CBSE Class 7 Science Advanced Chemistry Classification of Matter Long Answer Questions Set 01

Long Answer Questions

Teacher's Note:
1. Learn the table row by row.
2. Compressibility and diffusion are the favourite exam points.

Teacher's Note:
1. Always add exceptions in brackets.
2. Ten rows give a full-mark answer.

Question 3. Explain the following methods of separation with a suitable example. (a) distillation (b) filtration
Answer:
(a) Distillation separates a nonvolatile solid dissolved in a liquid. Example: copper sulphate solution.
1. Take the solution in a round-bottomed distillation flask fitted with a thermometer and a Liebig's condenser. Keep the flask on a tripod and heat it with a Bunsen burner.
2. The solution boils and water vapour passes into the condenser, where cold water in the jacket cools it back to liquid water.
3. The water is collected in the receiver, and copper sulphate remains in the flask.

(b) Filtration separates an insoluble solid from a liquid. Example: sawdust and water.
1. Fold a filter paper into a cone and fit it in a funnel, moistening it with a few drops of water.
2. Pour the mixture slowly into the cone and collect what drips in another beaker.
3. The clear liquid (filtrate) passes through, and the sawdust (residue) stays on the filter paper.

In simple words: Distillation boils off and re-condenses a liquid to leave the dissolved solid; filtration holds back an insoluble solid on paper.

Teacher's Note:
1. Draw and label both diagrams.
2. Name the filtrate, the residue and the receiver.

Question 4. Explain different processes of interconversions of states of matter.
Answer:
The state of matter depends on temperature and pressure. Changing them converts matter from one state to another. The changes are:
1. Melting: solid to liquid on heating, at the melting point (ice to water at 0°C).
2. Boiling: liquid to gas from the bulk of the liquid at the boiling point (water to vapour at 100°C). A similar change at the surface, at any temperature below the boiling point, is evaporation.
3. Condensation: gas to liquid on cooling (steam to water).
4. Solidification (freezing): liquid to solid on cooling, at the freezing point (water to ice at 0°C).
5. Sublimation: a solid changes directly to vapour without becoming liquid (camphor, iodine). The vapour on cooling gives back the solid, called the sublimate.

In simple words: Heating and cooling move matter between solid, liquid and gas through melting, boiling, condensation, freezing and sublimation.

Teacher's Note:
1. Draw the triangular figure of interconversion.
2. Give the example of water for each change.

Question 5. Describe the process of separation of constituents of gunpowder.
Answer:
Gunpowder contains potassium nitrate (nitre, \(\ce{KNO3}\)), carbon and sulphur.
1. Add carbon disulphide to gunpowder in a beaker. Sulphur dissolves in it.
2. Filter the mixture. The filtrate is the sulphur solution; the residue is carbon and nitre.
3. Pour the filtrate in a flat vessel and leave it in the open air. Carbon disulphide evaporates and sulphur is left.
4. Transfer the residue to hot water. Nitre dissolves in it, but carbon does not.
5. Filter again. Carbon is left on the filter paper as residue; dry it.
6. Evaporate the filtrate to dryness by heating. Solid nitre is obtained.
In simple words: Dissolve sulphur first, then dissolve nitre in hot water, leaving carbon behind.

Teacher's Note:
1. Remember the order of solvents: carbon disulphide first, hot water second.
2. Name what is left at each filtration.

Question 6. Give reasons for the following. (a) Leaves of plants appear to wilt in summer afternoons. (b) When a perfume is sprayed on hand, we feel cool. (c) Leaves of submerged aquatic plants contain wax coating. (d) Perspiration is greater in coastal areas than in non-coastal areas.
Answer:
(a) In the hot summer afternoon the rate of evaporation of water from the leaves is high. The leaves lose water faster than the roots can supply it, so they droop (wilt).
(b) A perfume is a volatile liquid. It evaporates quickly and takes the heat it needs from the skin, so the hand feels cool.
(c) The wax coating is water-repellent. Water does not stick to wax because the adhesive force between water and wax is weak compared with the cohesive force among water molecules. So water does not cling to or wet the leaves.
(d) Coastal air is humid, so sweat evaporates slowly and remains on the body for long. Dry non-coastal air makes sweat evaporate quickly, so we feel less perspiration there.
In simple words: Evaporation is quick in dry, hot air, takes heat away, and is slow in humid air.

Teacher's Note:
1. Part (c) uses cohesive and adhesive forces, the rest use evaporation.
2. Mention humidity in part (d).

Question 7. Explain sublimation with a suitable example.
Answer:
Sublimation is the process in which some solids change directly into vapour on heating, without passing through the liquid state. The vapour changes back into a solid on cooling. The solid obtained is the sublimate, and the vapour is the sublime.
Example: separation of ammonium chloride from common salt.
1. Put the mixture in a flat basin, cover it with an inverted funnel wrapped with wet filter paper, and plug the stem with cotton.
2. Place the basin on a sand bath and heat it gently.
3. Ammonium chloride sublimes, and its white vapour condenses on the cool funnel wall as white powder. Common salt remains in the basin.

In simple words: A sublimable solid turns to vapour on heating and back to solid on the cooler funnel wall, leaving the other solid behind.

Teacher's Note:
1. Camphor and iodine are other examples.
2. Label the funnel, the plug and the sand bath.

Teacher's Note:
1. Four parameters cover the question.
2. Linking packing, space and force makes the answer logical.

Question 9. Earthen pitchers are more effective in Hyderabad than in Chennai. Justify.
Answer:
The rate of evaporation of water depends on humidity (moisture in the air). The more the humidity, the less the rate of evaporation.
Chennai is a coastal city, so the air has more moisture and evaporation is slow. Hyderabad is a non-coastal city with drier air, so evaporation is faster.
A higher rate of evaporation cools the water in the earthen pitcher faster. So pitchers work better in Hyderabad.
In simple words: Dry air in Hyderabad lets water evaporate through the pot faster, so the water cools more.

Teacher's Note:
1. Humidity is the key word.
2. Evaporation takes heat from the water in the pot.

Question 10. How is milk powder prepared from milk?
Answer:
1. Milk is sprayed through an atomizer, which breaks it into very small droplets.
2. The droplets have a very large surface area, so the water in them evaporates rapidly at room temperature.
3. The water is entirely lost by evaporation, and the fat and solid particles are left behind.
4. These are converted into powder by adding other ingredients.
In simple words: Spraying milk into tiny drops makes the water evaporate quickly, leaving a dry powder.

Teacher's Note:
1. Larger surface area speeds up evaporation.
2. Only the water escapes, not the solids.

Question 11. Why is mercury used in a thermometer? Is it suitable for measuring high temperatures? Name the liquid which can be used for the measurement of high temperatures. Give reasons.
Answer:
Mercury expands on heating, and it has weak forces of attraction towards glass, so it does not stick to the glass tube. Hence it is used in thermometers.
It is not suitable for measuring high temperatures because it boils at a comparatively low temperature (about 357°C).
Gallium can be used for high temperatures, because it has a very high boiling point.
In simple words: Mercury is used because it expands regularly and does not stick to glass; for very high temperatures gallium is better.

Teacher's Note:
1. Link the answer to the boiling point of the liquid.
2. The adhesive force of mercury with glass is weak.

Question 12. Two thermometers A and B are dipped in water and alcohol respectively taken in two containers of similar dimensions at room temperature. Compare the temperatures shown by these two thermometers giving appropriate reasons.
Answer:
Alcohol evaporates much faster than water because its surface molecules have high kinetic energy and escape easily.
Evaporation causes cooling, so the alcohol cools more than the water.
So thermometer B (alcohol) shows a lower temperature than thermometer A (water). The mercury level falls more in B.
In simple words: Alcohol evaporates faster than water, so it cools more and its thermometer reads lower.

Teacher's Note:
1. Rate of evaporation depends on the nature of the liquid.
2. More evaporation means more cooling.

Question 13. Naturally occurring diamonds are sometimes found in different colours. Give reasons.
Answer:
Natural diamonds contain small amounts of impurities. These impurities have characteristic colours and retain their colours in the diamond. Hence naturally occurring diamonds appear in different colours.
In simple words: Impurities inside the diamond give it colour.

Teacher's Note:
1. Pure diamond is colourless.
2. Different impurities give different colours.