Question

A liquid is in equilibrium with its vapour in a sealed container at a fixed temperature. The volume of the container is suddenly increased.

1. What is the initial effect of the change on vapour pressure?
2. How do rates of evaporation and condensation change initially?
3. What happens when equilibrium is restored finally and what will be the final vapour pressure?

Answer 1

Liquid–vapour equilibrium:

A liquid in a sealed container remains in equilibrium with its vapour when the rate of evaporation equals the rate of condensation. At a fixed temperature, the vapour pressure depends only on temperature.

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(i) Initial effect on vapour pressure:

When the volume of the container is suddenly increased, the vapour spreads into the larger volume. As a result, the vapour pressure decreases momentarily.

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(ii) Change in rates of evaporation and condensation initially:

• The rate of evaporation remains the same initially because it depends only on temperature.
• The rate of condensation decreases because fewer vapour molecules strike the liquid surface per unit time due to lower vapour pressure.

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(iii) Restoration of equilibrium and final vapour pressure:

• Since evaporation rate is greater than condensation rate, more liquid evaporates.
• This continues until the rates of evaporation and condensation become equal again.
• At equilibrium, the vapour pressure becomes equal to the original vapour pressure.

Thus, although the vapour pressure decreases temporarily, the final vapour pressure remains unchanged as long as the temperature is constant.

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Conclusion:

An increase in volume at constant temperature causes a temporary disturbance in equilibrium, but the system finally restores equilibrium with the same vapour pressure.