HC Verma Questions and Solutions: Chapter 25: Calorimetry- 1 | HC Verma Solutions - JEE PDF Download

Q.1. Is heat a conserved quantity?

Yes, heat is a conserved quantity. Suppose two bodies A and B are at different temperatures, wherein A is higher and B is lower. When they are brought in contact, the heat given by A is equal to the heat gained by B. Thus, the heat is conserved in the system.
Here, we have ignored the heat exchanged with the surroundings. If we consider the surroundings, the heat of the system will be conserved.

Q.2. The calorie is defined as 1 cal = 4.186 joule. Why not as 1 cal = 4 J to make the conversions easy?

One calorie is defined as the heat required to raise the temperature of 1 g of water from 14.5°C to 15.5°C. Also, exactly 4.186 J of work is to be done on heating 1 g of water from 14.5°C to 15.5°C. So, we cannot round-off 4.186 J to 4 J as it will give significant difference in work done required to raise the temperature and heat required to raise the temperature.
Since work and heat are equivalent here, so on taking 1 calorie = 4 J, the validity of the equivalence of work and heat will get defied. Thus, just to make conversion easy, 1 calorie cannot be taken equal to 4 J.

Q.3. A calorimeter is kept in a wooden box to insulate it thermally from the surroundings. Why is it necessary?

A calorimeter is kept in a wooden box to insulate it thermally from the surroundings because in order to determine the specific heat capacity, the total heat transferred must be known. Heat must not be exchanged with the surroundings, otherwise the principle of calorimeter would not stand valid.

Q.4. In a calorimeter, the heat given by the hot object is assumed to be equal to the heat taken by the cold object. Does it mean that heat of the two objects taken together remains constant?

Yes, heat of the two objects taken together remains constant. If no heat is lost to the surroundings, the heat of the two bodies taken together actually remains conserved.

Q.5. In Regnault's apparatus for measuring specific heat capacity of a solid, there is an inlet and an outlet in the steam chamber. The inlet is near the top and the outlet is near the bottom. Why is it better than the opposite choice where the inlet is near the bottom and the outlet is near the top?

The inlet is near the top and the outlet is near the bottom because there is a loss of heat from the steam as it passes through the chamber. As the steam loses heat, a part of it condenses back to water and the cold steam gets denser and moves down towards the bottom.
But when done the other way round, the used steam does not pass through the chamber correctly and gets mixed up. This can result in discrepancy in the desired results.

Q.6. When a solid melts or a liquid boils, the temperature does not increase even when heat is supplied. Where does the energy go?

When a solid melts or a liquid boils, the heat supplied is actually used to break the bond forces between the molecules and bring them apart till the body changes its state completely. Thus, the energy is transferred to the molecules as kinetic energy and the temperature of body remains constant in the process.

Q.7.1. What is the specific heat capacity of melting ice?

The specific heat capacity of melting ice is 0.50 cal/g-°C or 2093 J/kg-K.

Q.7.2. What is the specific heat capacity of boiling water?

The specific heat capacity of boiling water is 0.46 cal/g-°C or 1926 J/kg-K.

Q.8. A person's skin is more severely burnt when put in contact with 1 g of steam at 100°C than when put in contact with 1 g of water at 100°C. Explain

Steam has greater energy than boiling water due to latent heat of vapourisation. The internal energy of the vapour at 100°C is greater than the internal energy of the boiling water at the same temperature. Thus, steam burns the skin more severely than boiling water.

Q.9. The atmospheric temperature in the cities on sea-coast change very little. Explain?

The atmospheric temperature in the cities on the sea-coast changes very little due to the following reasons:-

1) The specific heat capacity of water is four times greater than land. This means water heats and cools more slowly than land. Also, once the ocean has warmed, it gives up its heat much slower than the land.

2) The moisture content of the air over coastal locations is very high. Moisture in the form of water vapour is the predominate greenhouse gas in the troposphere. Water vapour prevents the loss of heat into space at night. Thus, the temperature during day and night is almost the same.

Q.10. Should a thermometer bulb have large heat capacity or small heat capacity?

The thermometer bulb should have large heat capacity because if it has lower heat capacity, it may expand at high temperatures resulting in false readings. Large heat capacity of the thermometer bulb ensures the correct reading of temperature by restricting the bulb from expanding.