freezingpoint

Freezing point is the temperature at which a liquid is in equilibrium with its solid phase under a given pressure. At this temperature, solid and liquid phases can coexist. For a pure crystalline substance, the freezing point equals the melting point; for water at 1 atm, this is 0°C.

Mixtures and solutions have lower freezing points than the pure solvent, a phenomenon called freezing point depression. The amount of depression depends on the number of dissolved particles, not their identity. The relation is approximately delta Tf = i·Kf·m, where i is the van’t Hoff factor, Kf is the solvent’s cryoscopic constant, and m is molality.

Impurities also affect nucleation—the formation of a solid phase. Impurities or surfaces can promote nucleation, causing freezing to start at higher temperatures; conversely, supercooling can delay freezing below the nominal freezing point when nucleation is suppressed.

Measurement methods include cryoscopy, differential scanning calorimetry, and capillary freezing point determinations. Freezing point data support practical applications in antifreeze formulations, food processing, cryopreservation, and meteorology. In engineering, controlling freezing points helps prevent ice formation and informs the design of lubricants and refrigerants.