Freezing Point Depression

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

Freezing Point Depression Examples

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.

The freezing point of vodka is lower than that of pure water. Vodka and other high-proof alcoholic beverages typically don't freeze in a home freezer. Yet, the freezing point is higher than that of pure ethanol (-173.5°F or -114.1°C). Vodka may be considered a solution of ethanol (solute) in water (solvent). When considering freezing point depression, look at the freezing point of the solvent.

Colligative Properties of Matter

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. So, for example, if both calcium chloride (CaCl₂) and sodium chloride (NaCl) completely dissolve in water, the calcium chloride would lower the freezing point more than the sodium chloride because it would produce three particles (one calcium ion and two chloride ions), while the sodium chloride would only produce two particles (one sodium and one chloride ion).

Freezing Point Depression Formula

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

Freezing Point Depression in Everyday Life

Freezing point depression has interesting and useful applications. When salt is put on an icy road, the salt mixes with a small amount of liquid water to prevent melting ice from re-freezing. If you mix salt and ice in a bowl or bag, the same process makes the ice colder, which means it can be used for making ice cream. Freezing point depression also explains why vodka doesn't freeze in a freezer.