Assertion (A): When a hot liquid is mixed with a cold liquid, the temperature of the mixer is undefined for some time and then becomes nearly constant.
Reason (R): If two bodies at different temperature are mixed in a calorimeter, the total energy of the two bodies remains conserved.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
When liquids of different temperatures are mixed, it takes time to reach thermal equilibrium, so temperature is not uniform initially. So (A) is true. In an ideal calorimeter, total energy is conserved. So (R) is true. However, (R) does not explain the time-dependent nature of temperature equilibration in (A).
Assertion (A): A bottle is filled with water at \(40^{\circ}\text{C}\text{ on opening it at moon, water will boil}\).
Reason (R): Atmospheric pressure on the surface of moon is zero and boiling point is proportional to pressure.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
The moon has a negligible atmosphere, leading to near-zero surface pressure. At such low pressures, water's boiling point decreases significantly. Consequently, water at \(40^{\circ}\text{C}\text{ would readily boil}\). Both (A) and (R) are true, and (R) provides the correct explanation for (A).
Assertion (A): Melting of solid causes no change in internal kinetic energy.
Reason (R): Latent heat is the heat required to melt a unit mass of solid.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
During melting at constant temperature, the average kinetic energy of molecules (related to temperature) remains constant.
Latent heat increases potential energy. So (A) is true. (R) correctly defines latent heat.
However, (R) does not explain why kinetic energy remains unchanged.
Assertion (A): If one gram of ice at \(0^{\circ}\text{C}\) is mixed with one gram of water at \(80^{\circ}\text{C}\), then the final temperature of mixture will be \(0^{\circ}\text{C}\).
Reason (R): Latent heat of ice is \(540\text{ cal/g}\).
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Heat to melt 1g ice at \(0^{\circ}\text{C}\text{ is }1 \times 80\text{ cal/g} = 80\text{ cal}\). Heat from 1g water cooling from \(80^{\circ}\text{C}\text{ to }0^{\circ}\text{C}\text{ is }1 \times 1 \times 80 = 80\text{ cal}\). All ice melts, final temperature is \(0^{\circ}\text{C}\). So (A) is true.
Latent heat of fusion of ice is \(80\text{ cal/g}\), not \(540\text{ cal/g}\). So (R) is false.