Matter in our surroundings – Class IX – In-text Solution (Part 4)

(Answers of In-text Question in NCERT Science Text Page 10)

1. Why does a desert cooler cool better on a hot dry day?

Ans: Desert cooler works on the principle of cooling effect of evaporation. In hot and dry days the moisture level is very low in atmosphere & the temperature is high hence it increases the rate of evaporation. Because of faster evaporation, cooler works better.

2. How does the water kept in an earthen pot (matka) become cool during summer?

Ans: 

• In summer moisture level is very low in the atmosphere & the temperature is also high, which therefore increases the rate of evaporation.
• Water from porous wall of earthen pot thus evaporates continuously, which lowers the temperature of water kept in the earthen pot due to the cooling effect of evapouration.
• That is why in summer, water kept in earthen pot becomes cool.

3. Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Ans: When acetone(used in nail-polish remover)/petrol/perfume is poured over palm, it evaporates quickly as these are volatile liquids. The evaporation lowers the temperature of palm and our palm feels cold.

4. Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Ans: When hot tea or milk is kept in a saucer, the liquid is exposed over a larger surface area as compared to in case of the liquid being kept in a cup. The larger surface area increases the rate of evaporation, thus enables faster cooling. That’s why we are able to sip hot tea or milk faster from a saucer rather than from a cup.

5. What type of clothes should we wear in summer?

Ans: We should wear cotton cloths in summer. As cotton cloths are good absorbent of sweat excreted in summer and also because they enable the sweat to evaporate. Because of evaporation of sweat from the surface of cotton cloth, we feel colder and comfortable.