In liquid gas equilibrium, the pressure of vapours above the liquid is constant at

Options

A. Constant temperature
B. Low temperature
C. High temperature
D. None of these

Answer

To solve the question regarding the pressure of vapors above a liquid in liquid-gas equilibrium, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Liquid-Gas Equilibrium**:
- In a closed container with a liquid (e.g., water), some molecules at the surface will have enough energy to escape into the gas phase. This process is called evaporation.

2. **Vaporization and Condensation**:
- As more molecules evaporate, the number of vapor molecules increases. Eventually, some of these vapor molecules will collide with the liquid surface and return to the liquid phase, a process known as condensation.

3. **Establishing Equilibrium**:
- At a certain point, the rate of evaporation (forward process) will equal the rate of condensation (backward process). This state is known as dynamic equilibrium.
- At equilibrium, the number of molecules evaporating and condensing remains constant over time.

4. **Vapor Pressure**:
- The pressure exerted by the vapor molecules above the liquid at equilibrium is called vapor pressure (Vp).
- This vapor pressure is dependent on the temperature of the liquid.

5. **Temperature Dependence**:
- The vapor pressure is constant at a given temperature. If the temperature changes, the vapor pressure will also change.
- Therefore, at a constant temperature, the vapor pressure remains constant.

6. **Conclusion**:
- The pressure of vapors above the liquid is constant at constant temperature.

### Final Answer:
The pressure of vapors above the liquid is constant at **constant temperature** (Option A).

---