NCERT 7th Class (CBSE) Science: Heat and Temperature

Question: What does bimetallic strip consists of?

Answer: The bimetallic strip consists of brass and iron.

Question: Why is vessel of water heated more quickly when heat is applied at the bottom than when it is heated at the top?

Answer: When the vessel is heated at the bottom the particles of water become lighter and rise up in the middle and cold particles of water which are heavier come down along the sides of the vessel. Convection currents are set-up and water becomes hot.

On the other hand, if heat is applied at the top, the upper portion of water becomes hot and hot molecules remain over the surface instead of going down. As water is a bad conductor of heat, the water at the bottom of the vessel remains unaffected.

Question: Why is a room heater always placed near the floor of a room and not near its ceiling?

Answer: Air is bad conductor of heat. If the room heater is placed near the ceiling then the ceiling will get heated and the rest air of the room will remain cold. But when the room heater is placed near the floor in contact with air it gets heated and becomes lighter so it goes up. The colder air takes its place and gets heated and goes up.

Question: What are the precautions needed while reading a laboratory thermometer?

Answer: The precautions are as follows:

1. The thermometer should be kept upright not tilted.
2. Bulb should be surrounded from all the substance of which the temperature is to be measured.
3. The bulb should not touch the surface of the container.

Question: Why do we use mercury in the thermometer in place of water?

Answer: We use mercury because of the following reasons:

1. Mercury expands more than water.
2. Mercury do not stick on the walls of the thermometer like water.
3. Mercury can be easily collected when thermometer breaks.