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Remember that Colligative Properties deals with what happens to the physical properties of a solvent compared to the physical properties of a solution made from this solvent. Most chemistry books only list freezing point depression and boiling point elevation as Colligative Properties. However, you could also consider another physical observation as Colligative Property.
If I add solute to a solvent to make a solution and compare the solvent's vapor pressure to the solution's vapor pressure, it has decrease. It is called Raoult's Law, which states that the vapor pressure of a solution (it is really the solution's solvent vapor pressure) is always less than the vapor pressure of the pure solvent.
Why: As you know vapor pressure has to do with liquid particles overcoming their IMF to go into the gas phase. If we look at the surface of the liquid and compare it to the surface of the solution, you see that there are solute particles (in the solution) that have replaced some liquid particles (if only had pure solvent) on the surface of the liquid. These solute particles do not go into the gas phase unlike the solvent particles. So at any given temperature, less potential surface particles that can go into the gas phase (becoming a vapor and causing a pressure we call vapor pressure) in the solution compared to the pure solvent (all the solvent could potentially go into the gas phase). So it only makes physical sense that the vapor pressure of the pure solvent will be greater than the solution. As you add more and more solute (increase concentration of the solution), the vapor pressure of the solution must be getting lower and lower.
Again, Remember, Colligative Properties have to do with you adding in solute particles to the solvent particles to form the solution. This will have an effect on the physical properties.