Question

why does the vapour pressure of solution decreases when non volatile solute is added

Answer 1

Dear student

Liquids at a given temperature vapourise and under equilibrium conditions the pressure exerted by the vapours of the liquid over the liquid phase is called vapour pressure.
If a non-volatile solute is added to a solvent to give a solution, the vapour pressure of the solution is only from the solvent. 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 and hence the fraction of the surface covered by the solvent molecules gets reduced due to presence of non-volatile solute. Consequently, the number of solvent molecules escaping from the surface is correspondingly reduced, thus, the vapour pressure is also reduced.

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