Decrease of Vapor Pressure

Be sure before you read this that you understand what vapor pressure is.

When a solute is added to a solvent several things happen.

First, since we can assume that there is an attraction between the solvent and the solute that is similar in strength (or stronger) than the solvent's attraction for itself, it should not surprise you that these additional attractions will make it more difficult for molecules of the solvent to break away and evaporate.

The resulting decrease in the amount of evaporation will cause a decrease in the vapor pressure (since amount and pressure are inversely related according to the Un-named Gas Law).

In addition, the number of particles on the surface (and therefore available to evaporate will be less (since some of those spaces will be occupied by solute particles). This will also decrease the number of solvent particles that can evaporate and therefore decrease the vapor pressure.

The math:

The vapor pressure of a solution can be found with the following formula:

where P_solution is the vapor pressure of the solution, X_solvent mole fraction of the solvent, and P_{pure solvent} is the vapor pressure of solvent when nothing is dissolved in it.

In simpler terms, imagine a solution of salt water. The vapor pressure of the solution is a measure of how well the water can evaporate with the salt mixed in. If the solution is 90% water (X_water=0.90), then the P_solution will be 90% of the pressure if the water was pure