When two substance A and B of same mass are heated under similar conditions number of free surfaces have been found to reduce to zero from one. If A maintains constancy in the temperature for a longer time than B during heating , then

Options

A. Latent heat of fusion of A is more than that of B.
B. latent heat of vapourisation of A is more than that of B
C. latent heat of fusion of B is more than that of A.
D. latent heat of vapourisation of B is more than that of A.

Answer

To solve the question, we need to analyze the information provided about substances A and B when they are heated. Here’s a step-by-step solution:

### Step 1: Understand the Initial Conditions
Both substances A and B are of the same mass and are heated under similar conditions. Initially, both substances have one free surface, indicating they are in a liquid state.

**Hint**: Recall that liquids have one free surface, while solids have multiple free surfaces and gases have no free surfaces.

### Step 2: Analyze the Change in Free Surfaces
As heating occurs, the number of free surfaces reduces to zero. This indicates that both substances have transitioned from a liquid state to a gas state, as gases have no free surfaces.

**Hint**: Remember that the transition from liquid to gas involves a change in state where the free surface characteristic of liquids disappears.

### Step 3: Compare the Temperature Behavior of A and B
The question states that substance A maintains a constant temperature for a longer time than substance B during heating. This suggests that A requires more energy to change its state from liquid to gas compared to B.

**Hint**: Think about what it means for a substance to maintain a constant temperature during a phase change.

### Step 4: Relate Temperature Constancy to Latent Heat
The fact that A maintains a constant temperature longer than B implies that A has a higher latent heat of vaporization. Latent heat of vaporization is the amount of heat required to convert a unit mass of a liquid into vapor without a change in temperature.

**Hint**: Recall that substances with higher latent heat of vaporization require more energy to change from liquid to gas, which results in a longer duration at a constant temperature.

### Step 5: Conclusion
Since A maintains a constant temperature longer than B, we conclude that the latent heat of vaporization of A is greater than that of B.

**Final Answer**: The latent heat of vaporization of A is greater than that of B.

### Summary of Options
1. **Latent heat of fusion and vaporization** - Incorrect (fusion is not relevant here).
2. **Latent heat of vaporization of A is more than that of B** - Correct.
3. **Latent heat of vaporization of B is more than that of A** - Incorrect.
4. **Latent heat of fusion of A is more than that of B** - Incorrect.

**Correct Option**: The second option is correct.