Class 9 Science


Exemplar Problem

Long Answer Questions

Question 23: You are provided with a mixture of naphthalene and sodium chloride by your teacher. Suggest an activity to separate them with well labeled diagram.

Answer: The mixture of naphthalene and sodium chloride can be separated by the process of sublimation as naphthalene is a sublime, i.e. it turns into vapour without changing in liquid and its vapour change into solid without changing into liquid, while sodium chloride is not a sublime.


Materials required: China glass, funnel, tripod, gas burner, spatula, cotton.


Inverte Funnel on Bowl kept on Burner

Using this process the mixture of naphthalene and sodium chloride can be separated.

Question 24: It is a hot summer day, Priyanshi and Ali are wearing cotton and nylon clothes respectively. Who do you think would be more comfortable and why?

Answer: Sweating is a natural mechanism to keep our body cool. Our body sweats a lot in the hot summer days. A cotton cloth absorbs sweat easily, while a nylon cloth does not. Evaporation of sweat absorbed by cotton cloth resulted in cooling effect to our body. The rate of evaporation also increases with the increase in surface area. When one wears cotton cloth in hot summer days it absorbs the sweat coming out from our body easily. The surface area of sweat also increases as it is absorbed by the cotton cloth which increases the rate of evaporation and one who wears cotton cloth feels more comfortable because of cooling effect due to evaporation of sweat. Hence, Priyanshi who wearing cotton cloth will feel more comfortable, while Ali does not.

Question 25: You want to wear your favourite shirt to a party, but the problem is that it is still wet after a wash. What steps would you take to dry it faster?

Answer: Wet cloths dried up because of evaporation. The rate of evaporation increases with the increase in temperature, surface area and wind speed. Hence, keeping the conditions to increase the rate of evaporation in mind, following steps should be taken to get the favourite shirt dried up quickly.

  1. We shirt should be spread up on the wire. This will increase the surface area of wet shirt which will increase the rate of evaporation.
  2. Speedy wind should be supplied to the shirt using a fan which will increase the rate of evaporation.
  3. If it is a day time, arrange the above combination in the sun, so that increase in temperature would increase the rate of evaporation.
  4. If it is not a day time, a hot iron can be used also before hanging the shirt on a wire.

Question 26: Comment on the following statements:

(a) Evaporation produces cooling.

Answer: In the process of evaporation, water molecules get some of the required kinetic energy to overcome the force of attraction between its particles to get escaped from atmospheric temperature and some of the kinetic energy from its neighbouring molecules. Because of getting some of the kinetic energy from neighbouring molecules, the temperature of neighbouring molecules of water decreases which is resulted in cooling effect. Hence, evaporation produces cooling.

(b) Rate of evaporation of an aqueous solution decreases with increase in humidity.

Answer: The moisture present in the atmosphere is called humidity. A dry atmosphere can hold more water molecules than a humid atmosphere. Hence, the rate of evaporation of aqueous solution decreases with the increase of humidity and vice versa. That’s why increase in humidity in the atmosphere decreases the rate of evaporation of an aqueous solution.

(c) Sponge though compressible is a solid.

Answer: There are lots of pores found in a sponge which are filled with air. When a sponge is compressed the air trapped in the pores expelled out, and when it is released, pores again filled with the air and a sponge comes in its original shape. Therefore, sponge is compressible because of air trapped in its pores and not because of the space between its particles as happens in the case of gas. Hence, sponge is a solid instead of it is compressible.

Question 27: Why does the temperature of a substance remain constant during its melting point or boiling point?

Answer: When heat is supplied to a solid, say ice, it starts melting. But temperature does not rise even after continuous supply of heat until all ice melts. The extra heat supplied, is used in melting of ice, which is called latent heat of fusion. Similarly, when a liquid, say water, starts boiling the temperature of water does not rise instead of continuous supply of heat. The extra heat supplied to the boiling water is used in the vapourisation of water, which is known as latent heat of vapourisation. That’s why, the temperature of a substance remains constant during its melting or boiling point.