Steve Marsden’s

Vapor pressure is a result of the kinetic energy of some surface particles overcoming the attractive forces that may exist in a liquid and making collisions with the walls of a container. We know from our discussion of phase changes that vaporization is endothermic (DH = +).

The process of vaporization also results in an increase in entropy (DS = +) since the particles in the gas phase have more degrees of freedom than those in the liquid phase. This tells us that the process of vaporization below the boiling point is aided by the tendency for entropy to increase. This is important since energy supplied by the surroundings to aid vaporization in the system tends to lower the entropy in the surroundings. So the entropy increase in the system is a necessary occurrence if vaporization is to follow the Second Law.

It is observed experimentally that dissolving a non-volatile solute in a solvent will lower the vapor pressure in proportion to the amount of solute added. How can this be explained using these ideas?