deepak sir 9811291604 is matter around us pure...chemical changes results in the formation of new...

Deepak sir 9811291604 Is matter around us pure

Is matter around us pure

POINTS TO REMEMBER Pure susbtances contain only one type of particles. Mixtures are constituted by more than one kind of pure form of matter (element or compound) mixed in any

proportion. A solution is a homogeneous mixture of two or more substances. The component present in larger proportion in a solution is called solvent. The component present in smaller proportion in a solution is called solute. The concentration of a solution is the amount of solute present per unit volume or per unit mass of the

solution/solvent. Alloys are solid solutions. Suspension is a heterogeneous mixture in which the solute particles don't dissolve and remain suspended through

the liquid. Colloidal solution is a homogeneous mixture in which the size of the particles is between 1 nm - 100 nm (1 nm = 10–

9 m). Gel is a colloidal solution of a liquid in a solid, e.g. butter, cheese, etc. An emulsion is a colloidal solution of two immissible liquids. The scattering of light by colloidal particles is called Tyndall effect. We employ the following common methods for the separation of components of a mixture :

• Evaporation • Crystallisation • Centrifugation • Distillation • Fractional distillation • Using separating funnel • Chromatography • Sublimation

During physical change, there is change in properties only. These changes are temporary. Chemical changes results in the formation of new chemical substance(s). These changes are permanent. A compound is a pure substance composed of atoms of two or more elements combined in a fixed proportion. Broadly, we can classify matter as below :

A.QUESTIONS FROM WITHIN THE CHAPTER

Q1. What is meant by a pure substance ? Ans. A substance which consists of a single type of particles and cannot be separated into other kind of matter by any physical

process is called a pure substance, e.g. NaCl, sugar, etc. Pure substances include elements and compounds.

Q2. List the point of differences between homogeneous and heterogeneous mixtures.


Ans. The main differences between homogeneous and heterogeneous mixtures are—

1. A homogeneous mixture has uniform composition throughout its mass while the composition of a heterogeneous mixture

varies throughout the bulk.

2. The particles of a homogeneous mixture are not physically distinct and cannot be seen by the naked eye, whereas in

heterogeneous mixture, the particles are physically distinct and can be seen with the naked eye.

Examples— Homogeneous mixtures—sodium chloride in water and alcohol in water.

Heterogeneous mixtures—sand and soil, sodium chloride and iron filings and oil and water mixtures.

Q3. Differentiate between homogeneous and heterogeneous mixtures with examples. Ans. Same as the previous question.

Q4. How are true solution, colloidal solution and suspension different from each other ?

Ans. Differences between true solution, suspension and colloidal solution:

True Solution Colloidal Solution Suspension

1. A true solution is a homogeneous

mixture of solute and solvent.

2. It is transparent.

3. The solute particles are very small,

i.e., less than 1 nm.

4. The particles are not visible even with

a powerful microscope.

5. The entire solution passes through

filter paper as well as semi-permeable

membrane.

6. The solute particles do not show

Tyndall effect.

7. The particles do not settle due to

gravity e.g. salt in water solution.

A colloidal solution appears to be homo-

geneous but actually it is a heterogeneous

mixture of solute and solvent

It is transluscent.

The solute particles are between 1-100

nm.

The particles are visible with the help of

microscope.

The particles can pass through ordinary

filter paper but not through a

semipermeable membrane.

The particles show Tyndall effect.

The particles do not settle due to gravity

e.g. blood.

It is a heterogeneous mixture.

It is opaque.

The solute particles are quite large i.e.,

more than 100 nm.

The particles are visible even with naked

eye.

The particles cannot pass through either a

filter paper or through a semipermeable

membrane.

They may or may not show Tyndall effect.

The particles may settle due to gravity e.g.

chalk powder in water.

Q5. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration

at this temperature.

Ans. Mass of solute (sodium chloride) = 36 g

Mass of solvent (water) = 100

g

:. Mass of the solution = Mass of solute+Mass of

solvent

= 36 g + 100 g = 136 g

Now concentration of the

solution =

Mass of solute

×100 Mass of

solution

= 36

×100 = 26.47 % 136

:. Concentration of the solution is 26.47%.


Q6. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than

25°C), which are miscible with each other ? Ans. We can separate a mixture of kerosene and petrol by fractional distillation.

Q7. Name the technique to separate

(i) butter from curd.

(ii) salt from sea water.

(iii) camphor from salt. Ans. (i) By centrifugation

(ii) By evaporation

(iii) By sublimation.

Q8. What type of mixtures are separated by the technique of crystallisation ? Ans. By the technique of crystallisation, we can separate a mixture of solid containing impurities and a liquid. By crystallisation,

a pure solid is separated in the form of its crystals from its solution.

Q9. Classify the following as chemical or physical changes :

— cutting of trees.

— melting of butter in pan.

— boiling of water to form steam.

— passing of eletric current through water and the water breaking down into hydrogen and oxygen gases.

— Dissolving common salt in water.

— Making a fruit salad with raw fruits.

— Burning of paper and wood. Ans. Cutting of trees—Physical change

Melting of butter in pan—Physical change

Boiling of water to form steam—Chemical change

Passing of eletric current through water and the water breaking down into hydrogen and oxygen gases—Chemical change

Dissolving common salt in water—Physical Change

Making a fruit salad with raw fruits—Physical Change

Burning of paper and wood—Chemical Change.

Q10. Try segregating the things around you as pure substances or mixtures. Ans. Mixtures—Air, milk, soil, butter, vegetables, paper, stainless steel utensils etc.

Pure substance—Distilled water, ice, sugar, salt, copper wire, mercury inside a thermometer etc

Is matter around us pure 2

B.EXERCISE GIVEN AT THE END OF THE CHAPTER

Q1. Which separation techniques will you apply for the separation of the following :

(a) Sodium chloride from its solution in water.

(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.

(c) Small pieces of metal in the engine oil of a car.

(d) Different pigments from an extract of flower petals.

(e) Butter from curd.

(f) Oil from water

(g) Tea leaves from tea

(h) Iron pins from sand

(i) Wheat grains from husk

(j) Fine mud particles suspended in water. Ans. (a) Evaporation (b) Sublimation (c) Filtration or sedimentation or centrifugation

(d) Chromatography (e) Centrifugation (f) Separating funnel (g) Filtration

(h) Magnetic separation (i) Winnowing (j) Centrifugation

Q2. Write the steps you would use for making tea. Use the words—solution, solvent, solute, dissolve, soluble, insoluble,

filtrate and residue. Ans. Step 1. Take some solvent, i.e. water in a pan and boil it.

Step 2. Put some tea leaves (solute) in a tea pot.

Step 3. Pour the boiling water into the pot and let it soak for a few minutes. It will form a solution.

Step 4. Put some sugar (solute) into a cup.

Step 5. Stir the solution in the tea pot.

Step 6. Filter this solution using a strainer and pour the solution, i.e., filterate into the cup.

Step 7. Now put some milk into the cup and stir it with a spoon.

The tea is ready for drinking.

The tea leaves (residue) will be left on the strainer. Sugar and milk are soluble solutes while tea leaves are insoluble solutes.


Q3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given

below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated

solution).

Substance disolved Temprature in K

283 293 313 333 353

Potassium nitrate

Sodium chloride

Potassium chloride

Ammonium chloride

21

36

35

24

32

36

35

37

62

36

40

41

106

37

46

55

167

37

54

66

(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of

water at 313 K ?

(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cook at room

temperature. What would she observe as the solution cools ? Explain.

(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature ?

(d) What is the effect of change of temperature on the solubility of a salt ? Ans. (a) At 313 K,

100 g of water will dissolve potassium nitrate at 313K = 62 g

:. 50 g of water will dissolve potassium nitrate at 313K = 31 g

(b) The solubility of potassium chloride at 353 K is 54 g/100 g of water and at room temperature (293 K), the solubility of

potassium chloride is 35 g/100 g of water. As the solution, potassium chloride cools from 353 K to 293 K, the crystals of

potassium chloride separate out and settle down at the base of the beaker. In other words, the dissolved potassium chloride gets

precipitated.

(c) From the table, we get the number of grams of the various substances dissolved (at 293 K) to form saturated solution which

are as under—

Potassium nitrate = 32 g

Sodium chloride = 36 g

Potassium chloride = 35 g

Ammonium chloride = 37 g

Thus, it is clear that at 293 K, ammonium chloride has highest solubility at this temperature.

(d) The rate of solubility is directly proportional to the temperature, i.e., it increases with the increase in temperature and vice-

versa.

Q4. Explain the following giving examples :

(a) saturated solution (b) pure substance (c) colloid (d) suspension. Ans. (a) Saturated solution. A solution in which no more amount of solute can be dissolved at a given temperature is called a

saturated solution.

(b) Pure susbtance. It is a homogeneous material consisting of a single type of particles with definite set of properties. Pure

substances include elements and compounds.

(c) Colloid. A colloid or colloidal solution appears to be homogeneous but actually is a heterogeneous mixture of solute and

solvent. The size of the solute particles lies between 1-100 nm and they are not visible to the naked eye. They neither dissolve

nor settle down in the solvent.

(d) Suspension. A suspension is a heterogeneous mixture in which the solute particles don't dissolve but remain suspended

throughout the bulk of the material. The size of the particles is more than 100 nm and they are visible even with the naked eye.

Q5. Classify each of the following as a homogeneous or heterogeneous mixture soda water, wood, air, soil, vinegar,

filtered tea. Ans. Homogeneous mixtures. Soda water, air, vinegar and filtered tea.

Heterogeneous mixtures. Wood and soil.

Q6. How would you confirm that a colourless liquid given to you is pure water ? Ans. Every liquid has a particular boiling point at 1 atmospheric pressure. If the given colourless liquid boils at exactly 100°C,

i.e., 373K at 1 atmospheric pressure, then it is pure water. If the boiling point is even slightly above 373 K, then it is

contaminated.

Q7. Which of the following materials fall in the category of a „pure substance‟ :

(a) Ice (b) Milk

(c) Iron (d) Hydrochloric acid

(e) Calcium oxide (f) Mercury


(g) Brick (h) Wood

(i) Air

Ans. Out of the given list, pure substances are :

(a) Ice – a compound (c) Iron – an element (d) Hydrochloric acid – a compound

(e) Calcium oxide – a compound (f) Mercury – an element.

Q8. Identify the solutions among the following mixtures :

(a) Soil (b).Sea water

(c).Air (d).Coal

(e).Soda water

Ans. Out of the given mixtures, solutions are :

(b) sea water (c) air and (e) soda water.

Q9. Which of the following will show “Tyndall effect” ?

(a) Salt solution (b) Milk

(c) Copper sulphate

solution (d) Starch solution.

Ans. Tyndall effect is shown by colloidal solutions. As milk and starch solution are colloids, they will show Tyndall effect.

Q10. Classify the following into elements, compounds and mixtures.

(a) Sodium (b) Soil

(c) Sugar solution (d) Silver

(e) Calcium carbonate (f) Tin

(g) Silicon (h) Coal

(i) Air (j) Soap

(k) Methane (l) Carbon dioxide

(m) Blood

Ans. Elements : Sodium, silver, tin and silicon

Compounds : Calcium carbonate, methane and carbon dioxide

Mixtures : Soil, sugar solution, coal, air, soap and blood.

Q11. Which of the following are chemical changes ?

(a) Growth of a plant (b) Rusting of iron

(c) Mixing of iron filing and sand (d) Cooking of food

(e) Digestion of food (f) Freezing of water

(g) Burning of a candle.

Ans. Out of the given changes, the following are the chemical ones :

(b) Rusting of iron (d) Cooking of food e) Digestion of food, and (g) Burning of candle

OTHER IMPORTANT QUESTIONS

VERY SHORT ANSWER QUESTIONS

Q1. What is a pure substance ? Ans. A substance made up of only a single type of particles is called a pure substance.

Q2. What are mixtures ? Ans. Mixtures are constituted by more than one kind of pure form of matter, i.e. pure substances.

Q3. What is a substance ? Ans. A substance is a kind of matter that cannot be separated into other kinds of matter by any physical process.

Q4. Are most of the naturally occuring materials on the earth substances ? Ans. No, they are mixtures.

Q5. What are the constituents of milk ? Ans. Milk is a mixture of water, fat and proteins.

Q6. Name any two substances that have the a uniform composition. Ans. Salt and sugar.

Q7. Which of the following is/are pure substance(s) ?

Ink, paper, water, milk, butter, ghee, sugar, blood Ans. Water and sugar.


Q8. Name the two types of mixtures. Ans. Homogeneous mixtures and heterogeneous mixtures.

Q9. What is a homogeneous mixture ? Ans. A mixture which has uniform composition throughout its mass is called a homogeneous mixture.

Q10. Give two examples of homogeneous mixtures. Ans. Sugar solution (sugar and water) and water solution (salt and water).

Q11. What is a heterogeneous mixture ? Ans. A mixture which does not have uniform composition throughout its mass is called a heterogeneous mixture.

Q12. Give two examples of heterogeneous mixtures. Ans. Mixture of iron filings and sulphur; and a mixture of salt and sand.

Q13. Define a solution. Ans. A homogeneous mixture of two or more substances is called a solution.

Q14. What is a binary solution ? Ans. A solution having two components is called a binary solution.

Q15. What is solute ? Ans. The component of a solution which is present in small amount and dissolved in another component is called solute.

Q16. What is solvent ? Ans. The component of a solution which is present in large amount and in which solute is disolved is called the solvent.

Q17. Give an example of a liquid in liquid type solution. Ans. Vinegar (acetic acid + water).

Q18. Give an example of a solid in liquid solution. Ans. A solution of sugar and water.

Q19. Give an example of a solid solution. Ans. Brass (an alloy having 70% copper and 30% zinc).

Q20. Give an example of a gas in liquid solution. Ans. Aerated drinks like soda water.

Q21. Define concentration of a solution. Ans. It indicates the exact amount of solute dissolved in an exact amount of solvent or solution.

Q22. What is an aqueous solution ? Ans. It is a solution in which water acts as a solvent.

Q23. What is a non-aqueous solution ? Ans. It is a solution in which the solvent is other than water.

Q24. What is solubility ? Ans. Solubility of a substance (solute) is the mass of the substance in grams which dissolves in 100 g of a solvent to form a

saturated solution.

Q25. What is a saturated solution ?

Ans. A solution is a said to be saturated if no more of solute can be dissolved in it at a given temperature and pressure.

Q26. What is a suspension ?

Ans. It is a heterogeneous mixture in which the solute particles do not dissolve and remain suspended throughout the liquid.

Q27. What is a colloidal solution ?

Ans. It is a heterogeneous mixture in which the size of the solute particles is between 1 nm (10–9

m) to 100 nm.

Q28. Give two examples of colloids.

Ans. Milk and jelly.

Q29. Why can't we see the colloidal particles with naked eyes ?

Ans. Because their size is from 1 nm to 100 nm, and we can't see such small particles with unaided eyes.

Q30. What is a gel ?

Ans. It is a colloidal solution of a liquid in solid e.g. butter, cheese etc.

Q31. What is an emulsion ?

Ans. A colloidal solution of two immiscible liquids is called an emulsion, e.g. milk.

Q32. What is Tyndall effect ?

Ans. The scattering of light by colloidal particles is called Tyndall effect.

Q33. How can we separate the coloured components (dye) from ink?

Ans. We can separate the dye (solute) from the solvent by evaporation, and then separate the components of the dye by

chromatography method.

Q34. How can we separate cream from milk ?

Ans. By centrifugation method.

Q35. How can we separate a mixture of two immiscible liquids ?

Ans. By using separating funnel.

Q36. How can we separate a mixture of two miscible liquids ?


Ans. By distillation.

Q37. What is filtration ?

Ans. When the insoluble component is separated by filtering the solution, it is called filtration.

Q38. What is chromatography ?

Ans. The process of separation of components of a mixture is called chromatography.

Q39. For the separation of what kinds of solutes is the process of chromatography used ?

Ans. Chromatography is used for the separation of those solutes that dissolve in the same solvent.

Q40. What is the essential condition for the separation of the components of a mixture by chromatography ?

Ans. For chromatography, it is essential that the components should be coloured.

Q41. How can we obtain different gases from air ?

Ans. By fractional distillation.

Q42. How can we obtain pure copper sulphate from an impure sample?

Ans. By crystallisation.

Q43. Give an application of crystallisation.

Ans. Purification of salt that we get from sea water.

Q44. How many elements are known today ?

Ans. More than 115.

Q45. Name the three types of elements.

Ans. Metals, non-metals and metalloids.

Q46. Give two examples of metals.

Ans. Silver and copper.

Q47. Which metal is liquid at room temperature ?

Ans. Mercury.

Q48. How many elements are gaseous ?

Ans. Eleven.

Q49. Name the substance that is composed of atoms of different elements in a fixed proportion.

Ans. Compound.

Q50. Is a new compound formed when a mixture is formed ?

Ans. No. Elements or compounds just mix together to form a mixture and no new compound is formed.

Q51. Can the constituents of a mixture be separated by physical methods ?

Ans. Yes, they can be separated easily by physical methods.

Is matter around us pure 4

SHORT ANSWER QUESTIONS Q1. What does the word „pure‟ means to a common man and to a scientist ?

Ans. For a common person pure means having no adulteration. But, for a scientist all these things are mixtures of different

substances and hence not pure, e.g. milk is actually a mixture of water, fat, proteins etc. When a scientist says that something is

pure, it means that all the constituent particles of that substance are the same in their chemical nature. So, a pure substance

consists of a single type of particles.

Q2. What is a pure substance chemically ?

Ans. Chemically a pure substance is a kind of matter that cannot be separated into other kinds of matter by any physical process.

It has the same colour, texture, taste and composition at a given temperature and pressure

Q3. Are the naturally occurring material in nature chemically pure substances ? Explain.

Ans. Most of the naturally occurring materials in nature are not chemically pure substances, and are mixtures of two or more

pure components e.g. sea water, minerals, soil etc., which are all mixtures.

Q4. Describe the term mixture. What are the two kinds of mixtures ?

Ans. A mixture is formed when two or more substances are mixed at random. The constituents of a mixture are not chemically

combined and can be separated into pure substances even by physical processes. For example, sea water is a mixture of

dissolved salts and water. On heating, the water evaporates and solid salt is left behind.

Mixtures are of two types— (i) Homogeneous mixtures, which have uniform composition throughout their mass, e.g. sugar

solution and salt solution. (ii) Heterogeneous mixtures, which don't have uniform composition throughout their mass, e.g. a

mixture of salt and sand.

Q5. Make a list of the various types of homogeneous and heterogeneous mixtures with an example of each. Ans. (i) Homogeneous mixtures

(a) Solid solution. Alloys, e.g., brass, steel, bronze.

(b) Liquid solution. Sugar solution in water, e.g. sea water.

(c) Gaseous solution. e.g. air.

(ii) Heterogeneous mixtures

(a) Solid + Solid : sugar and sand

(b) Solid + Liquid : suspensions


(c) Solid + Gas : Smoke

(d) Liquid + Liquid : Milk

(e) Liquid + Gas : Soap bubbles.

Q6. Why is air a mixture and not a pure substance ? Ans. Air is a homogeneous mixture of several gases. The following points substantiate the fact :

1. The amounts of gases present in air at different places varies only marginally.

2. The different gases present in air are separable by a physical process i.e., fractional distillation of liquid air.

3. No definite chemical formula can be assigned to air.

4. The properties of air are the properties of its constituents, e.g. Air turns lime water milky due to the presence of CO² in it.

Q7. What is (i) a solute, (ii) solvent and (iii) solubility ? Ans. (i) Solute. The substance(s) present in smaller proportion in a solution is called solute.

(ii) Solvent. The substance present in larger proportion in a solution is called solvent.

(iii) Solubility. The maximum amount of the solid substance (solute) that can be dissolved in a given amount of the solvent is

termed its solubility at a given temperature.

Q8. Write the solute and the solvent in the following solutions :

(i) Sugar solution (ii) vinegar (iii) coke (iv) tincture of iodine. Ans. (i) In sugar solution, sugar is the solute and water is the solvent.

(ii) In vinegar, acetic acid is the solute and water is the solvent.

(iii) In coke, carbon dioxide is the solute and water is the solvent.

(iv) In tincture of iodine, solid iodine is the solute and alcohol is the solvent.

Q9. Give the main properties of a solution. Ans. 1. A solution is a homogeneous mixture.

2. The particles of a solution are smaller than 1 nm in diameter and can't be seen by naked eyes.

3. Because of their extremely small size, the particles of a solution do not scatter a beam of light passing throught the solution,

thus making the path of light in the solution invisible.

Q10. When a ray of light passes through a solution, it is not visible. Why ? Ans. The particles of a solution are smaller than 1 nm (10

–9 nm) in diameter. Such small particles do not scatter a beam of light

passing through them. So, the path of light is not visible in a solution.

11. What are alloys ? Ans. Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still,

an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For

example, brass is a mixture of approximately 30% zinc and 70% copper.

Q12. What do you mean by concentration of a solution ? Ans. The amount of solute present in a given mass or volume of solution or solvent is called concentration of the solution.

Concentration

=

Amount of solute

Amount of

solution

Or Concentration

=

Amount of solute

Amount of

solvent

Q13. Can you test whether a given solution is saturated or not ? How? Ans. Yes. We can do so by adding a pinch of the solute dissolve and stirring the solution. If the solid solute does not dissolve

and settles at the bottom of the container, it means that the solution is saturated. But, if the solute gets dissolved in the solution,

then it means that the solution is unsaturated.

Q14. What is the concentration of a solution which contains 12 g of urea in 160 g of solution ?

Ans. Mass of urea (solute) = 12 g

Mass of solution = 160 g

:. Mass percentage of

solution =

Mass of solute

×100 Mass of

solution

= 12

×100=7.5% 160


Q15. A saturated solution of a salt had been prepared at 25°C. 5 g of salt was recovered when 25 g of this solution was

evaporated completely. Calculate the amount of salt that was dissolved in 1000 gram of water while preparing the

solution. What is the solubility of this salt at 25°C ? Ans. Mass of the solution = 25 g

Mass of the salt obtained = 5 g

:. Mass of water = 25 g – 5 g = 20 g

Now, Mass of salt in 1000 g

water =

Mass of salt

×1000 Mass of

water

= 5g

×1000g = 250g 20g

Thus, 1000 g of water dissolved 250 g of salt. Q16. Why does a saturated solution becomes unsaturated on heating? Ans. On heating, the solubility of the salt increases and thus more salt can be dissolved at a higher temperature. This means that

the earlier amount of the dissolved salt is not the maximum. Thus, a saturated solution becomes unsaturated on heating.

Q17. How are aqueous solutions different from non-aqueous solutions? Ans. True solutions obtained in water are aqueous solution, e.g., vinegar. True solutions in organic liquids like alcohol, acetone

etc. are non-aqueous solutions, e.g., amino acids dissolved in acetone etc.

Q18. What is a suspension ? Ans. A suspension is a heterogeneous mixture of a solid in a liquid. It is formed when a liquid contains small solid particles

which are not soluble in it mixture. They are visible to the naked eye. If such a mixture is left undisturbed, the solid particles

slowly settle down to the bottom. But if the particle size of the solid is greater than 10–7

m (100 nm) and it is insoluble in the

given liquid, then the solid particles remain suspended in the liquid. This is called a suspension, e.g. mud water and sand water.

Q19. What are colloidal solutions ? Ans. A colloidal solution is neither a true solution nor a suspension. The particle size in a colloidal solution is between 1 nm to

100 nm. These particles are not visible to the naked eyes. But they are large enough to scatter the light that passes through the

dispersion medium, i.e. the medium of the colloidal solution, e.g. in fog, water droplets form the dispersed phase and air is

dispersing medium. Examples of colloidal solutions are milk, ink and blood.

Q20. Give the properties of colloids. Ans. The main properties of colloids are—

1. Colloids are heterogeneous mixtures.

2. The size of particles of a colloid is too small to be individually seen by naked eyes.

3. Colloids are big enough to scatter a beam of light passing through it and make its path visible.

4. They do not settle down when left undisturbed.

Q21. What do you mean by dispersed phase and dispersion medium ? Ans. In a colloidal solution, the colloidal particles constitute dispersed phase, while the medium in which the colloidal particles

are dispersed is called the dispersion medium.

For example, in the colloidal solution of starch in water, starch particles constitutes the dispersed phase and water constitutes the

dispersion medium.

Q22. What do you understand by Tyndall effect ? Why do we observe Tyndall effect when sunlight passes through a

dense forest ? Ans. The illumination of a beam of light due to scattering on collision with colloidal particles is called Tyndall effect. In a dense

forest, we see fog (tiny water droplets suspended in air) in the early morning. As these droplets are large enough to scatter beam

of light, Tyndall effect can be observed when sunlight passes through dense forest.

Q23. Why do we separate mixtures ? Ans. We separate mixtures :

1. to remove an undesirable component like stones etc. from wheat and rice.

2. to remove harmful components like insects from grains.

3. to remove soluble impurities from water.

4. to obtain a pure sample of a substance, and

5. to obtain a useful component such as butter from milk.

Q24. Name the methods commonly employed for separation of components of a mixture.


Ans. These are :

1. Evaporation 2. Crystallisation 3. Centrifugation 4. Distillation 5. Fractional distillation 6. Using separating funnel

7. Chromatography, and 8. Sublimation

How do we separate the non-volatile component from its volatile solvent ? Give some practical application of the method

employed. Ans. We separate the non-volatile component, i.e. the solute from its volatile solvent by evaporation method.

Some practical applications of evaporation are :

1. Drying of clothes.

2. Obtaining salt from sea water, and

3. Reducing the moisture content of foodgrains.

Q26. Where is the technique of evaporation used on a large scale ? Ans. The technique of evaporation is used on a large scale to obtain common salt from sea water. For this, the collected sea

water is put in large containers and is left in the open on sunny days. After a few days, the salt remains as residue whereas the

water evaporates. After that, the salt is further purified to get the common salt that we use in our food.

Q27. What do you mean by centrifugation ? On what principle is it based ? Ans. The process of separating lighter or heavier particles from a liquid by rotating the mixture at a very high speed is called

centrifugation.

Centrifugation is based on the principle that the denser particles are forced to the bottom and the lighter stay at the top when they

are spun at high speeds.

Q28. Give three applications of centrifugation. Ans. 1. It is used in diaries and homes to separate butter from cream.

2. It is used in washing machines to squeeze out water from the wet clothes.

3. It is used in diagnostic laboratories to test blood and urine samples.

Q29. Give two applications of separating funnel. Ans. 1. To separate a mixture of oil and water.

2. To extract iron from its ore the lighter gangue is removed from the top, leaving behind the molten iron at the bottom of the furnace.

Q30. How can a mixture of salt and ammonium chloride be separated? Ans. A mixture of salt and ammonium chloride is heated in a china dish till dense white fumes of ammonium chloride appear. Then an inverted

funnel is placed on the china dish. Ammonium chloride being sublime, will directly change from solid to gaseous state on heating, and the

vapours of ammonium chloride will deposit on the inner walls of the funnel. Thus, salt will be left behind in the china dish. This process is

called sublimation.

Q33. What is distillation and its importance ? Ans. The conversion of a liquid into the vapour and condensing the vapours back into liquid is known as distillation.

Importance.

(1) This method is used for the separation of components of a mixture containing two miscible liquids which boil without decomposition and

have sufficient differences in their boiling points.

(2) To separate a volatile component of a solution from a non-volatile component.

Q34. Which process is used to separate a mixture of two or more miscible liquids for which the difference in boiling point is less than

25 K ? Ans. To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K, fractional distillation

process is used, e.g. for the separation of different gases from air. The apparatus is similar to that for simple distillation, except that a

fractionating column is fitted in between the distillation flask and the condenser.

Q35. How is air separated into its constituents ? Ans. As we know, air is a homogeneous mixtures of several gases which have boiling points much below the room temperature. So separate

the different constituents of air, it is first of all brought to liquid form under higher pressure and reduced temperatures. Then, it is slowly

warmed up in fractionating column at a constant high pressure. Then we obtain different fractions of air in the gaseous form at different

temperatures.

Q31. By which methods you would separate camphor from sodium chloride ? Ans. 1. Sublimation. The mixture of camphor and sodium chloride is heated in a

china dish covered with an inverted funnel closed from the lower end. Camphor,

being sublime, changes directly to vapour state and the vapours collect on the inner

wall of the funnel. Sodium chloride is left as residue in china dish.

2. Filtration. Dissolve the mixture of camphor and NaCl in water. Sodium chloride

dissolves in water while camphor remains as residue. Then filter the contents. The

residue left on filter paper is camphor. Sodium chloride is then recovered from

water by evaporation.

Q32. What types of mixtures are separated by simple distillation ? Ans. Simple distillation is used for the separation.

1. Two miscible liquids that boil without decomposition and have sufficient

difference in their boiling points, like water and alcohol.

2. A solution containing a volatile component and a non-volatile component.


Q36. Briefly describe the process used to purify a solid from its impurities. How will you obtain pure sugar from a sample of impure

sugar ? Ans. The process of crystallization is used to purify a solid from its impurities. Take about 5 gm of a sample of impure sugar dissolved in

minimum amount of water and filter. Filter the impurities. Now saturate the solution by heating it in a china dish, till some solid starts

appearing.

Now cover the solution with filter paper and leave it undisturbed for a day. We obtain the crystals of pure sugar which are separated either by

decantation or filtration.

Q37. Give two advantages and disadvantages of crystallization over evaporation. Ans. Advantages :

1. We get fine crystals of the substance. 2. We can safely purify the solids which decompose on heating.

Disadvantages :

1. The liquids needs to be purified for further use. 2. Some substance remains with the liquid/solvent.

Q38. Write the composition of air. Which gas forms the liquid first as the air is cooled, and which gas boils up first when air boils up

first ? Ans. Air contains approximately

78% - Nitrogen

21% - Oxygen

1% - A mixture of other gases (like 0.03 % CO2).

As the air is cooled, oxygen will form liquid first. Nitrogen will boil up first when air boils.

Q39. Give two differences between simple distillation and fractional distillation.

Simple distillation Fractional distillation

1. By simple distillation, we can separate two miscible

liquids having sufficient difference in their boiling

points.

2. It is done by using an air condenser or a water

condenser.

1. By fractional distillation, we can separate a mixture of two or

more miscible liquids for which the difference in the boiling

points is less than 25°C.

2. A fractionating column is fitted in between the distillation flask

and the condenser for the process.

Q40. Give the main characteristics of a physical change. Ans. 1. No new substance is formed during a physical change.

2. A physical change is temporary and reversible.

3. The physical properties of the substance may change during a physical change.

4. There is no change in the composition of the substance during a physical change.

Q41. What is a chemical change ? Give a few examples. Ans. A chemical change is a permanent change in which the composition of the substance changes, giving rise to one or more new substances

with different properties.

Examples—(1) Formation of curd.

(2) Rusting of iron, and (3) Burning of wood.

Q42. Differentiate between physical and chemical changes. Ans. Differences between physical and chemical change :

Physical changes Chemical changes

1. Physical changes are temporary.

2. Physical changes take place only in properties such as

colour, physical state, density etc.

3. Chemical properties of the substance remain

unchanged even after the change.

4. We can obtain back the original substance(s) easily.

1. Chemical changes are permanent.

2. Chemical changes result in the formation of new chemical substance(s).

3. New substance(s) with different properties are formed.

4. Original substance(s) cannot be obtained back easily.

Q43. What is a compound ? Give two examples. Ans. A pure substance composed of atoms of two or more elements combined in a fixed proportion, is called a compound.

Example—H2O, NaCl

Q44. Write two differences between an element and a compound. Ans. 1. An element is composed of atoms of one kind only, while a compound is composed of atoms of different elements in a fixed proportion

by weight.

2. An element cannot be broken into more simpler form by chemical methods, while a compound can be decomposed into its constituents by

chemical methods.

Q45. Differentiate between mixtures and compounds.


Ans.

Mixtures Compounds

1. Elements or compounds just mix together and do not

react.

2. No new substance is formed.

3. A mixture shows the properties of the constituent

substances.

4. A mixture has a variable composition.

5. The constituents of a mixture can be separated easily by

physical methods.

1. Elements or compounds combine and react.

2. A new substance is formed.

3. The composition of the new substance is always fixed.

4. The new substance has totally different properties.

5. The constituents of a compound can be separated only by

chemical reactions.

Q46. Give two reasons why hydrogen is considered an element. Ans. Hydrogen is considered an element because :

1. Hydrogen has only one kind of atoms.

2. Hydrogen cannot be broken into new substances by ordinary physical or chemical methods.

Q47. Give two reasons why water is regarded as a compound. Ans. Water is regarded as a compound because

1. It contains atoms of two element i.e. H and O in a fixed proportion, i.e. 2 : 1.

2. Like all other compounds water has a fixed boiling point.

Q48. Give the main characteristics of compounds. Ans. 1. Compounds are the substances formed by chemical combination of atoms of two or more elements.

2. The constituents are present in a fixed ratio.

3. The constituent elements cannot be separated by physical processes.

4. A chemical reaction takes place during the formation of a compound

LONG ANSWER QUESTIONS

Q1. How does the meaning of a pure substance differ for a layman and a chemist ? Ans. For a layman, i.e. a pure substance means that it is not adulterated, i.e. pure milk to him means that it is not mixed with water or other

adulterants to thicken it. But for a chemist, milk is not a pure substance. It is a mixture of water, fats and proteins. These different materials in

milk are obtained from different sources. In the same way, for a common man pure fruit juice means that it is not mixed with fruit juice of

other fruits or water or salt. But for a chemist, fruit juice is a mixture of water and several other substances.

Q2. Give two differences between :

(a) Aqueous and non-aqueous solutions.

(b) Saturated and non-saturated solutions. Ans. (a) Differences between aqueous solution and non-aqueous solutions:

Aqueous Solution Non-aqueous Solution

1. Solutions prepared in water are called

aqueous solutions.

2. These solutions ionize easily and hence can

conduct electricity.

Example—Sugar solution and salt solution.

1. The solutions formed by dissolving substances in liquids other

than water, like alcohol, acetone etc. are called non-aqueous

solutions.

2. These solutions do not ionize and therefore do not conduct

electricity.

Example—Solution of sulphur dissolved in carbon disulphide and

solution of iodine dissolved in alcohol.

(b) Differences between saturated and unsaturated solutions :

Saturated Solution Unsaturated Solution

1. Saturated solution is one in which no more solute

can be dissolved at a given temperature.

2. In a saturated solution, the solute starts separating

from the solution in the form of crystalline solid when

the temperature of the solution is decreased.

1. An unsaturated solution is one in which no more amount

of solute can be dissolved without raising the temperature.

2. When the temperature of un-saturated solution is

decreased, it becomes saturated.

Q3. Give three differences each between : (a) True solution and Colloidal solution (b) Suspensions and Colloids. Ans. (a) Difference between true solution and colloidal solution :

True Solution Colloidal Solution

1. A true solution is a homogeneous mixture.

2. Size of solute particles is 10–8 cm.

3. It does not show Tyndall effect as it does not

scatter light.

Example : Salt solution and sugar solution.

1. A colloidal solution appears to be homogeneous but actually

is heterogeneous.

2. Size of solute particles is between 10–7 cm and 10–5 cm.

3. It shows Tyndall effect as it scatters a beam of light when the

latter is passed through it and makes its path visible.

Example : Milk and blood.


(b) Difference between Suspensions and Colloids :

Suspension Colloids

(or Colloidal solutions)

1. The particles of suspensions settle down on

keeping it for sometime.

2. Size of solute particles is larger than 10–5 cm.

3. The particles of suspensions can be seen with

naked eyes.

Examples : muddy water and chalk powder.

1. The particles of a colloidal solution do not settle down.

2. Size of solute particles is between 10–7 cm and 10–5 cm.

3. The particles of a colloidal solution cannot be seen with

naked eyes.

Examples : Milk and blood.

Q4. Give some common examples of colloids having different dispersed phase and dispersing medium in a tabulated form. Ans.

Dispersed phase Dispersing

medium Type Examples

Liquid

Solid

Gas

Liquid

Solid

Gas

Liquid

Solid

Gas

Gas

Liquid

Liquid

Liquid

Solid

Solid

Solid

Aerosol

Aerosol

Foam

Emulsion

Sol

Foam

Gel

Solid Sol

Fog, Clouds, Mist

Smoke, Automobile, Exhaust

Shaving Cream

Milk, Face Cream

Milk of Magnesia, Mud

Foam, Rubber, Sponge, Pumice

Jelly, Cheese, Butter

Coloured Gemstone, Milky Glass

Q8. What is centrifugation. How is it carried out ? Ans. The process of separating suspended particles from a liquid (like colloids) by churning the liquid at a high speed is called centrifugation.

It is based on the principle that denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.

The mixture is taken in a closed bottle and rotated at a high speed. The heavy particles settle at the bottom while the lighter particles remain

behind. For instance, to separate cream from milk, milk is churned for 2-3 minutes. We see that the cream collects at the centre and being

lighter than milk, floats at the top of the mixture. Thus, cream is separated from milk by centrifugation.

Q9. How will you show that the dye is blue/black ink is a mixture ? Ans. 1. We can take a thin strip of filter paper and draw a line on it approximately 3 cm above the lower edge with the help of a pencil.

2. Put a small drop of ink at the centre of the line and let it dry for some time.

3. Lower the filter paper into a glass jar containing water such that the drop of ink on the paper is just above the water level and leave it

undisturbed for some time.

4. Watch carefully, as the water rises up on the filter paper and reaches the ink drop, dissolves its components and rises upwards along them.

Q5. How do we obtain coloured components, i.e. dye from Blue/Black ink ? Ans. We can obtain dye from blue/black ink by the following process—

1. Fill half a beaker with water and put a watch glass on the mouth of the beaker.

2. Put few drops of ink on the watch glass.

3. Put the beaker on a stand. Put a burner below the stand and start heating the beaker. We

don't heat the ink directly as it may catch fire or the watch glass may break. We will see that

evaporation is taking place from the watch glass.

4. Continue heating as the evaporation goes on and stop heating when we see any further

change on the watch glass.

We observe that water evaporates, leaving behind a residue of dye.

Q6. Demonstrate filtration with the help of an experiment. Ans. Filtration. When one component of a mixture is soluble in water and the other

component is insoluble in water, the soluble component gets dissolved and the insoluble

component is separated by filtering the solution.

Experiment. Set up the apparatus as shown the figure below. Pass the solution through the

filter paper fitted in the funnel. We see that during filtration, the insoluble solid substance is

retained at the filter paper as residue while the liquid that is free from any suspended matter

passes through the filter paper and gets collected as filtrate.

We can obtain the soluble component from the filtrate by evaporation.

Q7. How will you separate by using a separating funnel ? Ans. Separation of liquid-liquid mixture by using separating funnel. This method is used

when the two liquids are immiscible, e.g. a mixture of carbon disulphide and water can be

separated by this method.

The mixture is taken in a separating funnel and the funnel is allowed to stand for sometime.

On standing, the liquid with higher density forms the lower layer whereas the liquid having

lower density forms the upper layer. Then the two liquids are taken out from the separating

funnel in separate conical flasks.


Q10. Describe the physical properties of (a) metals and (b) non-metals. Ans. (a) Physical properties of metals.

1. Metals are amorphous solids (Mercury is an exception which is a liquid at room temperature).

2. Metals are very hard (Sodium is an exception). Hardness varies from metal to metal.

3. Most of the metals have a metallic lustre. They have shining surfaces.

4. Metals show malleability, i.e. they can be beaten into sheets.

5. Metals are ductile because i.e. they can be drawn into wires of desired diameter.

6. Metals have very high melting points.

7. Metals possess tensile strength, and can be stretched up to some degree without fracturing.

(b) Physical properties of non-metals.

1. Non-metals are crystalline solids, liquids or gases.

2. They are less hard than metals.

3. They have a dull lustre, and do not reflect light well.

4. They are poor conductors of heat and electricity.

5. They are non-malleable and non-ductile.

6. Solid non-metals are brittle. They can be broken into pieces when hammered.

7. They have low boiling and melting points.

Q11. Describe distillation. How is it carried out ?

Ans. Distillation is a process employed to obtain pure liquid from its solution. It can be defined as the conversion of impure liquid into vapours

by evaporation and then condensation of the vapours to get pure liquid.

Impure liquid or mixture is taken in a flask, fitted with a condenser, as shown in the figure. On heating, the liquid gets evaporated and the

vapours pass to the delivery tube, where they condense into liquid. The pure condensed liquid is then collected in receiver. As the solid remains

behind in the flask, it gets separated.

This method is generally used for the separation of components of a mixture containing two miscible liquids that boil without decomposition

and their boiling points are quite different (more than 25-30°C).

We take the mixture of ammonium chloride and common salt in a china dish and cover it with an inverted funnel with its open end closed with

cotton wool. The walls of china dish are kept cooled. On heating, ammonium chloride sublimes and condenses on the cooler parts whereas

common salt is left behind. Then, with the help of a knife, we scrap off the fine powder of ammonium chloride deposited on the funnel.

MULTIPLE CHOICE QUESTIONS

Q1. Which of the following is a heterogeneous mixture ?

(a) A mixture of salt and water

(b) A mixture of sugar and water

(c) A mixture of alcohol in water

(d) A mixture of oil and water

Q2. The particles of which of the following do pass through the filter paper ?

(a) True solutions (b) Colloidal solutions

(c) Suspensions (d) All of these

Q3. The size of colloidal particles varies from

(a) 1-100 nm (b) 10-50 nm

(c) 1-10 nm (d) 10-100 nm

Q4. Tyndall effect is shown by a

(a) solution (b) true solution

Q12. Describe briefly how (a) Evaporation and (b) Sublimation is carried out. Ans. (a) Evaporation is a process of changing a liquid into its gaseous state by heating it at a

temperature below its boiling point, e.g., NaCl (common salt) can be obtained from a

mixture of common salt and water by evaporation.

The mixture is heated in a china dish using a sand bath. When water vaporises, NaCl is left

behind.

(b) Sublimation is used to the separate the components of a mixture in which one

component undergoes sublimation whereas the other does not, e.g. a mixture of ammonium

chloride and common salt can be separated by this method.


(c) colloidal solution (d) suspension

Q5. Solution of soap in water is a

(a) colloidal solution (b) true solution

(c) suspension (d) None of these

Q6. A gel is a colloidal solution of a

(a) liquid in solid (b) solid in solid

(c) gas in solid (d) None of these

Q7. Which of the following may be termed as a pure substance ?

(a) Aerosol (b) Sodium chloride

(c) Soil (d) All the above

Q8. A mixture of two miscible liquids can be separated by

(a) using a separating funnel (b) distillation

(c) centrifugation (d) evaporation

Q9. Dye can be separated from ink by



Q10. We can separate a mixture of two immiscible liquids by



Q11. We can separate cream from milk by



Q12. Which method will we employ to obtain pure copper sulphate from an impure sample ?

(a) Evaporation (b) Crystallisation

(c) Fractional distillation (d) Sublimation

Q13. We can obtain different gases from air by

(a) Evaporation (b) Crystallisation

(c) Fractional distillation (d) Sublimation

Q14. Which of the following metals is liquid at room temperature ?

(a) Sodium (b) Gold

(c) Nickel (d) Mercury

Q15. A mixture of ammonium chloride and sand can be separated by

(a) Sublimation (b) Fractional distillation

(c) Evaporation (d) Crystallisation

ANSWERS 1. (d) 2. (c) 3. (a) 4. (c) 5. (a) 6. (a) 7. (b) 8. (b) 9. (d) 10. (a) 11. (c) 12. (b) 13. (c) 14. (d) 15. (a)

deepak sir 9811291604 is matter around us pure...chemical changes results in the formation of new...

Documents