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Science | Class 10th

CBSE Board Paper 2018

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CBSE Board Paper 2018 Set - 1

General Instructions:

1. The question paper comprises two Sections, A and B. You are to

attempt both the sections.

2. All questions are compulsory.

3. All questions of Section A and Section B are to be attempted

separately.

4. There is an internal choice in three questions of three marks

each, two questions of five marks each in Section A and in one

question of two marks in Section B.

5. Question numbers 1 and 2 in Section A are one-mark questions.

These are to be answered in one word or in one sentence.

6. Question numbers 3 to 5 in Section A are two-marks questions.

These are to be answered in about 30 words each.

7. Question numbers 6 to 15 in Section A are three-marks

questions. These are to be answered in about 50 words each.

8. Question numbers 16 to 21 in Section A are five-marks

questions. These are to be answered in about 70 words each.

9. Question numbers 22 to 27 in Section B are based on practical

skills. Each question is a two-marks question. These are to be

answered in brief.

Time allowed: 3 Hours Max Marks: 80

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1. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?

Section A

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2. Write the energy conversion that takes place in a hydropower plant.

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3. A compound ‘X’ on heating with excess conc. sulphuric acid at 443 K gives an unsaturated compound ‘Y’. ‘X’ also reacts with sodium metal to evolve a colourless gas ‘Z’. Identify ‘X’, ‘Y’ and ‘Z’. Write the equation of the chemical reaction of formation of ‘Y’ and also write the role of sulphuric acid in the reaction.

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4. (a) Name one gustatory receptor and one olfactory receptor present in human beings.

(b) Write a and b in the given flow chart of neuron through which information travels as an electrical impulse.

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5. If the image formed by a spherical mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a labelled ray diagram to support your answer.

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6. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.

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7. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid.

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8. (a) Why are most carbon compounds poor conductors of electricity?

(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound.

9. Name the hormones secreted by the following endocrine glands and specify one function of each:

(a) Thyroid (b) Pituitary (c) Pancreas

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The pH of a salt used to make tasty and crispy pakoras is 14. Identify the salt and write a chemical equation for its formation. List its two uses.

OR

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10. Write one main difference between asexual and sexual mode of reproduction. Which species is likely to have comparatively better chances of survival – the one reproducing asexually or the one reproducing sexually? Give reason to justify your answer.

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11. State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.

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12. Show how would you join three resistors, each of resistance 9Ω so that the equivalent resistance of the combination is (i) 13.5 Ω, (ii) 6 Ω?

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OR

(a) Write Joule’s law of heating.

(b) Two lamps, one rated 100 W; 220 V, and the other 60 W; 220 V, are connected in parallel to electric mains supply. Find the current drawn by two bulbs from the line, if the supply voltage is 220 V.

13. A. List the factors on which the resistance of a conductor

in the shape of a wire depends.

B. Why are metals good conductors of electricity whereas glass is a bad conductor of electricity? Give reason.

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State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.

OR

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16. (a) Write the steps involved in the extraction of pure metals in the middle of the activity series from their carbonate ores.

(b) How is copper extracted from its sulphide ore? Explain the various steps supported by chemical equations. Draw labelled diagram for the electrolytic refining of copper.

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15. What is a dam? Why do we seek to build large dams? While building large dams, which three main problems should particularly be addressed to maintain peace among local people? Mention them.

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14. Students in a school listened to the news read in the morning assembly that the mountain of garbage in Delhi, suddenly exploded and various vehicles got buried under it. Several people were also injured and there was traffic jam all around. In the brain storming session the teacher also discussed this issue and asked the students to find out a solution to the problem of garbage. Finally they arrived at two main points– one is self management of the garbage we produce and the second is to generate less garbage at individual level.

(a) Suggest two measures to manage the garbage we produce.

(b) As an individual, what can we do to generate the least garbage? Give two points.

(c) List two values the teacher instilled in his students in this episode.

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17. (a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.

(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.

(c) State Modern periodic law.

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18. (a) Mention any two components of blood.

(b) Trace the movement of oxygenated blood in the body.

(c) Write the function of valves present in between atria and ventricles.

(d) Write one structural difference between the composition of artery and veins.

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OR

(a) Define excretion.

(b) Name the basic filtration unit present in the kidney.

(c) and label the following organs of

excretory system which perform following functions:

(i) form urine.

(ii) is a long tube which collects urine from kidney.

(iii) store urine until it is passed out.

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20. (a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.

(b) Why do stars twinkle? Explain.

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OR

(a) Write the function of each of the following parts of human eye:

(i) Cornea (ii) Iris (iii) Crystalline lens (iv) Ciliary muscles

(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.

21. (a) State Fleming’s left hand rule.

(b) Write the principle of working of an electric motor.

(c) Explain the function of the following parts of an electric motor.

(i) Armature (ii) Brushes (iii) Split ring

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19. (a) Write the function of following parts in human female reproductive system :

(i) Ovary (ii) Oviduct (iii) Uterus

(b) Describe in brief the structure and function of placenta.

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Section B

22. A student added few pieces of aluminium metal to two test tubes A and B containing aqueous solutions of iron sulphate and copper sulphate. In the second part of her experiment, she added iron metal to another test tubes C and D containing aqueous solutions of aluminium sulphate and copper sulphate.

In which test tube or test tubes will she observe colour change? On the basis of this experiment, state which one is the most reactive metal and why.

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23. What is observed when a solution of sodium sulphate is added to a solution of barium chloride taken in a test tube? Write equation for the chemical reaction involved and name the type of reaction in this case.

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24. List the steps of preparation of temporary mount of a leaf peel to observe stomata.

25. Name the process by which an amoeba reproduces. Draw the various stages of its reproduction in a proper sequence.

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26. An object of height 4.0 cm is placed at a distance of 30 cm from the optical centre ‘O’ of a convex lens of focal length 20 cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ‘O’ and principal focus ‘F’ on the diagram. Also find the approximate ratio of size of the image to the size of the object.

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27. The values of current (I) flowing through a given resistor of resistance (R), for the corresponding values of potential difference (V) across the resistor are as given below:

Plot a graph between current (I) and potential difference (V) and determine the resistance (R) of the resistor.

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A student is viewing under a microscope a permanent slide showing various stages of asexual reproduction by budding in yeast. Draw diagrams of what he observes. (in proper sequence)

OR

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1. • We know that violet colour of the flowers is a dominant trait and

the white colour of the flowers is recessive.

• When pea plant bearing white flowers is bred with a pea plant bearing white flowers the resulted F1 progeny would consist of all violet flowers.

• The result can be explained by looking at the following figure:

• The dominant violet flower is represented by VV where as the recessive white flower his represented by vv.

• On cross breeding, the two plants we get plants having the characters Vv, Vv, Vv and Vv. Since V (i.e. violet colour) is dominant all flowers are violet.

2. Hydropower plant is an example of a traditional source of energy in which the kinetic energy of the flowing water is converted into electricity. The plant transforms the potential energy of water stored at height in the reservoir of a dam into

Solutions (Set-1)

Section A (Solutions)

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kinetic energy of flowing water. As the water run down through a pipe to the turbine, where electricity is produced through a suitable process.

3. A compound X reacts with concentrated sulphuric acid at 443K to form an unsaturated compound Y. The reaction is –

Further X reacts with sodium metal to evolve a colourless gas Z

Here Z is hydrogen gas(H2)

Y is an unsaturated compound formed by reaction of X and conc. H2SO4. Hence, H2SO4 is acting as a dehydrating agent here.

X is Ethyl alcohol (C2H5-OH) and Y is an unsaturated hydrocarbon. Y is ethene(C2H4).

X-C2H5-OH/Ethanol/Ethyl alcohol

Y-C2H4 / Ethene

Z-H2/ Hydrogen

Equation:

Role of Sulphuric acid:

It is used as a dehydrating agent.

4. Gustatory receptors are the receptors that provide a sense of taste. The gustatory receptor in human beings is the tongue.

Olfactory receptors are responsible for providing the sense of smell. In human beings, nose is an olfactory receptor.

(b) a. Cell body

b. Axon

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The information, acquired at the end of the dendritic tip of a nerve cell sets off a chemical reaction that creates an electrical impulse. This impulse travels from the dendrite to the cell body, and then along the axon to its end.

5. When an object is placed in front of a convex mirror for all position, the image thus formed is always erect, virtual in nature and diminished.

Ray Diagram-

The two position considered to study the nature of image formed by the convex mirror are described with the help of ray diagrams (a) and (b) in the figure above. In the first ray diagram, the object is kept at infinity and in the second case the object is placed as a finite distance from the mirror. The nature of image formed in the first case, ray diagram (a) is virtual, highly diminished but erect. The second ray diagram, (b), shows similar results. The image formed is diminished and virtual but erect. Hence, the correct answer is convex mirror.

6. Example of decomposition by sunlight-

Example of decomposition by heat-

Example of decomposition by electricity-

The above decomposition reactions are taking place with the help or sunlight, heat or electricity.

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7. 2ml of sodium hydroxide reacts with zinc metal to give a gas. The gas formed is Hydrogen (H2) and the compound formed is sodium zincate. The reaction is

To test the presence of the gas, bring the mouth of the test tube near a burning matchstick. If the flame extinguishes with a ‘pop’ sound, then the presence of Hydrogen gas is confirmed.

When Zn reacts with dilute solution of a strong acid, H2 gas will be evolved.

Example:

The salt used is sodium bicarbonate or sodium hydrogen carbonate. Its chemical formula is NaHCO3.

Sodium hydrogen carbonate can be formed by using NaCl as a base raw material. The reaction is given as:

NaHCO3 is used -

(i)- In soda-acid fire extinguishers.

(ii)- as an ingredient in antacids.

8. (a) Carbon compounds are poor conductors of electricity as-

(i)- Carbon compounds majorly form covalent compounds and

Not ionic compounds

(ii)-as they form covalent compounds, ions are absent as they

Can’t dissociate into ions so they conductivity is poor.

OR

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(b) Cyclohexane is saturated compound in which carbon atoms are arranged in a ring.

The structure of cyclohexane is-

The number of single bonds is 18.

9. a) Thyroid secretes thyroxin hormone.

Function: Thyroxin regulates carbohydrate, protein and fat metabolism in the body so as to provide the best balance for growth.

b) Pituitary secretes the growth hormone.

Function: Growth hormone regulates growth and development of the body.

c) Pancreas secretes insulin.

Function: helps in regulating blood sugar levels.

10. The primary or the main difference between asexual and sexual reproduction is that sexual reproduction involves two parents whereas asexual reproduction requires only one. Also in sexual reproduction male and female gametes are formed which fuse to give rise to an offspring. However, in asexual reproduction, there is no gamete formation.

• Species that reproduce via sexual reproduction would be able to survive better. This is because sexual reproduction gives rise to variations in offspring.

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• Variations allow an organism to survive and adapt to new environments better.

• If a population of reproducing organisms were suited to a particular niche and if the niche were drastically altered, the population could be wiped out. However, if some variations were to be present in a few individuals in these populations, there would be some chance for them to survive.

• For example, if there were a population of bacteria living in temperate waters, and if the water temperature were to be increased by global warming, most of these bacteria would die, but the few variants resistant to heat would survive and grow further.

11. The laws of refraction, also known as Snell’s law of refraction describes the behavior of light as it goes across an interface from a one medium to another.

The following two statements are the laws of refraction of light:

(i) The incident ray, refracted ray and the normal to the interface of the two separate media at the point of incidence, all lies in the same plane.

(ii) The ratio of the sine of angle of incidence to the sine of angle of incidence for a given pair of media and the colour of light is always constant.

If i and r are the angles made by the incident ray and the refracted ray, respectively, at the point of incidence, then we can write;

The term absolute refractive index is defined as the ratio of the speed of light in the vacuum to the speed of light in a certain medium. It can also be expressed as,

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nm =

The ability of a lens to converge or diverge the light rays, depending on the nature of the lens is measured in terms of the power of the lens. It is defined as the inverse of focal length, described as;

The SI unit of power of a lens is called Dioptre. It is often denoted by letter D. Since, the focal length of a lens is expressed in meters, then, power is expressed in Dioptre. Thus, the power of a lens of a focal length of 1m is 1D, and its unit is m-1.

The question has given two lenses with focal length values as following;

Let’s call it f1 and f2.

f1 = 40 cm

f2 = -20cm

Now, the corresponding values of power of these lenses are, say P1 and P2.

Power of a lens is defined as,

Therefore, for the lens 1,

P1 = 1

40

100

=100

40= 2.5 𝐷

The value of power in this case is positive in nature that signifies that the lens is converging in nature.

Similarly, for the lens 2,

OR

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P2 = 100

−(20) = -5 D

The power of this lens is negative, that speaks about the nature of the lens. A lens with a negative power is diverging in nature in conclusion, a lens with negative power is always diverging in nature and for positive power the converging capacity of the lens is significant.

12. Given: Three resistors, each of resistance 9Ω

The objective of the question is to arrange the resistor in such a way that the equivalent resistance of the circuit is equal to following values,

(i)13.5 Ω

If we arrange the two resistors in parallel and the third resistor in series, the equivalent resistance of the combination will be equal to 13.5 Ω.

The equivalent resistance of the parallel combination of the two resistors is given by,

𝑅𝑝 =𝑅1𝑅2

𝑅1 + 𝑅2

As per question,

R1 = R2 = R3 = 9 Ω

𝑅𝑝 =9×9

9+9= 4.5 Ω

Therefore, upon substituting the value of R1 and R2 in Req, it is equal to 4.5 Ω

Now, the equivalent resistance of the combination is a series combination of Req and the third resistor,

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Req = Rp + R3 = 4.5 + 9 = 13.5 Ω

(ii) 6 Ω

Similarly, If we put 2 resistors in series, parallel with the third resistor, the equivalent

of the resultant combination will be equal to 6 Ω.

The series combination of two resistor is equal to, Rs = R1 + R2 = 9 + 9 = 18

The equivalent resistance of the entire combination is a parallel combination of Rs

and R3, that is equal to,

𝑅𝑒𝑞 =(𝑅𝑠𝑅3)

𝑅𝑠+𝑅3=

18×9

18+9= 6Ω

(a) Joule’s law of heating explains the amount of heat generated due to current within a wire, its mathematical expression can be explained in following cases;

(i) It is directly proportional to the square of current for a given value of resistor and for a fixed period of time, H∝ I2

(ii) When the heat dissipated is proportional to the resistance for a given current and for a fixed period of time, H∝ R

(iii) It is directly proportional to the time for which the current flow across the resistance, H∝ t

When all these conditions are merged together, the resulting expression for heat dissipation is,

OR

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(b) Given: Two lamps, let’s say A and B, with following power and voltage rating are given;

Lamp A: Power Rating = P = 100 W; Voltage Rating = V = 220V.

Lamp B: Power Rating = P = 60 W; Voltage Rating = V = 220V.

We need to determine the current drawn by the bulbs from the line, if the supply voltage is equal to 220V.

As we know the power dissipated by a bulb is a product of voltage and current, so it can be written as,

So the current drawn by the bulb from the 220V supply is equal to,

For bulb A,

Similarly, for bulb B,

Hence, the voltage drawn by bulb A and bulb B from the line is equal to 0.45 A and 0.27 A, respectively

13. A. The resistance of a wire depends mainly on three factors:

(a) It is directly proportional to the length of the wire, R ∝ l

(b) It is inversely proportional to the cross-sectional area of the

wire, R ∝ 1

𝐴

(c) It depends upon the material out of which the wire is made, which is called the resistivity of the wire.

(d) Resistance of a wire also varies with temperature.

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The relationship of wire with these aforementioned factors can be mathematically expressed in the following way,

Where ρ is the resistivity of the wire, A is the cross-sectional area of wire and l is the length of the wire.

B. Metals are considered as a good conductor of electricity as they contain delocalised electrons that are not firmly held by the atom. Instead, they can move freely through the positive metal ions. When a potential difference is applied across the metal, the electrons are pushed away towards the positive terminal of the cell, away from the negative terminal of the battery. On the other hand, glass is a poor conductor of electricity as it contains no loosely bound free electrons. Consequently, glass being an insulator is incapable of conducting electricity.

14. a. Two ways in which we can manage the garbage we produce is by:

(i) Separating the waste produced. Waste can be separated into dry waste and wet waste or into biodegradable and non biodegradable wastes.

(ii) properly disposing the garbage at places specified by the municipality. Waste produced can further be dealt with in different ways depending on the type of waste produced. For example, if the waste is natural, it can be composted, if waste generated is biodegradable it can be subjected to vermicomposting.

b. (i) To generate less waste, we can follow the three Rs, refuse, reuse and recycle. We can practise recycling.

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(ii) We can reuse items instead of throwing them away after single use. We can refuse to buy or use items that generate a lot of waste.

c. The teacher instilled the following two values in his students:

(i) He created awareness about the environmental effects of waste and taught them about individual responsibility regarding waste generation.

(ii) He also taught them about being responsible citizens by telling them to generate less waste.

15. A dam is a barrier constructed to hold back water and raise its level to form a reservoir.

• We seek to build large dams so that they can be used to generator electricity and be as a water supply for irrigation purposes.

• When building dams some things need to be kept in mind.

• Dams cannot be constructed everywhere. They’re often constructed in hilly terrains.

• Dam construction leads to a loss of a large amount of agricultural land, so when constructing dams, this should be kept in mind.

• Construction of dams also leads to destruction of habitats as lands get submerged in water. So some kind of rehabilitation process needs to be in effect.

• Also, there needs to be some form of compensation and rehabilitation facility in place for local people because construction of dams leads to displacement of the local population.

16. a) Pure metals are extracted from their carbonate ores through the following steps:

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1. Concentration of ore:

Gangue or matrix first needs to be removed from the ore keeping in mind the differences in physical and chemical properties of gangue and ore.

2. Calcination:

The carbonate ores must be heated strongly in absence of air to convert them into metal ores.

For example:

3. Reduction:

Next, reduction is carried out using a more reactive metal or carbon.

For example:

This happens because carbon has a greater binding affinity for oxygen than Zinc does.

Finally, the obtained metal is refined by electrolysis of their salt solutions.

b) Copper glance is the ore of copper sulphide. It is first roasted and then the remaining Cu2O is reduced using Cu2S in the tank.

This way, copper metal is extracted.

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Electrolytic refining of copper: The electrolyte is a solution of acidified copper sulphate. The anode is impure copper, whereas, the cathode is a strip of pure copper. On passing electric current, pure copper is deposited on the cathode.

17. a) 1. Dobereiner’s method of classification:

Advantage: This was the first time that metals were grouped on the basis of their similar chemical properties. Three Dobereiner’s triad could be found which had special characteristics.

When arranged in increasing atomic masses, the atomic mass of the middle element was equal to the average of the atomic masses of the other two elements.

For example: Li – 6.9 u

Na – 23 u

K – 39.1 u

This encouraged others to classify elements on the basis of chemical properties and atomic masses.

Disadvantage: He was able to only place nine such elements in three triads and therefore this method wasn’t efficient enough for a study.

2. Newland’s law of octaves:

Advantage: Newland was able to place 56 elements known at the time in his classification system. This system also, for the first time, allowed period recurrence of properties to be studied.

Disadvantage: His classification system worked only for the lighter elements and the properties matched only for elements till Calcium. He also placed elements like Cobalt and Nickel in the same slot.

3. Mendeleev’s classification:

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Advantage: In his classification, Mendeleev arranged elements on the basis of increasing atomic masses and similar formula for hydrides and oxides. He also left gaps in his classification table for undiscovered elements.

Disadvantage: Mendeleev placed elements with greater atomic mass prior to ones having lesser atomic mass for the similarity of properties. However he couldn’t justify his reason for doing so.

b) Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.

c) The modern periodic law states that the properties of elements are the periodic function of their atomic number.

18. (a) The two components of blood are:

• Plasma

• Red blood cells

(b) The movement of oxygenated blood in the body can be depicted by the following flow chart:

Lungs → Left part of the heart → aorta → various organs

• The deoxygenated blood enters the heart through veins. The deoxygenated blood reached the lungs where it gets oxygenated.

• The oxygenated blood leaves the lungs via the pulmonary vein and returns to the left side of the heart.

• The blood then enters the left atrium, gets pumps through the mitral valve and then into the left ventricle.

• The left ventricle pumps the oxygenated blood to the different organs and body parts.

(c) The valves present between the atria and ventricles of the heart prevent the blood from flowing back when a contraction takes place.

(d) Arteries have thick elastic walls while veins are thin walled.

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(a) Excretion is the biological process of removal of toxic waste generated by the body. The waste produced is in form of excess water, salts, urea, uric acid etc.

(b) Nephrons are the basis filtration unit of the kidney. Each nephron includes a filter, called the glomerulus, and a tubule.

(c) (i) urine is formed by kidneys.

(ii) urine collection tube: Ureter

(iii) Stores urine: Urinary bladder

19. (a) (i) Ovary: A pair of ovaries is present in the human female reproductive system. Ovaries produces ova (eggs/female gametes) Ovaries are also responsible for producing estrogen, which is a female hormone involved in the development of secondary sexual characters.

(ii) Oviduct: It is responsible for carrying the mature ovum to the uterus. The oviduct is also the place where fertilization takes place.

(iii) Uterus: Uterus or the womb is the place where the fertilised egg gets implanted and it nurtured. It holds and supports the developing foetus.

(b)Placenta is an organ that develops in the uterus during pregnancy. It is like a disc which is embedded in the uterine wall. On the foetal side of tissue, villi are present and on the

OR

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maternal side there are blood spaces. The umbilical cord arises from the placenta.

Function: The placenta provides nutrition to the developing foetus from the mother. It is a connection between the mother and the foetus. The waste generated by the foetus is also removed by transferring them into the mother’s blood through the placenta.

20. (a) As the student is unable to clearly see distant objects, it suggests that the student is suffering from an eye defect. The defect is referred to as Myopia or short sightedness. The far point for a myopic person is closer than the infinity [fig(a)]. This defect is caused by elongation of the eye ball or due to excessive curvature of the eye lens. The image formed in this case is in front of retina but not at the retina itself resulting into the blurriness of the image.

This defect is corrected using a concave lens that diverges the ray coming from the object and shift the image to the retina, which in turn improves the clarity of the image produced by the eye. As shown in the figure the concave lens diverges the incoming rays thereby fixing the aberration.

(b) The twinkling of the star is governed by the phenomena of refraction due to the earth’s atmosphere. As the light originating from the star enters the earth’s atmosphere, it is

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refracted by a certain angle, which bends the light. The bending of the light causes the star to change its position and the brightness, hence the twinkle.

(a)

Fig – Human Eye

(i) Cornea: Cornea is a smooth, thin dome shaped membrane that covers the pupil, iris and anterior chamber a human eye. The primary function of the cornea is to let the incoming ray enter the eye and converge the light rays. It accounts for almost one-third of the power of the eye lens. Also, cornea protects the eye from minor injury and abrasions.

(ii)Iris: The iris is a pigmented tissue located behind the cornea that gives colour to the eye. It is responsible for regulating the amount of light entering the eye by varying the size of the pupil and also perpetually adapting to the light changes in the surrounding. The iris allows the near point reading.

OR

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(iii) Crystalline lens: Human eye consists of clear jelly-like transparent material, shaped as a convex lens that converges the light and brings it to the retina for vision. It is suspended by the ligaments attached in between the ciliary body that varies the curvature of the lens, which in turn changes the power.

(iv) Ciliary muscles: Ciliary muscles are composed of tissue located at the periphery of the eye. The role of ciliary muscles is to change the curvature of the lens that can thus change the focal length. When you are looking at an object placed closer to the eye, the ciliary muscles contract. The contraction of muscles increases the curvature of the lens as a result the lens become thicker; as a result the focal length of the eye lens decreases. Thus, it enable us to see the object clearly.

(b) The sun appears reddish early in the morning due to the refraction of the light rays. At the time of sun rise the light rays travel a longer distance before reaching our eye. As a result the light rays with shorter wavelength gets scattered away by the thick layer of atmosphere at the horizon, leaving only the rays of longer wavelength i.e. Red. During the day time, the sun is situated overhead. Therefore, the only a small fraction of blue and violet colour of shorter wavelength get dispersed in the atmosphere and the sun appears to be yellow.

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21. Fleming’s left-hand rule is a method to determine the direction of force acting on the conductor that is kept in a uniform magnetic field. It is not a process to determine the magnitude of the force but the direction of any of the three parameters; current, magnetic field and the force when the direction of two of the three parameters are known.

It states “Hold your thumb, the forefinger and the second finger at a right angle to each other. If the forefinger represents the direction of the field and the second finger represents that of the current then thumb gives the direction of the force”

Fig – Fleming’s Right-Hand Rule

(b) A motor is a device that converts electrical energy into mechanical energy.

The principal of Electric Motor: When a current carrying coil is placed in a magnetic field and current is passed through it, the coil experiences a force.

The direction of force is determined using Fleming’s left-hand rule.

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The coil ABCD is placed in between two magnetic poles in such way that the arms of the coil AB and CD are perpendicular to the direction of the magnetic field. As the coil draws current from the battery source, it experiences a force, determined using Fleming’s left-hand rule. The forces cause the coil to rotate continuously generating useful mechanical energy.

(c) (i)Armature: Armature is a mechanical component of an electric motor. The armature is supplied with the current and it experiences a force.

The function of the armature can mainly be described in the following ways:

(a)The armature is responsible for carrying the current across the magnetic field.

(b)The armature is the element in the electric motor that induces and sustains the rotary motion in the electric motor.

(ii)Brushes: Brushes, sometimes known as motor brush, are a small component of an electric motor that enables the delivery of current from the stationary wire to the rotating wire in an electric motor. They are usually made up of carbon material. In the electric motor, the brushes are mounted on the stationary wire circuit of the motor as shown in the figure, X and Y. They operate together with the slip rings to accomplish the commutation of current in the electric motor.

(iii)Slip rings: Slip rings are a device that reverses the direction of flow of current in an electric motor through a circuit is known as a commutator. P and Q are the split rings as shown in the figure. The reversal of current also, in turn, reverses the direction of force acting on the armature. Earlier the upward force acting on the coil segment AB is now pushed down, similarly, the downward force acting on the coil segment CD is reversed to upward force.

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22. Test tube A:

Iron sulphate is a green coloured solution. However, the resultant aluminium sulphate is colourless.

Test tube B:

Copper sulphate is a blue coloured solution. However, the resultant aluminium sulphate is colourless.

Test tube C:

No reaction takes place here because aluminium is more reactive than iron.

Test tube D:

Here, iron replaces copper and form ferrous sulphate which is a pale green coloured solution because iron is more reactive than copper.

Colour changes were observed in test tubes A, B and D.

Therefore, reactivity in decreasing order is: Al > Fe > Cu

Aluminium is the most reactive metal because it displaced both iron and copper from their compounds.

Section B (Solutions)

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23. When a solution of sodium sulphate is added to a solution of barium chloride the following reaction takes place:

The barium sulphate formed precipitates at the bottom of the test tube in the form of a white precipitate.

The type of reaction taking place here is a double displacement reaction or precipitation reaction.

Observation: A white precipitate of BaSO4 is formed in the test tube.

24. 1. A healthy leaf is removed from a potted plant.

2. The peel from the lower surface of the leaf was removed with the help of forceps and kept in a watch glass containing water.

3. A few drops of safranin stain were put in the watch glass.

4. After a few minutes the peel was taken out and placed onto a clean glass slide.

5. Next, a drop of glycerine was put over the peel and a clean coverslip was gently placed over it with the help of a needle.

6. Excess stain and glycerin were removed with the help of a blotting paper.

7. The slide was observed under a microscope.

25. An amoeba reproduces via the process known as binary fission.

After replicating its genetic material through mitotic division, the cell divides into two equal sized daughter cells.

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• Yeast is a unicellular organism. Yeast reproduces via the asexual process known as budding.

• Budding usually occurs when food is available in plenty.

• In budding, one or more outgrowths appear on one side, which increases in size.

• The nucleus divides through mitosis, one daughter nucleus passes into the bud and the other remains in the parent cell.

• The bud either separates off from the parent cell or new bud appears before its separation from the parent cell resulting in the formation of branched or unbranched chain of buds.

OR

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26. Given: Height of the object = AB = 4 cm

Object’s distance from optical centre = u = -30 cm (distance in the direction opposite to the incident ray is taken to be negative)

Focal length of the convex lens = F = 20 cm

Let the distance of the image formed from the optical centre be v and the height of it be A’B’.

The expression to determine the distance of the image formed is given by,

As mentioned in the question the object is placed at 30 cm from the optical centre O of a convex lens with a focal length of 20 cm, therefore substituting the value of u. We get,

⇒

⇒

We obtain the value of v, which is equal to 60 cm.

Now the magnification of the image is defined as,

Therefore, ratio of the height of image to the height of the object is 2:1

Hence, the ray diagram will look like this,

Ray Diagram

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27. According to the Ohm’s law the current generated through a conductor is directly proportional to the potential difference or voltage applied across the conductor. It can be described with following mathematical expression,

The above equation can be rearranged into,

The question has provided with a table that show the values of current I through a resistor R for different values of voltage V.

Graph :

Since, the value of the resistor is the slope of the V-I graph, which is equal to 5Ω

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