Freezing point

From Glossary of Meteorology

freezing point

(Also called apparent freezing point.) The temperature at which a liquid solidifies under any given set of conditions.

It may or may not be the
same as the melting point or the more rigidly defined true freezing point or (for water) ice point. It is not an equilibrium property of a substance; it applies to the liquid phase only. The freezing point is somewhat dependent upon the purity of the liquid; the volume and shape of the liquid mass; the availability of freezing nuclei; and the pressure acting upon the liquid. The freezing point is a colligative property of a solution and becomes proportionately lower with an increasing amount of dissolved matter. Therefore, since natural water almost invariably contains some solutes, its freezing point is found to be slightly below 0°C. For example, bulk samples of normal seawater freeze at about -1.9°C (28.6°F).