A cylinder filled with a movable piston contains liquid water in equilibrium with water vapour at `25^@C` . Which one of the following operations results in a decrease in the equilibrium vapour pressure ?

Options

A. Moving piston downward a short distance
B. Removing a small amount of vapour
C. Removing a small amount of the liquid water
D. Dissolving salt in the water

Answer

To solve the problem of determining which operation results in a decrease in the equilibrium vapor pressure of water in a cylinder with a movable piston, we can analyze the effects of different operations on the vapor pressure.

### Step-by-Step Solution:

1. **Understanding Vapor Pressure**:
- Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid at a given temperature. At 25°C, water has a specific vapor pressure that depends only on the temperature.

2. **Effect of Moving the Piston Downward**:
- If the piston is moved downward, the volume of the gas decreases, which increases the pressure. According to Le Chatelier's principle, increasing the pressure will favor the side with fewer moles of gas, which in this case means more liquid will condense. Thus, this operation does not decrease the vapor pressure; it actually increases it.

3. **Effect of Reducing the Volume**:
- Reducing the volume of the cylinder will also increase the pressure of the vapor. Similar to moving the piston downward, this will lead to more condensation of vapor into liquid, thus not decreasing the vapor pressure.

4. **Removing a Small Amount of Vapor**:
- If a small amount of vapor is removed from the system, the equilibrium will shift to produce more vapor until a new equilibrium is established. This does not decrease the vapor pressure significantly, as the system will adjust back to its original vapor pressure.

5. **Removing a Small Amount of Liquid Water**:
- Removing a small amount of liquid water will not affect the vapor pressure significantly because the vapor pressure is determined by the amount of vapor present, not the liquid.

6. **Dissolving Salt in Water**:
- When salt (a non-volatile solute) is dissolved in water, it decreases the number of water molecules at the surface available to escape into the vapor phase. This leads to a decrease in the vapor pressure of the water. This is due to the colligative properties of solutions, where the presence of a solute lowers the vapor pressure of the solvent.

### Conclusion:
The operation that results in a decrease in the equilibrium vapor pressure is **dissolving salt in water**.