Why is the vapour pressure of a solvent lowered on the addition of non-volatile solute into it ?

Explanation

Liquids at a given temperature vapouries and under equilibrium conditions the pressure exerted by the vapours of the liquid over the liquid phase is called vapour pressure [Fig.(a)] . In a pure liquid the whole surface is occupied by the molecules of the liquid. If a non - volatile solute is added to a solvent to given a solution [Fig (b)] , then vapour pressure of the solution is from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. In the solution , the surface has both solute and solvent molecules , thereby the fraction of the surface covered by the solvent molecules gets reduced . As such the number of solvent molecules escaping from the surface is reduced, thus, the vapour pressure is also reduced.