Two open beakers one containing a solvent and the other containing a mixture of that solvent with a non volatile 
solute are together sealed in a container. Over time

The pure solvent solution will try to maintain higher vapour pressure in the sealed container and in return the solvent vapour molecules will condense in the solution of non-volatile solute as it maintains an equilibrium with lower vapour pressure.

(Lowering of vapour pressure is observed when a non volatile solute is mixed in a volatile solvent). 

This will eventually lead to increase in the volume of solution and decrease in the volume of solvent.