Freezing Point Depression

Freezing point depression occurs when the freezing point of a liquid is lowered by adding another compound to it. The solution has a lower freezing point than that of the pure solvent.

For example, the freezing point of seawater is lower than that of pure water. The freezing point of water to which antifreeze has been added is lower than that of pure water.

Freezing point depression is a colligative property of matter. Colligative properties depend on the number of particles present, not on the type of particles or their mass.

Freezing point depression can be calculated using the Clausius-Clapeyron equation and Raoult's law. In a dilute ideal solution the freezing point is:

Freezing Point_total = Freezing Point_solvent - ΔT_f

where ΔT_f = molality * K_f * i

K_f = cryoscopic constant (1.86°C kg/mol for the freezing point of water)

i = Van't Hoff factor