Freezing point depression means the freezing point of a solution is less than the freezing point of the pure solvent. That means a solution must be cooled to a lower temperature than the pure solvent in order for freezing to occur. If a non-volatile solute in the pure liquid then the freezing point of the liquid is lowered as, the vapour pressure of the liquid is decreased, due to presence of non volatile solute particles at the surface of the liquid. Consequently, the vapour pressure of the solid phase of pure liquid will be lowered.
The change in freezing point of the solvent in a solution from that of the pure solvent is directly proportional to the molal concentration of the solute:
delta T = Kf m
T is the change in freezing point of the solvent,
Kb is the molal freezing point depression constant, and
m is the molal concentration of the solute in the solution.