A liquid is in equilibrium with its vapour in a seated container at a fixed temperature. The volume of the container is suddenly increased.
(a) What is the initial effect of the change on vapour pressure?
(b) How do rates of evaporation and condensation change initially?
(c ) What happens when equilibrium is restored finally and what will be the final vapour pressure?

Explanation

a. Initially, the vapour pressure will decrease.
b. The rate of evaporation remains constant at constant temperature in a closed vessel. However, the rate of condensation will be low initially because there are fewer molecules per unit volume in the vapour phase and hence the number of collisions per unit time with the liquid surface decrease.
c. When equilibrium is restored rate of evaporation =rate of condensation. the final vapour pressure will be same as it was originally,