freezing point depression

Meaning: A solution freezes at a lower temperature than the pure solvent. Dissolved solute particles make the liquid more disordered and lower the solvent's tendency to leave the liquid, so the solution must be cooled further before the solvent crystallizes. The solute generally stays in the liquid instead of joining the solid. Because only the number of particles matters, it is a colligative property, like boiling point elevation, vapor pressure lowering, and osmotic pressure.

Equation: ΔTf = i Kf m, where ΔTf is the drop in freezing point (K or °C), i is the van't Hoff factor (dimensionless, particles per formula unit), Kf is the solvent's freezing point depression constant (K·kg/mol), and m is molality (mol solute per kg solvent).

On the MCAT: Expect a ranking question, such as which of 0.1 m glucose, NaCl, or CaCl₂ freezes at the lowest temperature (CaCl₂, i ≈ 3). Other setups ask for the freezing point of a salt or antifreeze solution, or ask you to read a phase diagram where the solid-liquid line shifts to lower temperature. Road salt and antifreeze are the usual examples.

Common confusion: Students forget i for ionic compounds or use molarity instead of molality. ΔTf is the size of the drop, so the new freezing point is the pure solvent's value minus ΔTf. The ideal i is a maximum, because ion pairing in real solutions makes the effect slightly smaller.