Boiling Point Elevation/Freezing PointDepression
Lowering the vapor pressure of a substance has an obvious effecton boiling point; the boiling point goes up. The BP increases becausemore energy is required for the solvent's vapor pressure to reach theexternal pressure.
The logic/picture which was used to explain vapor pressurelowering can be used to explain the change in boiling point. Thereare fewer solvent molecules at the surface capable of vaporizing sothe vapor pressure drops. Thus, the temperature of the boiling pointmust be increased.
Another consequence of lowered vapor pressure is a decrease infreezing point. A picture can be used to explain this too.
Instead of examining the vapor-liquid interface welook at the solid-liquid interface.
At a solid's normal melting point is defined for materials under 1atm. of pressure as the temperature at which the solid is inequilibrium with the liquid.
At the freezing point the rate at which the solid melts is equalto the rate at which the liquid freezes A. If a solute isintroduced the rate at which the solid melts does not change, butsince there are fewer liquid molecules next to the solid the rate atwhich the liquid freezes drops--(B) three molecules melt andonly one freezes--there is no longer an equilibrium and the solidmelts. To reestablish the equilibrium the rate at which the solidmelts must be lowered; i.e., the temperature must be lowered.
The change in both the freezing point and the boiling point isdirectly proportional to the amount of material dissolved in thesolution
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Because K is positive for both boiling point elevation andfreezing point depression the 
for BP elevation and the for freezing point depression must be calculated differently.
A simple way to remember this is that the for the freezing depression is the amount the freezing point isdepressed (or lowered), and the for boiling point elevation is the amount the boiling point iselevated (or raised). If you remember the direction then dealing withthe signs is easy.
It is important to note that Kb and Kf areconstants for a given material, but vary from material tomaterial.
Kf for water is 1.86°Ckgmol-1; whereas, Kf forbenzene is 5.12 °Ckgmol-1.
