Question

A non volatile solute ________ (decrease/increase) the rate of evaporation of the solvent and thereby lowers the vapour pressure of solvent.

Answer 1

In case of pure water, the surface area is completely occupied by water molecules. In case of solution, the surface area is partly occupied by water molecules and partly occupied by non volatile solute.

Hence, the escaping tendency of water molecules from pure water is greater than the escaping tendency of water molecules from a solution because of more surface area. Thus, the rate of evaporation of the solution is less than the rate of evaporation of water.

So, when a non-volatile solute is added to water, its vapor pressure is lowered.