surrounding environment is dirty, crowded and damped

with insufficient light and air. A baby of them died instantly after it was born. They

often catch cold and suffer from fever and stomach diseases. Now their working

ability is decreasing for different diseases and they have many problems.

a. What is called the place where Rahima lives?

b. Why do the family members of Rahima and Kamal often suffer from different

diseases?

c. Explain how the family of Rahima Begum would get rid of this situation?

d. Discuss the causes of Rahima to live in Dhaka in this way.

X 100 The number of death of all ages in a year



Chapter - 24

Flood, River Erosion andDraught in Bangladesh

Geographical LocationBangladesh is situated near the northern part of the torrid zone between 200 34' and

260 38' north latitudes and 880 01' and 920 41' east longitudes. It is one of the biggest

delta regions of the world. This huge delta is formed by the alluvial soil drained by the

Padma, the Jamuna and the Meghna. The area of Bangladesh is 1,47,570 square

kilometres. To the north and north-east of this country lies some hills that are not very

high and next to these are the provinces of India. There are the high Himalayas at a

little further to this. To the north-east are the Garo, the Khashia and the Joyantia

mountains ranges. To the east and south-east is the Arakan Yama. The Bay of Bengal

in the south extends southwards to merge with the Indian Ocean. On the west lies the

extensive plain land of India. Because of such location, the winter is not long lasting

in Bangladesh. The intensity of cold is also less. There is plenty of rainfall during the

monsoon. The Himalayas protects Bangladesh from the cold waves flowing from

Siberia. During the monsoon, air filled with moisture flows from the south of the Bay

of Bengal. This air mixed with moisture and clouds are obstructed by the Himalayas,

the Arakan Yama, the Garo, the Khashia and the Joyantia group of mountains,

resulting in abundant rainfall in Bangladesh.

Natural CharacteristicsBesides the geographical location, Bangladesh is endowed with some other unusual

natural characteristics. For this reason, dreadful tidal wave and flood occur frequently

in Bangladesh. The reasons for tidal wave and flood are as follows:

a) The concave coastal belt along the Bay of Bengal

b) To the south of the coastal belt lies the stepladder-wise extensive slope of the sea bed

c) Greater beight of waves during high tide in the coastal area of Chittagong as compared

to other places

d) The gradual slope of the plain land from north to the southern coast

e) The continuous siltation in the river bed due to the alluvial soil carried by the river

t) Elevation of seawater level along the coastal belt under the influence of monsoon winds

Climate

Located in the torrid zone with the distinctive configuration of land, the climate of

Bangladesh is mainly equable. Because of this equable of climate, the weather

prevalent in Bangladesh is both favourable and unfavourable. The favourable weather

keeps the nature of Bangladesh evergreen with abounding crop.

The unfavourable weather brings sudden calamity every year. The unfavourable

weather is manifested as sea cyclone and tidal wave and flood, draught, kalboishakhi

tornado and excess rainfall.

According to meteorologists, four major seasons prevail in Bangladesh. These are pre-

monsoon (March, April, May), monsoon (June, July, August, September), post-

monsoon (October, November) and winter (December, January, February). These four

seasons have great influence over the weather in Bangladesh. Seasonal influence

along with overall long-term weather forms the overall climate of this country. So, the

climate of Bangladesh can be called the climate of tropical seasons.

The annual rainfall in all parts of Bangladesh is not the same. There is a statistical

data for the average rainfall over the last thirty years. According to the statistics,

Sylhet has the highest average annual rainfall. The avarage rainfall is 4500 millimetre.

The second highest rainfall recorded at Cox's Bazar and Chittagong areas. The

average annual rainfall is 3000 millimetre. The areas with lowest annual rainfall are

Rajshahi, Pabna and Kushtia. In these areas the annual average rainfall is little over

1400 millimetre. The average annual rainfall in Dhaka and the adjoining areas is about

2000 millimetre.

In Bangladesh, the temperature in all seasons at all places is not the same. The

temperature is the highest in the pre-monsoon and the lowest in winter. The highest

average temperature during pre-monsoon (summer) in the northern and north-western

regions of the country rises up to 440-450 and in the rest of the regions it rises up to

410-420 Celsius. On 18 May 1972, the highest temperature at Rajshahi rose up to 45.10

Celsius. The winter temperature in the northern and central regions of Bangladesh is

180-210 and sometimes falls as low as 40-50 Celsius. On 4 February 1968, the lowest

temperature of the country fell at 2.50 Celsius at Srimongol.

FloodWhat is flood: According to the hydrologists, flow of water more than the drainage

capacity through the outlet is flood. During monsoon, there is excess rainfall in the

upstream and within Bangladesh. When there is excess rainfall, there is tremendous

increase in the pressure of water in the basins of major rivers and their tributaries. As

a result, the flow of water in the rivers goes beyond their holding capacity. The

country is then flooded.

Types of flood: Four types of flood occur in Bangladesh. These are as follows:

(1) Flash flood that occurs in the mountainous rivers in the north-eastern and eastern

regions of the country,

(2) Flood that occur due to obstruction to outflow of excess water following excess rainfall,

Flood, River Erosion and Draught in Bangladesh 215

(3) Flood in the major rivers due to excess rain in the monsoon and

(4) Flood due to cyclonic tidal surge in the coastal and estuary regions.

Causes of flood: A total of 230 rivers including the Padma, the Jamuna and the

Meghna rivers, flow through Bangladesh. Among these rivers, excess rainfall in the

basins of the Padma, the Jamuna and the Meghna is the main reason for flood in the

country. Of the annual rainfall over the country, 80 percent occur during the four

months of June-September. The area of the drainage basin of these three rivers is

16,00,000 square kilometres altogether of which only 7.5 percent area is in

Bangladesh. The monsoon water of this large basin flows through Bangladesh to the

Bay of Bengal. So, even if there is less rainfall during the monsoon in Bangladesh but

excess rainfall elsewhere in the river basin causes flood in Bangladesh. This happened

in the flood of 1998. Flood in Bangladesh may take the form of moderate, terrible or

colossal intensity. The usual cycle of occurrence of the moderate, terrible and colossal

floods are 3-4, 6-7 and 30-35 years respectively.

In addition to the excess rainfall in the basin, there are some other factors that help

increase the height of floodwater. These factors are:

(1) Increase in water due to melting of snow under the influence of monsoon rains in

the Himalayas,

(2) The depth of rivers decrease due to siltation of the alluvial soil carried by rivers

each year,

(3) Deforestation in the hilly areas on the other side of the country's geographical

borders,

(4) Construction of roads, highways, railways, dams, embankments and so on,

obstruct the natural flow of water. This obstruction to flow, together with the gradually

changing south-westerly seasonal winds, increase the average a natural height of

water near the coastal belt along the Bay of Bengal.

Occurrence of colossal flood: In Bangladesh, colossal flood occurred only few years

back in the two consecutive years of 1987 and 1988. The flood of 1988 was the most

colossal one in the history. This flood started from about the end of August and lasted

till the middle of September. The flood covered 1,20,973 square kilometre area in 60

districts. The number of affected people were 4 crore 67 lakh and casualty was

2,329 persons.

In 1998, the most terrible flood of the century occurred in Bangladesh. This flood

lasted for about three months and inundated two-third area of the country. This was

the longest lasting flood of the century. The height of water level in the big rivers

surpassed all the previous records. In this terrible flood, more than one thousand

216 General Science

people died and there was extensive damage to houses, roads, bridges and culverts,

crops, farming and other properties. Recalling the past, this dreadful flood was a

strong blow to the overall economy and infrastructure of the country.

Probable solution of the flood problem: Because of the geographical location, each

year about 35 percent of the area of Bangladesh is inundated by normal flood. The

people of this country are living with flood since long.

In this stage of development, it is not possible to stop or totally control flood. But we

can take short and medium term preparations. As a result, life and property and the

natural environment can be saved to a great extent from being damaged.

The measures are mentioned below:

a) Strengthening institutional infrastructure:

- To strengthen the institutes that identify disaster and give early warning,

- Improving the data collection methods,

- Develop dependable modern data-bank.

b) Undertake structural strategy to control flood:

- As a part of the overall flood control programme, construction of more than one

water reservoir,

- Increase the depth of rivers by dredging or digging to increase water holding capacity

of the major rivers,

- Construction of embankment.

Measures to combat flood: To combat flood, the following measures have to be

taken in pre-flood, flood and post-flood periods to reduce to a great extent the amount

of damage due to flood.

Pre-flood perioda) Construction of houses on high lands,

b) Not to construct houses along the river sides outside the protection embankment,

c) Plant more trees around the houses to reduce loss due to flow of flood water,

d) Surround the mud-made houses with closely placed bamboo sticks or firm wood-

made enclosures,

e) Install of tube-wells in a place that may not be submerged by flood water,

f) Make beforehand arrangement of where to shift people, cattle head or valuables in

case of flood,

g) Store at home some amount of puffed rice, flattened rice, molasses and other dry

food during flood prone months,


h) Make the boat useable, if present,

i) Maintain liaison with local Disaster Management Committee/ volunteers.

During flood

a) Take every step after lot of thought and deliberation without losing patience,

b) If house is submerged in water, then take shelter in nearby high place/ embankment/

shelter centre,

c) Before moving for shelter the important items should be wrapped with polythene

and secured in the deck under the roof,

d) In case a boat is not at hand for use in flood, prepare a raft with banana tree,

e) Drink tube-well water or boiled water. If this is not possible, then use water purifying

tablets or alum,

1) Carefully keep the bottle of carbolic acid with the lid open so that snakes do not

enter the house during flood,

g) Pay special attention towards children who do not know how to swim,

h) Take vaccination against diseases before their outbreak during floods,

i) For the security of flood affected village, form volunteer force in liaison with local

Disaster Management Committee.

Post-disaster period-

a) As soon as flood water recedes, return to homestead and make it habitable,

b) Take initiatives to make own land cultivable,

c) Soon after flood, take vaccination for prevention of disease before being affected

by disease,

d) Contact voluntary organisation for help in rebuilding the houses.

River Erosion

The role of river in the life of the people in Bangladesh is unlimited. For centuries, life

and living of the people has revolved around river for fishing, farming, transportation

and business. As a result, human habitation has developed much along the riverbanks

compared to other places.

Just as rivers have been of great benefit to people over ages, in the same way the

number of problems originating from these is no less. For Bangladesh, river erosion is

a great problem. At least 70 lakh people have been affected by river erosion during the

period 1970-1990. River erosion occurs at about 130 places of riverbanks in

218 General Science

Bangladesh. Several lakh people are affected each year by it. Their normal life is

severely interrupted.

Causes and places of river erosion: Bangladesh is formed of alluvial soil. So, the

flow of river regulates the process of river erosion. River erosion occurs regularly in

75 rivers including the Padma, the Jamuna and the Meghna. The villages, market

places and cities by the banks of rivers are always threatened by river erosion.

Maximum erosion occurs along the banks of the Jamuna and the Meghna. The gradual

erosion of the Jamuna and the Meghna has put the districts of Sirajganj and Chandpur

respectively at risk. In spite of throwing big boulders and concrete blocks in the rivers,

it is becoming increasingly difficult to save the two towns. Research shows that

erosion occurs more in wide rivers. Figures from 1983-84, 1984-85 and 1985-86 fiscal

years show that of the total 462 upazilas 94, 85 and 55 upazilas respectively suffered

loss due to river erosion. The affected districts are Barisal, Bhola, Jhalokathi,

Chittagong, Cox's Bazaar, Bagerhat, Chapainawabganj, Bogra, Jamalpur, Tangail,

Faridpur, Madaripur, Laxmipur, Rangpur, Gaibandha, Kurigram, Pabna, Sirajganj,

Manikganj and Chandpur.

Measures to prevent erosion: River erosion can be prevented by different measures.

The measures are:

a) Constructing embankment along the riverbanks,

b) Throwing large number of big boulders and concrete blocks along the riverbanks,

c) Planting large number of trees along the riverbanks.

Draught

Draught is a condition that is said to occur when there is excessive increase in the

demand of water, long term scarcity of water or when there is no source for supply of

necessary water. Usually, by draught is meant a state where there is scarcity of

humidity in the atmosphere due to natural or artificial causes leading to a weather

condition in which there is no rainfall over a prolonged time.

Causes and nature of draught: In Bangladesh, draught is seen to occur before and after

the seasonal winds, that is, in the pre and post-monsoon periods when there is no rainfall

for a prolonged time. The influence of high pressure annulus in the torrid zone is

considered to be the main cause of draught in this country. Bangladesh falls in the grip of

draught in the year when this high pressure annulus is intense and its influence extends

over Bangladesh and adjoining areas up to the Bay of Bengal for a prolonged time.


In Bangladesh, the draught is usually of short and medium duration. This type of

draught mainly disrupts agricultural activities. When the weekly rainfall is 25-50

percent less than the normal, the draught is of medium intensity and when the weekly

rainfall is less than the normal by more than 50 percent, it is termed as severe draught.

Effects of draught: In Bangladesh, the effects of draught become severe in some

years. Land, canal, beel, pond, nala, etc. all dry up. As a result, agricultural activity is

threatened and there is severe shortage of food. In order to provide food to the huge

population, the Government has to import food from the development budget on

emergency basis. There is possibility of famine if for some reason a delay occurs in

the import of food.

Like many other countries of the world, draught is not long lasting in Bangladesh. In

the country, draught is irregular and non-periodic. The whole of Bangladesh is not

affected by draught at the same time. In the great draught that occurred in Bangladesh

in 1957 and 1979, the country's 46.54 percent area and 53.03 percent people and

42.04 percent area and 43.90 percent people respectively were affected.

Measure for prevention of draught: To prevent draught, long term and immediate

plans need to be taken at the state, regional and village levels. The measures to be

included in the workplan are as follows:

a) Extensive aforestation,

b) Stopping deforestation,

c) Excavation of dead ponds and storage of adequate water,

d) A void wastage of water

Exercise


1. What is flood?

a. Flow of water from melted ice

b. Movement of huge clouds in the monsoon sky

c. Flow of water beyond the holding capacity of river

d. Excess rainfall

2. River erosion is much at the bank of Jamuna, because-

i. continuous silting in the bottom of the river

ii. depth of this river is more

iii. this river is more wider

220 General Science


a. i b. ii

c. iii d . i, ii & iii

3. The main cause of drought in Bangladesh is considered as-

i. extended continental shelf at the costal area of southern part

ii. the effect of tropical high pressure

iii. extensive use of vestige fuel


a. i b. ii

c. i ii d. ii, iii

Creative Questions

1. Bangladesh is a land of rivers. A total of 230 rivers including the Padma, the

Meghna, the Jamuna are flowing over this country. The effect of these rivers on

the socio-economic condition and natural environment of Bangladesh is very

important. In rainy season, though the water carried by these rivers causes flood,

flood becomes dangerous and destructive in nature in some years. As a result we

face a great loss.

a. What is flood?

b. Explain one of the main causes of flood in Bangladesh.

c. Make a list of the loses in your locality caused by recent devastating flood.

d. Describe the effect of rivers in Bangladesh.

2. Jamal's house is at a village by the Padma at Harirampur thana in the district of

Manikganj. He cannot pay his school fees as the financial condition of his family

is very poor. But he has heard from his father that their economic condition was

not so. Just before ten to twelve years, his father was a rich farmer. His father had

enough cultivatable land and he grew a lot of crops there . But now they have no

land. The cause of it is river erosion.

a. How many rivers in Bangladesh erode regularly?

b. Explain a cause of river erosion.

c. Describe why river erosion is responsible for the present condition of Jamal?

d. Write down the demerits of flood?


Chapter - 25

AIDSAIDS is a deadly disease in the present world. The entire human civilization is facing

a severe threat because of this deadly disease. Under the geographic and socio-

economic conditions of Bangladesh, there is a high probability that AIDS may spread

in epidemic form in our country too. That is why all of us need to know about AIDS.

The word AIDS is an acronym of the words Acquired Immune Deficiency Syndrome.

It is a kind of viral disease. The name of this virus is HIV; the full name is Human

Immunodeficiency Virus.

DIV Stands for:

H Human Human being

I Immunodeficiency Deficiency in the body's capacity to prevent diseases.

V Virus Virus

That is, the virus that reduces the capacity of the human body to prevent diseases is HIV.

AIDS Stands for:

A Acquired Acquired from others

I Immune Capacity of the body to prevent diseases.

D Deficiency Shortage/lack of

S Syndrome A symptom of a disease or a set of such symptoms.

In other words, AIDS is such a state of the body of an HIV-infected person when the

immune system is gradually reduced and at one stage it is totally damaged.

HIV is a very small, special type of virus that causes AIDS in human beings. After

entering into human body through several ways, this virus slowly destroys the original

immune system of the body. At one stage, the immunity of the infected person is

reduced too much. This last stage of HIV infection is AIDS. Because of reducing

immune capacity of the body, other diseases very easily attack an AIDS patient. No

effective antidote against AIDS has yet been discovered. So, death is the ultimate

consequence of an AIDS patient. That is why AIDS is called a killer disease or a

deadly disease.

HIV can be transmitted into human body through several ways. For example:

I) Through receiving HIV infected blood;

2) Using infected needle and syringe, infected instruments in operation or through

transplantation of an infected organ (kidney, cornea etc);

3) Through breastfeeding of HIV infected mother.

HIV does not spread through air, water, food or normal or causal touch. In order to

provide a clear idea, the ways through which HIV does not spread, are mentioned below:

I) Taking care of an HIV infected person or an AIDS patient;

2) Living in the same room;

3) Using bed of the AIDS patient;

4) From sneezing or coughing;

5) Using her/his cloth;

6) By touching;

7) Shaking hands, embracing each other or playing together;

8) Sharing the same toilet/urinal used by AIDS patient;

9) Using same dish or plate;

10) Sharing food with an AIDS patient and bathing in the same pond or bathroom;

11) If mosquitoes or other insects bite.

The ways through which HIV does not spread have been pictorially shown below:

Though AIDS is not yet alarming in our country, experts believe that for various

reasons it may soon take the form of an epidemic. In Bangladesh already some people

have died of this disease. For preventing the spread of AIDS, development of self-

consciousness is essential.

This self-consciousness means-

- Appropriate knowledge and awareness about

one's own body

- Appropriate knowledge and awareness about

HIV/AIDS

- Awareness about one's own right (social and economic)

- Appropriate knowledge about morality and values

Learners' task:

Inform (at least five) brothers-sisters!

relatives/ neighbours of your age about

the issue of HIV/AIDS. Discuss with

them about any aspect of self-

consciousness. Listen what they say, write

a report on the subject and present it in

the classroom.


- Awareness about one's own strengths and weaknesses

- Clear idea about what is good and bad for oneself

- Consciousness in selecting right friend/ good friend.

If we raise consciousness on the above subjects, we would be saved from being

attacked by AIDS.

We will also inform others about this. A beautiful society will develop if we all

become conscious.

We need to know further that the country can be substantially saved from the AIDS

epidemic through adherence to religious and social commandment and prohibition.

ExerciseMultiple Choice Questions1. Which one is the elaboration of AIDS?

a. Acquired Immune Deficiency Syndrome

b. Acquired Deficiency Syndrome

c. Acquired Immuno Deficiency Symptom

d. Acquired Immuno Deficit Syndrome

2. Elaboration of HIV-

a. Human Influenza Virus

h. Human Immunodeficit Virus

c. Human Induced Virus

d. Human Immunodeficiency Virus

3. HIV/AIDS spreads-

a. through air

b. by using the bed of AIDS patient

c. through food

d. by receiving HIV contaminated blood

Creative Questions1. Rima is 16 years old. A few years ago, she got an accident and for her treatment

she was unconsciously injected blood from an unknown man. Recently she often

feels sick. She always suffers from fever and loose motion. Her weight is

reducing. Doctor told her for blood test. Examining the report Doctor told Rima's

mother that her daughter is found to be HIV positive and he advised her to get

admitted into hospital quickly.

a. What is HIV?

b. Describe the main characteristic of that virus.

c. Explain why the doctor did prescribe Rima to get admitted into a hospital.

d. What are the things we need do be aware of to prevent HIV / AIDS? Discuss.

224 General Science

prescribed by national curriculum and textbook board as a

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