CBSE Class 7 Science Advanced Chemistry Classification of Matter Short Answer Questions Set 02

Short Answer Questions

Question 1. Sodium catches fire easily and chlorine is a harmful gas. But sodium chloride is indispensable in our daily meal. Give reasons.
Answer:
Sodium chloride is a compound formed by the chemical combination of sodium and chlorine.
In a compound the elements do not retain their own properties. So sodium chloride does not catch fire like sodium and is not harmful like chlorine, and it is safe to eat.
In simple words: Sodium chloride has properties of its own, quite different from sodium and chlorine.

Teacher's Note:
1. Elements lose their properties when they form a compound.
2. Water, a compound of two gases, is another example.

Question 2. Sand and sawdust are mixed with water. Name the techniques that can separate sand and sawdust from water.
Answer:
Sand is heavier than water and settles down, so it can be separated by sedimentation and decantation.
Sawdust floats on water, so it can be separated by filtration.
In simple words: Sand settles and is decanted; sawdust floats and is filtered.

Teacher's Note:
1. Decide whether the solid sinks or floats.
2. Use two methods one after the other.

Question 3. Two test tubes X and Y are filled with water and mercury, respectively. After these two liquids were poured, water drops on the inner walls of X were observed but no mercury drops in Y were seen. Give reasons.
Answer:
Between water and glass the adhesive forces are stronger than the cohesive forces between water molecules. So water sticks to the glass and leaves drops on the wall.
In mercury the cohesive forces are stronger than the adhesive forces between mercury and glass. So mercury does not stick to the glass, and no drops remain.
In simple words: Water clings to glass; mercury does not.

Teacher's Note:
1. Compare adhesive and cohesive forces for each liquid.
2. This is also the reason for concave and convex meniscus.

Question 4. Cotton clothes can be made more easily wet than synthetic clothes. Explain.
Answer:
Cotton and water attract each other strongly, i.e. the adhesive force between water and cotton is greater than the cohesive force among water molecules.
Water therefore spreads over and soaks into cotton easily. Synthetic fibres do not attract water as strongly, so they wet less easily.
In simple words: Water sticks well to cotton, so cotton gets wet easily.

Teacher's Note:
1. Link wetting with adhesive forces.
2. Cotton also absorbs sweat for the same reason.

Question 5. The thermal expansion of solids is the least among the three states of matter. Explain.
Answer:
In solids, molecules are packed very closely and attract each other with strong forces.
On heating, they can move only slightly farther apart, so solids expand very little compared with liquids and gases.
In simple words: Tightly held molecules in a solid cannot spread out much when heated.

Teacher's Note:
1. Close packing and strong forces limit expansion.
2. Gases expand the most.

Question 6. Camphor pellets should be preserved in air-tight containers. Give reasons.
Answer:
Camphor is a sublimable substance: at normal atmospheric pressure it changes directly from solid to vapour.
If kept open, the pellets slowly turn into vapour and disappear. An air-tight container stops the vapour from escaping.
In simple words: Open camphor turns to vapour and is lost, so it is kept in a closed container.

Teacher's Note:
1. Sublimation happens without melting.
2. Naphthalene balls are stored in the same way.

Question 7. During summer vacation Ravi decided to go to her grandparent's place at Delhi. Since it was summer, Ravi's mother advised him to carry only cotton clothes. Why did she say so? Give reason.
Answer:
Cotton absorbs sweat. When the sweat in the cloth is exposed to the air, it evaporates.
Evaporation causes cooling, so the body feels cool. Hence cotton clothes are comfortable in summer.
In simple words: Cotton soaks up sweat, which evaporates and cools the body.

Teacher's Note:
1. Evaporation takes heat from the body.
2. Synthetic cloth does not absorb sweat as well.

Question 8. Why are droplets of water observed on the outer walls of a glass tumbler containing ice?
Answer:
The ice melts by absorbing heat from the air around the tumbler, so the glass surface becomes cold.
The water vapour present in the air touches the cold wall, loses heat and condenses into tiny water droplets on the outer wall.
In simple words: Water vapour from the air condenses on the cold glass.

Teacher's Note:
1. The water does not leak out from inside the glass.
2. Condensation is gas to liquid on cooling.

Question 9. When Rita opened the perfume bottle in the bedroom without the permission of her mother, how did her mother come to know while watching TV in the drawing room?
Answer:
The perfume vapour has fast-moving molecules with large gaps between them. These properties let the gas diffuse easily into the air.
The scent spread from the bedroom to the drawing room by diffusion, so her mother could smell it.
In simple words: Perfume vapour diffused through the air to the other room.

Teacher's Note:
1. Gases diffuse because molecules move fast.
2. Diffusion lets us smell things from far away.

Question 10. Jack was getting late for the school, so his mother advised him to pour hot milk from a glass into a saucer and then drink. Why did she say so? Give reason.
Answer:
The rate of evaporation increases with the surface area of the liquid. The saucer exposes a larger surface of milk than a glass.
More molecules escape from the surface, and evaporation causes cooling. So the milk cools quickly in the saucer.
In simple words: A wider surface makes the milk evaporate and cool faster.

Teacher's Note:
1. Surface area controls the rate of evaporation.
2. Pouring into a plate also works.

Question 11. Why do we observe fog and mist in winter mornings?
Answer:
In winter the morning temperature is very low.
The moisture (water vapour) present in the air condenses into tiny droplets of water, which float in the air and appear as fog and mist.
In simple words: Cold air makes the water vapour condense into tiny droplets.

Teacher's Note:
1. This is a daily example of condensation.
2. Fog droplets are suspended in air.

Question 12. When Vasu was running high temperature her mother was nursing him by placing a wet cloth on his forehead till the temperature has gone down. Justify what purpose did the mother's action serve.
Answer:
Evaporation is the change of a liquid into vapour, and the heat needed for it is taken from the surroundings.
The water in the wet cloth evaporates slowly, taking heat from the forehead. So the body temperature goes down.
In simple words: Water evaporating from the wet cloth takes heat from the body and cools it.

Teacher's Note:
1. Evaporation causes cooling.
2. A cloth soaked in water spreads water over a large area.

Question 13. A student has a mixture consisting of charcoal and sulphur powder. He adds a certain reagent where he observes that one of the components goes into the solution state. Name the techniques by which the components can be separated.
Answer:
The reagent is carbon disulphide, which dissolves sulphur but not charcoal.
1. Charcoal is separated by filtration; it remains on the filter paper.
2. Sulphur is recovered from the filtrate by evaporation of the carbon disulphide.
In simple words: Dissolve the sulphur, filter off the charcoal, then evaporate the solvent.

Teacher's Note:
1. The same idea is used for gunpowder.
2. Carbon disulphide is hazardous, so it is used only in a laboratory.

Question 14. Mother took her two daughters Bhavani and Shivani to a swimming pool in summer vacation. They both enjoyed swimming hours together. When they came out of swimming pool, both were shivering and fighting for the towel. Explain the reason for their shivering immediately after they came out of the swimming pool.
Answer:
When they come out, water droplets are present on their bodies. These droplets evaporate slowly from the skin.
Evaporation causes cooling, since the heat is taken from their bodies. Hence they feel cold and shiver.
In simple words: Water on the skin evaporates and takes body heat away, so they feel cold.

Teacher's Note:
1. Evaporation cools the surface it leaves.
2. A dry towel stops this cooling.

Question 15. Establish logically that water is a compound.
Answer:
A pure substance has molecules made of the same kind of atoms. All molecules of water are the same, and each is made of two hydrogen atoms and one oxygen atom.
These two elements are combined chemically in a fixed proportion by mass. A pure substance whose molecules contain different kinds of atoms combined in a fixed ratio is a compound. Hence water is a compound.
In simple words: Water is made of hydrogen and oxygen joined in a fixed ratio, so it is a compound.

Teacher's Note:
1. Mention both the 'different atoms' and 'fixed proportion' points.
2. The ratio of hydrogen to oxygen by mass is fixed.