There is some liquid in a closed bottle. The amount of liquid remains constant as time passes. The vapor in the remaining part.

Options

A. (a)must be saturated
B. (b)must be unsaturated
C. (c)may be unsaturated
D. (d)there will be no vapour.

Answer

To solve the question, we need to analyze the situation of a closed bottle containing a liquid and vapor. The key points to consider are:

1. **Understanding the System**: In a closed bottle with liquid and vapor, there is a dynamic equilibrium between the liquid and vapor phases. Molecules from the liquid can evaporate into the vapor phase, and vapor molecules can condense back into the liquid phase.

2. **Constant Amount of Liquid**: The problem states that the amount of liquid remains constant over time. This indicates that the rate of evaporation (liquid to vapor) is equal to the rate of condensation (vapor to liquid).

3. **Saturation Concept**:
- If the vapor is **saturated**, it means that the vapor pressure is at its maximum for the given temperature, and the number of molecules evaporating from the liquid equals the number of molecules condensing back into the liquid. This results in no net change in the amount of liquid.
- If the vapor is **unsaturated**, there would be more evaporation than condensation, leading to a decrease in the amount of liquid over time, which contradicts the given condition of constant liquid amount.

4. **Conclusion**: Since the amount of liquid remains constant, the vapor must be saturated. Therefore, the correct answer is that the vapor must be saturated.

### Step-by-Step Solution:
1. **Identify the System**: A closed bottle with liquid and vapor.
2. **Analyze the Condition**: The amount of liquid remains constant over time.
3. **Understand Evaporation and Condensation**: In a closed system, molecules can transition between liquid and vapor.
4. **Apply the Concept of Saturation**:
- If the vapor is saturated, the rate of evaporation equals the rate of condensation.
- If the vapor is unsaturated, evaporation would exceed condensation, leading to a decrease in liquid.
5. **Draw a Conclusion**: Since the amount of liquid does not change, the vapor must be saturated.

### Final Answer:
The vapor in the remaining part must be saturated.