Question Answers

Quantity of Heat

1. A medical officer prescribes to put wet clothes on the forehead of a person suffering from high fever, why?

The specific heat capacity of water is quite high i.e. 4200 J Kg^-1 K^-1. So, a small amount of water can absorb large amount of heat. A cloth soaked in cold water absorbs large amount of heat when it is kept on the forehead of a person suffering from high fever. This helps in lowering the temperature of the body. This is why medical officer prescribes to put wet clothes on the forehead of a person suffering from high fever.

2. Why does the temperature of ice fall when salt is added to it?

When salt is added to ice, the mixture cools below 00C but remains in liquid. The quantity of heat required to melt the mixture is obtained from the release of heat with the decrease in temperature of the mixture. Thus, the temperature of the ice fall.

3. If you add heat to an object, do you necessarily increase its temperature? Justify your answer.

It is not necessary that the temperature of the object will increase with addition of heat. The amount of heat added may be used to change the state of the object from solid to liquid or liquid to gas at a particular constant temperature.

4. Water in earthen pot remains cold in summer. Explain, why?

Earthen pots are porous, so water molecules come out to the surface through these pores. The water molecules on the surface of the pot absorb heat form the water inside the pot and evaporate. Thus, heat is lost from the water and as a result the temperature of the water decreases. Hence the water in earthen pot remains cold in summer.

5. Why do you feel cool in the mouth when you eat halls?

Hall contains mint which is a volatile substance. When we eat halls, mint vaporizes frequently from the halls. The heat required for the vaporization is absorbed from the surrounding organs of the mouth. So, the temperature of the mouth organs decreases. Hence we feel cool in the mouth.

6. Why does a food cook faster in pressure cooker than open vessel with boiling water?/h6>

When food is cooked in pressure cooker, the water vapour pressure is high than in open vessel. This increases the boiling point of the water above 100°C. So, more amount of heat can be absorbed by food before boiling of water. As a result, the food is cooked faster in pressure cooker.

7. Explain why a person feels cool when he pours a little spirit on his hand.

Spirit is a volatile substance. When it is poured on hand, it absorbs heat from the body portion in contact with it and evaporates faster. So, the temperature of the portion falls. Hence the person feels cool.

8. If you wet your hand and pickup an ice tray that is below 0°C, your hand may stick to it. This does not happen to wood. Why?

When we pick up an ice tray that is below 0°C with wet hand, the tray absorbs heat from the water in between the hand and the tray. When the temperature of the water lowers to 0°C, it freezes to form ice and gets attached to both hand and the tray. Hence hand may stick to the tray. But in case of wood, it is a bad conductor of heat. So, no heat is absorbed through the wood.

9. When you come out of swimming pool, you feel cold. Why?

When we come out of the swimming pool, our body is wet. The water molecule on our body surface evaporated absorbing heat from our body. Thus the temperature of the body decreases. Hence we feel cold.

10. Why is spark produced when two stones are striken against one another?

When two stones are striken against one another, heat energy is produced due to friction. This heat energy is converted to light energy and thus spark is produced.

11. Why is the groundnut fried along with sand?

Groundnut should be fried with low heat. When it is fried with sand, most of the heat is absorbed by sand as it has large specific heat capacity. Sufficient heat is absorbed by the groundnut and thus is fried well.