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. What is the initial effect of the change on vapour pressure?

Answer 1

Hint: Vapour pressure can be defined as the pressure that is formed by the vapour of the liquid over the surface of the liquid. The vapour pressure depends upon the temperature and pressure changes.

Complete step by step answer:
Vapour pressure is the pressure that is exerted by a gas in equilibrium with a solid or liquid in a closed container at a given temperature.

The Le Chatelier's principle can be stated as " When any system at equilibrium for a long period of time is subjected to a change in concentration, temperature, volume, or pressure, 
(1) the system changes to a new equilibrium, and 
       (2) this change partly counteracts the applied change."
Basically, equilibrium tries to nullify the change done by us and the system tries to go back to equilibrium state. In this question as well, equilibrium will try to shift vapour pressure in such a way that the change is negotiated.

-In this question, it is given that a liquid is kept in a sealed container. And this liquid is in equilibrium with its vapour form. Now, the lid of the container is slightly moved upwards so as to increase the volume of the container, while keeping the container sealed.

-When the volume of the container is increased, what happens is that the vapours that were exerting pressure on the liquid, gets some space and starts moving in the upward direction and the vapour pressure decreases initially.

-This is because as soon as the volume of the container is increased, the density of the vapours decreases which accounts for the decreases in the vapour pressure. Basically there are a lesser number of molecules per unit volume present in the vapour phase.

Note: Remember that the question is asking about the initial conditions, of when the volume is increased. Once the volume is increased, the equilibrium is again attained as the liquid tends to convert into vapours and as a result the vapor pressure attains the same value it had before.