CBSE Class 9 Science Is matter around us pure Sure Shot Questions Set 01

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ASSIGNMENT QUESTIONS SET - 1
CHAPTER - 2
IS MATTER AROUND US PURE

1. On the basis of composition, how matter is classified?

Answer: On the basis of composition, matter is classified into pure substances and mixtures. Pure substances are further divided into elements and compounds.

Teacher's Note:
a) Pure substances have a fixed composition and fixed properties.
b) A mixture contains two or more substances that are not chemically combined.

2. Mention whether the following statements are true or false. Correct the false statements.
a) An aqueous solution of copper sulphate is homogeneous.
b) Milk is a pure substance.
c) A molecule of sulphur is monoatomic.

Answer:
a) True. The copper sulphate is evenly dissolved in water, so the solution has the same composition throughout.
b) False. Milk is a mixture of water, fats, proteins, sugars and minerals. Corrected: Milk is a mixture.
c) False. A molecule of sulphur (S₈) contains eight sulphur atoms, so it is polyatomic. Corrected: A molecule of sulphur is polyatomic.

Teacher's Note:
a) A homogeneous mixture looks uniform throughout, and a solution is an example.
b) Milk looks uniform but can be separated into cream and water-based parts by physical methods.
c) Monoatomic means one atom per molecule, as in helium or neon.

3. What is meant by a pure substance?

Answer: A pure substance is matter made of only one kind of particles, which have the same composition throughout. It has fixed physical and chemical properties.

Teacher's Note:
a) Pure water, pure gold and pure oxygen are examples of pure substances.
b) Even a small amount of impurity makes a substance impure.

4. What are the characteristics exhibited by a pure substance?

Answer:
1. A pure substance has a fixed composition.
2. It has definite physical properties, such as a fixed melting point and boiling point.
3. It has uniform properties throughout the sample.

Teacher's Note:
a) Fixed melting and boiling points are the most useful test of purity.
b) A pure substance cannot be separated into other substances by physical methods.

5. What are different categories of pure substance?

Answer: Pure substances are of two categories: elements and compounds.

Teacher's Note:
a) An element contains only one kind of atom, for example, oxygen.
b) A compound contains two or more elements chemically combined, for example, water.

6. Name two properties of a substance to check its purity?

Answer: Melting point and boiling point.

Teacher's Note:
a) A pure substance melts and boils at fixed temperatures.
b) An impure substance melts over a range of temperature and boils at a higher temperature.
c) Density is another property that can be used to check purity.

7. Define mixture.

Answer: A mixture is a substance made of two or more elements or compounds that are mixed together but not chemically combined.

Teacher's Note:
a) The components of a mixture keep their own properties.
b) The components can be separated by physical methods.

8. What are the kinds of mixture?

Answer: Mixtures are of two kinds: homogeneous mixtures and heterogeneous mixtures.

Teacher's Note:
a) A homogeneous mixture has uniform composition throughout, for example, sugar solution.
b) A heterogeneous mixture has visibly different parts, for example, sand and water.

9. What are the characteristics of mixture?

Answer:
1. A mixture does not have a fixed composition.
2. The components of a mixture keep their own properties.
3. The components can be separated by physical methods such as filtration or evaporation.
4. The components are not chemically combined, and no new substance is formed.

Teacher's Note:
a) A mixture of salt and water can have different proportions of salt.
b) No energy change of the kind seen in a chemical reaction takes place when a mixture is made.

10. List the points of differences between homogeneous and heterogeneous mixtures.

Answer:

Teacher's Note:
a) Look at the mixture under light: a homogeneous mixture looks the same from every part.
b) Heterogeneous mixtures often settle or separate on standing.

11. Identity which of the following is homogenous mixture or heterogeneous one. Also identify the type of constituents in mixture (e.g. gas in gas, gas in liquid, gas in solid etc.)
a. Air
b. Water and Oil (N2 + O2 )
c. Hydrogen in Palladium
d. Aerated Water (CO2 + H2O)
e. Chalk in water
f. Ethyl Alcohol in Water
g. Alloys e.g. brass
h. Dust (e.g. fine sand) in water
i. Sand + iron fillings
j. Sand + ammonium chloride
h. milk
i. Mercury in amalgamated Zinc

Answer:

a. Air: homogeneous mixture; gas in gas.
b. Water and oil: heterogeneous mixture; liquid in liquid (immiscible).
c. Hydrogen in palladium: homogeneous mixture; gas in solid.
d. Aerated water: homogeneous mixture; gas in liquid.
e. Chalk in water: heterogeneous mixture; solid in liquid.
f. Ethyl alcohol in water: homogeneous mixture; liquid in liquid.
g. Alloy (brass): homogeneous mixture; solid in solid.
h. Dust in water: heterogeneous mixture; solid in liquid.
i. Sand and iron filings: heterogeneous mixture; solid in solid.
j. Sand and ammonium chloride: heterogeneous mixture; solid in solid.
h. Milk: heterogeneous mixture (a colloid); liquid in liquid.
i. Mercury in amalgamated zinc: homogeneous mixture; liquid in solid.

Teacher's Note:
a) The text "(N2 + O2)" in item b is the composition of air, so it is not part of water and oil.
b) Two items are labelled "h" in the paper; both are answered above in the order printed.
c) Milk looks uniform to the eye, but under a microscope the fat droplets are visible, so it is classed as heterogeneous.

12. What are the constituents of brass?

Answer: Brass is an alloy of copper and zinc.

Teacher's Note:
a) Brass is made by mixing copper and zinc in suitable proportions.
b) Alloys are harder than their pure metals and are widely used.

13. Alloys cannot be separated by physical means, though it is considered mixture, Why?

Answer: The constituent metals of an alloy are so intimately mixed, and they form a single uniform solid, that they cannot be separated by simple physical methods.

Teacher's Note:
a) Alloys are homogeneous mixtures of metals with other metals or non-metals.
b) Their metals can only be separated by chemical methods.

14. What are elements?

Answer: Elements are pure substances made of only one kind of atoms.

Teacher's Note:
a) Oxygen, iron and copper are examples of elements.
b) An element cannot be broken down into a simpler substance by chemical reactions.

15. How elements are further classified?

Answer: Elements are classified into metals, non-metals and metalloids.

Teacher's Note:
a) Metals are generally lustrous, malleable and good conductors.
b) Non-metals are generally dull, brittle and poor conductors.
c) Metalloids show properties of both metals and non-metals, such as silicon and germanium.

16. What is a compound? Give an example.

Answer: A compound is a pure substance formed by two or more elements chemically combined in a fixed ratio by mass. Example: water (H₂O).

Teacher's Note:
a) Water always contains hydrogen and oxygen in the ratio 1:8 by mass.
b) The properties of a compound are different from those of its elements.

17. State the differences between compounds and mixtures.

Answer:

Teacher's Note:
a) Salt is a compound, but a mixture of salt and sand is a mixture.
b) Look for a fixed ratio and new properties to identify a compound.

18. What is a solution? What are the properties of a solution?

Answer:
A solution is a homogeneous mixture of two or more substances.
Properties:
1. It is homogeneous and its composition is uniform throughout.
2. Its particles are smaller than 1 nm and cannot be seen.
3. The particles do not scatter light, and the solution is transparent.
4. The solute does not settle on standing, and it cannot be separated by filtration.

Teacher's Note:
a) Sugar dissolved in water is a solution, and sugar does not settle when left undisturbed.
b) Filter paper cannot separate the solute from a true solution.

19. What is meant by solute and solvent?

Answer: The solute is the substance that dissolves in a solvent. The solvent is the substance in which the solute dissolves, and it is usually present in the larger amount.

Teacher's Note:
a) In a salt solution, salt is the solute and water is the solvent.
b) Water is called the universal solvent because it dissolves many substances.

20. Identify solute and solvent in the following solutions. Also mention the physical state of solute and solvent.
(a) Sugar in water
(b) Urea in water
(c) Ammonium chloride in water
(d) Ethyl alcohol in water
(e) Carbon Di-Oxide in water (soda water)

Answer:

Teacher's Note:
a) In (d), both substances are liquids, so the one present in the larger amount, water, is taken as the solvent.
b) In a solution of a gas in a liquid, the gas is always the solute.

21. What is meant by Solubility?

Answer: Solubility is the maximum amount of a solute (in grams) that can be dissolved in 100 g of a solvent at a given temperature to form a saturated solution.

Teacher's Note:
a) Solubility always refers to a particular temperature.
b) The unit of solubility is grams of solute per 100 g of solvent.

22. What factors affect the solubility of solvent and solute?

Answer:
1. Nature of the solute and the solvent. Like dissolves like.
2. Temperature. Solubility of most solids increases with temperature.
3. Pressure. It has a major effect on the solubility of gases in liquids.

Teacher's Note:
a) Common salt dissolves well in water but not in petrol, which shows the role of nature.
b) Solubility of gases in water decreases when temperature rises.

23. Why do fish go in deep waters during day light?

Answer: During the day, the surface water becomes warmer. Oxygen is less soluble in warm water, so the surface water has less dissolved oxygen. Fish move to deeper, cooler water, which has more dissolved oxygen.

Teacher's Note:
a) Fish need dissolved oxygen to breathe, and the amount depends on temperature.
b) The solubility of a gas in a liquid decreases as temperature increases.

24. Based on the type of solvent, how solutions are classified?

Answer: Solutions are classified as aqueous solutions, where water is the solvent, and non-aqueous solutions, where a solvent other than water is used.

Teacher's Note:
a) Salt water is an aqueous solution.
b) Iodine dissolved in alcohol is a non-aqueous solution.

25. Based on the amount of solute in the given solution, how solutions are classified?

Answer: Solutions are classified as unsaturated, saturated and supersaturated, according to the amount of solute dissolved at a given temperature.

Teacher's Note:
a) An unsaturated solution can still dissolve more solute.
b) A supersaturated solution holds more solute than it can normally hold at that temperature.

26. What are aqueous solutions?

Answer: Aqueous solutions are solutions in which water is the solvent.

Teacher's Note:
a) Sugar solution and salt solution are aqueous solutions.
b) Most biological processes take place in aqueous solutions.

27. What are non-aqueous solutions?

Answer: Non-aqueous solutions are solutions in which a solvent other than water is used, for example, alcohol, ether, kerosene or benzene.

Teacher's Note:
a) Paints and varnishes are often prepared using non-aqueous solvents.
b) Iodine dissolves in alcohol to form a non-aqueous solution.

28. When we open the cap of a cola drink (or any carbonated beverage), why does excess of bubbles come out?

Answer: The cola drink contains carbon dioxide dissolved under high pressure. When the cap is opened, the pressure falls, the solubility of carbon dioxide decreases, and the excess gas escapes as bubbles.

Teacher's Note:
a) The solubility of a gas in a liquid increases with pressure.
b) Warm drinks lose their fizz faster because the solubility of the gas also falls with temperature.

29. Why air is a mixture not a compound? Give reasons.

Answer:
1. Air does not have a fixed composition, and the amounts of its gases vary from place to place.
2. The gases in air keep their own properties, such as the smell and the supporting of combustion by oxygen.
3. The gases in air can be separated by physical methods, such as fractional distillation of liquid air.

Teacher's Note:
a) Nitrogen and oxygen are present in air, but they are not chemically combined.
b) No new substance is formed when air is made.

30. What are the advantages of preparing solutions?

Answer:
1. Substances in solution react faster, because the particles are well mixed.
2. Medicines in solution are absorbed by the body more quickly.
3. Solutions are easy to handle, measure and transport.

Teacher's Note:
a) Syrups and liquid medicines are given in solution form for this reason.
b) Solids in solution can be measured accurately by volume.

31. What is an unsaturated solution?

Answer: An unsaturated solution is a solution in which more solute can be dissolved at the given temperature.

Teacher's Note:
a) A small amount of salt in a glass of water gives an unsaturated solution.
b) Adding more solute to it still dissolves the solute.

32. What is saturated solution? Explain with an example.

Answer: A saturated solution is a solution which holds the maximum amount of solute at a given temperature, so no more solute can be dissolved. Example: when common salt is added to water at 25°C until some salt stays undissolved, the solution is saturated.

Teacher's Note:
a) A saturated solution and undissolved solute can exist together.
b) Saturation depends on temperature, so a solution can become unsaturated when it is heated.

33. What is supersaturated solution?

Answer: A supersaturated solution contains more solute than it can normally hold at that temperature. It is unstable, and the extra solute separates out as crystals when a seed crystal is added or the solution is disturbed.

Teacher's Note:
a) A hot saturated solution of sodium acetate is cooled to make a supersaturated solution.
b) Supersaturated solutions are prepared by cooling a hot saturated solution slowly.

34. What are suspensions? Explain with an example.

Answer: A suspension is a heterogeneous mixture in which particles of a solid are spread throughout a liquid or gas but do not dissolve. Example: chalk powder mixed in water.

Teacher's Note:
a) The particles of a suspension are larger than 1000 nm and visible to the eye.
b) Shake the bottle well before use, because the particles settle on standing.

35. What are the properties of suspensions?

Answer:
1. A suspension is heterogeneous.
2. The particles are visible to the naked eye.
3. The particles settle down on standing.
4. The particles scatter light and are separated by filtration.

Teacher's Note:
a) Muddy water is a common example of a suspension.
b) The particles are too big to pass through filter paper.

36. Explain with an example what is a colloid?

Answer: A colloid is a heterogeneous mixture in which the dispersed particles are between 1 nm and 1000 nm in size and do not settle on standing. Example: milk, in which fat droplets are dispersed in water.

Teacher's Note:
a) Colloids look uniform but are not true solutions.
b) Smoke, fog, butter and jelly are also examples of colloids.

37. What are the physical states of dispersed phase and dispersion medium of a cloud?

Answer: In a cloud, the dispersed phase is liquid (tiny water droplets), and the dispersion medium is gas (air).

Teacher's Note:
a) Clouds are colloids of liquid droplets in air.
b) The dispersed phase is the substance that is spread out, and the dispersion medium is the substance that surrounds it.

38. What are the physical states of dispersed phase and dispersion medium of a fog?

Answer: In fog, the dispersed phase is liquid (water droplets), and the dispersion medium is gas (air).

Teacher's Note:
a) Fog is a colloid similar to a cloud, formed near the ground.
b) Fog droplets scatter light, which reduces visibility.

39. What are the properties of a colloid?

Answer:
1. The particles scatter light, which shows the Tyndall effect.
2. The particles show Brownian movement.
3. The particles do not settle on standing.
4. The particles cannot be separated by filtration, but can be separated by ultra-filtration or centrifugation.

Teacher's Note:
a) The random zigzag motion of colloidal particles is due to collisions with the molecules of the medium.
b) Colloidal particles often carry an electric charge, which keeps them from joining together.

40. How are sol, solution and suspension different from each other?

Answer:

Teacher's Note:
a) A sol is a colloid in which a solid is dispersed in a liquid, for example, starch in water.
b) Suspension particles are visible, while solution particles are invisible.

41. What is Tyndall effect? Doee true solution exhibit Tyndall effect.

Answer: The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a beam of light visible. A true solution does not exhibit the Tyndall effect.

Teacher's Note:
a) A torch beam passing through a dusty room shows a visible light path because of this effect.
b) Solution particles are too small to scatter light, so the beam is not seen.

42. "Tyndall effect can be observed when sunlight passes through the canopy of dense forest. or we see a rich red sunset." Explain how this occurs

Answer:
1. Sunlight passing through the canopy of a dense forest meets tiny droplets and dust particles in the air, which are colloidal in size.
2. These particles scatter the light, making the path of the sunlight visible.
3. At sunset, sunlight travels a longer path through the atmosphere. Most of the blue light is scattered away, and the red light reaches the eye, giving a red sunset.

Teacher's Note:
a) The blue light is scattered more strongly than red light by the particles in the air.
b) The visible light path through the forest is an example of the Tyndall effect.

43. What do you mean by strength of the solution?

Answer: The strength of a solution is the amount of solute present in a given amount of the solution. It is also called the concentration of the solution.

Teacher's Note:
a) A solution with more solute in the same volume is more concentrated.
b) Concentration can be expressed in several ways, as in the next question.

44. What are the various methods to express concentration of a solution?

Answer:
1. Mass by mass percentage (w/w %) = mass of solute / mass of solution × 100.
2. Mass by volume percentage (w/v %) = mass of solute / volume of solution × 100.
3. Volume by volume percentage (v/v %) = volume of solute / volume of solution × 100.
4. Parts per million (ppm) = mass of solute / mass of solution × 10⁶.

Teacher's Note:
a) Always check whether the question asks for the mass of the solution or only the solvent.
b) Parts per million is used for very dilute solutions, such as pollutants in water.

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

Answer:
1. Mass of solute (NaCl) = 36 g.
2. Mass of solvent (water) = 100 g.
3. Mass of solution = 36 + 100 = 136 g.
4. Concentration (w/w %) = \( \frac{36}{136} \) × 100 = 26.47%.
5. Hence, the concentration of the saturated solution is 26.47% by mass.

Teacher's Note:
a) The mass of the solution is the mass of solute plus the mass of solvent, not the solvent alone.
b) At 293 K the solution is saturated, so 36 g is the solubility of sodium chloride at this temperature.
c) Keep at least two decimal places for percentage answers.

46. Calculate the mass of glucouse and mass of water required to make 200g of 25% solution of glucouse.

Answer:
1. Concentration = 25%, so the mass of glucose in 100 g of solution is 25 g.
2. Mass of glucose required = \( \frac{25}{100} \) × 200 = 50 g.
3. Mass of water required = 200 - 50 = 150 g.

Teacher's Note:
a) Check the answer by adding the two masses: 50 g + 150 g = 200 g.
b) Water is the solvent, so its mass is the solution mass minus the glucose mass.

47. A solution contains 40 mL of ethyl alcohol mixed with 100mL of water. What is the concentration of the solution in terms of volume by volume percentage?

Answer:
1. Volume of solute (ethyl alcohol) = 40 mL.
2. Volume of solution = 40 + 100 = 140 mL.
3. Volume by volume percentage = \( \frac{40}{140} \) × 100 = 28.57%.

Teacher's Note:
a) This answer assumes the volumes are additive, which is the standard assumption at school level.
b) If the solvent volume were used alone, the value would be 40%, which is incorrect for volume by volume percentage.

48. What are different ways to separate solid mixtures?

Answer: Handpicking, threshing, winnowing, sieving, magnetic separation, sedimentation and decantation, filtration, evaporation, sublimation, crystallisation and chromatography.

Teacher's Note:
a) The method chosen depends on the properties of the components, such as size, density or magnetism.
b) Sublimation is useful when one component turns directly from solid to vapour.

49. Describe sieving method.

Answer:
1. The mixture is placed on a sieve with holes of a particular size.
2. The sieve is shaken, so the fine particles pass through the holes.
3. The bigger particles are left on the sieve, and the two parts are separated.

Teacher's Note:
a) Flour is sieved to remove bran and bigger particles.
b) The method depends on a difference in particle size.

50. Winnowing works on what property?

Answer: Winnowing works on the difference in density (weight) of the particles.

Teacher's Note:
a) Light husk is blown away by wind, while heavier grains fall straight down.
b) Farmers use winnowing to separate grain from husk.

51. What are the reasons for separating the constituents of a mixture?

Answer:
1. To obtain a pure substance from a mixture.
2. To use each constituent for its own purpose.
3. To remove harmful or unwanted substances.
4. To recover valuable components.

Teacher's Note:
a) Metals are separated from their ores so that they can be used.
b) Removing impurities from drinking water makes it safe to use.

52. When it is useful to apply sedimentation and decantation? Give an example where these methods are used? Explain the process.

Answer:
1. These methods are useful when the solid is insoluble and heavier than the liquid, and the two are kept in a container.
2. The mixture is left undisturbed, so the heavier particles settle at the bottom.
3. The clear liquid above is then carefully poured out, leaving the solid behind. Example: separating sand from water.

Teacher's Note:
a) Decantation must be done slowly so that the settled solid is not disturbed.
b) Water treatment plants use sedimentation before filtration.

53. What principle is applied in centrifugation? Give examples where this method is applied to separate mixtures.

Answer: Centrifugation works on the principle that particles of different densities move through a liquid at different rates when spun fast. Heavier particles move outward and settle faster. Examples: separation of cream from milk and separation of blood cells from plasma.

Teacher's Note:
a) A washing machine uses the same principle to remove water from clothes.
b) Centrifugation separates a mixture quickly, which sedimentation cannot do.

54. How crystallization is better than evaporation?

Answer:
1. Crystallisation gives pure crystals of the solid, while evaporation leaves a solid mass with impurities.
2. Crystallisation separates the dissolved solid from the solution in a regular form that can be collected and dried.
3. Crystallisation can be used to purify a solid that has impurities dissolved in it.

Teacher's Note:
a) Evaporation of a salt solution leaves the solid with some impurities still present.
b) Crystals form slowly on cooling, and slow growth gives purer crystals.

55. Name the technique to separate (i) butter from curd (ii) salt from sea-water (iii) camphor from salt

Answer:
(i) Butter from curd: centrifugation (or churning).
(ii) Salt from sea-water: evaporation.
(iii) Camphor from salt: sublimation.

Teacher's Note:
a) Churning separates butter, which is lighter, from the buttermilk.
b) Camphor turns directly from solid to vapour on heating, and it is collected again as a solid.

56. What is Chromatography?

Answer: Chromatography is a technique for separating the components of a mixture, based on the different rates at which they move through a medium (such as paper) carried by a solvent.

Teacher's Note:
a) Components that stick less to the paper travel farther up.
b) Chromatography is used to separate inks, dyes and plant pigments.

57. A good method to separate alum (phitkari) from impure samples is (a) Filtration (b) Sedimentation (c) Crystallization (d) Sublimation

Answer: (c) Crystallization

Teacher's Note:
a) Alum dissolves in hot water and forms crystals as the solution cools.
b) Insoluble impurities are removed by filtration during the process.

58. A boy buys common salt from the market which is contaminated with Ammonium Chloride (NH4Cl) and sand. The procedure he should adopt to obtain pure NaCl is the following :
(a) to mix the sample in water and evaporate the solution
(b) to mix the sample in water and evaporate the decanted solution
(c) to mix the sample in acetone and evaporate the decanted solution
(d) to heat the sample, then mix in water and evaporate the decanted solution

Answer: (d) to heat the sample, then mix in water and evaporate the decanted solution

Teacher's Note:
a) Ammonium chloride sublimes on heating, so it is removed as vapour.
b) Salt dissolves in water and sand settles, so the clear solution is decanted and evaporated.

59. Define Brownian movement in colloids.

Answer: Brownian movement is the random, zigzag motion of colloidal particles in a dispersion medium, caused by the unequal collisions of the medium's molecules with the particles.

Teacher's Note:
a) Brownian movement is seen clearly under a microscope.
b) It provides evidence that the molecules of a liquid are in constant motion.

60. Name the following :
(a) a lustrous liquid metal.
(b) a liquid non-metal
(c) a metal which can be cut with a knife
(d) a non-metal which is good conductor of electricity.
(e) an element which melts when kept on the palm.
(f) the best conductor of heat.

Answer:
(a) Mercury.
(b) Bromine.
(c) Sodium (or potassium).
(d) Graphite.
(e) Gallium.
(f) Silver.

Teacher's Note:
a) Mercury is the only metal that is liquid at room temperature.
b) Graphite is a form of carbon and conducts electricity through its free electrons.
c) Gallium has a melting point of about 30°C, just above room temperature.

61. How many elements are there which are in gaseous state at room temperature?

Answer: There are 11 elements which are gases at room temperature.

Teacher's Note:
a) The gaseous elements include hydrogen, nitrogen, oxygen, fluorine and chlorine.
b) The noble gases helium, neon, argon, krypton, xenon and radon are also gases.

62. Name the elements are in liquid state at room temperature.

Answer: Mercury and bromine are the two elements that are liquid at room temperature.

Teacher's Note:
a) Mercury is a metal, and bromine is a non-metal.
b) Very few elements are liquid at normal temperature.

63. Who used the term 'element' first time?

Answer: Robert Boyle first used the term 'element'.

Teacher's Note:
a) Boyle is also known for Boyle's law of gases.
b) The idea of elements was developed further by later chemists such as Lavoisier.

64. Who gave the first explanatory definition of 'element'?

Answer: Antoine Lavoisier gave the first explanatory definition of an element.

Teacher's Note:
a) Lavoisier is often called the father of modern chemistry.
b) He defined an element as a substance that cannot be broken down by any chemical means.

65. Identify the following as mixture or compound. (i) blood (ii) common salt (iii) sugar (iv) brass

Answer:
(i) Blood: mixture.
(ii) Common salt: compound.
(iii) Sugar: compound.
(iv) Brass: mixture (alloy).

Teacher's Note:
a) Blood contains cells, plasma and other substances that are not chemically combined.
b) Common salt (sodium chloride) and sugar (sucrose) have fixed compositions.

66. Sasha heats a container carrying Nitrogen and Oxygen. After heating at very high temperature, it gives Nitric oxide. Identify what are mixtures and/or compounds before and after the reaction.

Answer:
1. Before the reaction, nitrogen and oxygen are both elements, and their mixture is a mixture of two elements.
2. After the reaction, nitric oxide (NO) is formed, which is a compound of nitrogen and oxygen.
3. Any gas left unreacted keeps the mixture of elements together with the compound.

Teacher's Note:
a) Nitrogen and oxygen are elements, so a mixture of them is a mixture of elements.
b) Nitric oxide has a fixed composition, so it is a compound.

67. In beaker A, sugar cubes are dissolved into water while in beaker B, crushed cubes are taken. In which beaker the rate of dissolution is faster?

Answer: In beaker B, the crushed cubes dissolve faster.

Teacher's Note:
a) Crushing increases the surface area of the sugar exposed to water.
b) Stirring and warming the water also increase the rate of dissolution.

68. Identify solute and solvent in the following solutions :
(i) aerated drinks
(ii) tincture of iodine
(iii) lemon water

Answer:
(i) Aerated drinks: solute is carbon dioxide (gas); solvent is water (liquid).
(ii) Tincture of iodine: solute is iodine (solid); solvent is alcohol (liquid).
(iii) Lemon water: solute is lemon juice (acid), which is dissolved in water; solvent is water (liquid).

Teacher's Note:
a) Carbon dioxide is the gas that makes the drink fizzy.
b) Iodine dissolves in alcohol to form tincture of iodine.
c) Water is the solvent in most drinks, because it is present in the largest amount.

69. State the principle of each of the following methods of separation of mixtures.
(i) centrifugation method.
(ii) separation using separating funnel.

Answer:
(i) Centrifugation: particles of different densities move at different rates when the mixture is spun at high speed, so the heavier particles settle first.
(ii) Separating funnel: two immiscible liquids of different densities separate into two layers, and the heavier layer can be drained out from the bottom.

Teacher's Note:
a) Centrifugation is faster than sedimentation for fine particles.
b) Oil and water are separated in a separating funnel because they do not mix.

70. Why solutions do not exhibit Tyndall effect?

Answer: The particles of a solution are very small, less than 1 nm, so they are too small to scatter light. Therefore, the path of the light beam is not visible.

Teacher's Note:
a) The Tyndall effect needs particles large enough to scatter light, which colloids have.
b) A solution is transparent for this reason.

71. What is an emulsion? Give examples

Answer: An emulsion is a colloid in which a liquid is dispersed in another liquid. Examples: milk, butter, and face cream.

Teacher's Note:
a) In milk, fat droplets are dispersed in water.
b) Oil and water do not mix, but an emulsifier helps to keep them together as an emulsion.

72. What are the differences and similarities between concentration and solubility?

Answer:
Similarity: both describe the amount of solute present in a solution, and both are measured with respect to a fixed quantity of solution or solvent.
Difference: solubility is the maximum amount of solute that dissolves in 100 g of solvent at a given temperature, and it gives a saturated solution. Concentration is the amount of solute present in a given amount of solution, and it can have any value, including values for unsaturated solutions.

Teacher's Note:
a) Solubility depends on temperature, but concentration can be changed by adding more solvent.
b) A saturated solution has a concentration equal to its solubility value.

73. Give examples of liquids that are
(i) completely miscible
(ii) partially miscible
(iii) practically immiscible

Answer:
(i) Completely miscible: ethyl alcohol and water.
(ii) Partially miscible: phenol and water.
(iii) Practically immiscible: oil and water.

Teacher's Note:
a) Miscible liquids mix in all proportions to form a single layer.
b) Partially miscible liquids dissolve in each other only up to a limit, and then form two layers.