Lowering of Vapour Pressure
If we take a pure liquid in a closed container, we find that a part of the liquid evaporates and fills the available space with its vapour. The vapour exerts a pressure on the walls of the container and exists in equilibrium with the liquid. This pressure is referred as the vapour pressure of the liquid.
When a non-volatile solute is dissolved in the solvent so that a dilute and homogeneous solution results, then again the vapour pressure of the solution will be made up of entirely from the solvent since the solute does not evaporate. This vapour pressure of the dilute solution is found to be lower than the vapour pressure of the pure solvent.
From Fig. it may be seen the surface of a dilute solution is partly occupied by solute molecules, thereby the number of solvent molecules at the surface being reduced. Consequently the vapour pressure of the solvent molecules gets lowered on the surface of the solution.
The relationship between the vapour pressure of the solution and its
concentration is given by a French chemist named Francois Marie Raoult (1886).
According to Raoult's law, at constant temperature the vapour pressure of the
solution (p) is directly proportional to the molefraction of the solvent (X1)
present in the solution.