Lowering of vapour pressure of a solution : When a nonvolatile solute is added to a pure solvent, the surface area is covered by the solute molecule decreasing the rate of evaporation, hence its vapour pressure decreases. This decrease in vapour pressure is called lowering of vapour pressure.
If P0 is the vapour pressure of a pure solvent (liquid) and P is the vapour pressure of the solution, where P < P0 , then, (P0 – P) is the lowering of the vapour pressure.
