Chapter 1: Matter in Our Surroundings

You are provided with a mixture of naphthalene and ammonium chloride by your teacher. Suggest an activity to separate them with well-labelled diagram.

Soln:

Naphthalene is a non-polar compound which will not dissolve in water. Similarly, ammonium chloride is a polar compound which is soluble in water. Naphthalene is volatile in room temperature whereas ammonium chloride is volatile at high temperature.

The mixture of Naphthalene and ammonium chloride can be separated by decantation of aqueous mixture. Naphthalene remains undissolved in water and can be taken out in a funnel. The filtrate of ammonium chloride can be evaporated to obtain dry ammonium chloride.

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?

Soln:

Hot summer leads to a lot of sweating. Cotton absorbs moisture whereas Nylon will not absorb the moisture as efficiently as cotton does. Hence cotton absorbs the sweat faster that Nylon does. Evaporation of moisture from the cotton cloth will give a cool feeling, especially when the wind blows. Hence, it can be concluded that Priyanshi will be more comfortable in summer days.

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?

Soln:

Drying of clothes is due to the process of evaporation hence the following steps should be taken to dry the shirt faster.

1. Dry the shirt under a fan with high speed. With an increase in the speed of fan, water vapours will move away. This decreases water vapour in the surroundings which will increase the evaporation and the shirt gets dried faster.
2. Spread the shirt on a hanger which will increase surface area. Increase in the surface area makes the shirt dry quickly.
3. Dry the shirt in sunlight. Because of high-temperature water evaporates faster. Thus the shirt gets dried easily.
4. Iron the shirt. Heat increases the speed of vaporization and evaporation. This dries the shirt faster.

26. Comment on the following statements:

(a) Evaporation produces cooling.

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

(c) Sponge though compressible, is a solid.

Soln:

a) When a liquid evaporates it takes latent heat from the object it touches. This will make the object cooler. Hence evaporation causes cooling.

b) When there is an increase in the humidity water vapour in the atmosphere will be more. Hence the atmosphere will not take water vapours easily which decreases the process of evaporation.

c) Sponge is a solid because it has definite shape and volume which does not change until you press it. It has minute pores in which air is filled. When you press it, air passes out through pores this makes it a compressible solid.

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

The temperature of a substance remains constant during melting and boiling points till the completion of melting and boiling because of the latent heat of fusion used by the substances. Latent heat of fusion helps to overcome the force of attraction between particles of solid to change into a liquid when they melt. Hence temperature remains constant.

In the same way during the formation of vapours, latent heat helps solid substance to convert into a gaseous state. Hence temperature of a substance remains constant at boiling point.