Let us suppose a small amount of a non-volatile solute (sugar) is dissolved in a solvent (water). When evaporation of this solution takes place, only water evaporates as the other component (i.e., sugar) is non-volatile. The vapor pressure of the solution is found to be less than that of the pure solvent. The lowering of vapor pressure in this case can be explained on the basis of reduced tendency of the solvent molecules to go into vapor phase.
This is because some sugar molecules occupy position of water molecules at the surface of the liquid and they do not have the tendency to go into the vapor state.
fig 3.4
There are lesser number of solvent molecules in the solution and in the vapor as compared to the pure solvent.
