Freezing Point, temperature at which a liquid congeals into the solid state at a given pressure (see Pressure; Temperature).

The freezing point of a pure (unmixed) liquid is essentially the same as the melting point of the same substance in its solid form and may be regarded as the temperature at which the solid and liquid states of the substance are in equilibrium. If heat is applied to a mixture of liquid and solid substance at its freezing point, the temperature of the substance remains constant until it has become completely liquefied, because the heat is absorbed not in warming the substance but in providing the latent heat of fusion. Similarly, if heat is abstracted from a mixture of liquid and solid substance at its freezing point, the substance will remain at the same temperature until it has become completely solid, because heat is given off by the substance in its change from the liquid to the solid state. Hence, the freezing point or melting point of a pure substance may also be defined as the temperature at which freezing or melting continues once it has commenced.

All solids melt when heated to their melting points, but most liquids can remain liquid even though cooled below their freezing points. A liquid may remain in this supercooled state for some time. This phenomenon is explained by molecular theory, which conceives the molecules of a solid as being well ordered and the molecules of a liquid as being disordered. To solidify, a liquid must have a nucleus (a point of molecular orderliness) around which the disordered molecules can crystallize. The formation of a nucleus is a matter of chance, but once a nucleus forms, the supercooled liquid will solidify rapidly.

The freezing point of a solution is lower than the freezing point of the pure solvent before introduction of the solute (substance dissolved). The amount that the freezing point is lowered depends on the molecular concentration of the solute and on whether the solution is an electrolyte. Nonelectrolytic solutions have higher freezing points for a given concentration of solute than do electrolytes. The molecular weight of an unknown or unidentified substance may be determined by measuring the amount by which the freezing point of a solvent is lowered when a known amount of the unidentified substance is dissolved in it. This process of determining molecular weights is called cryoscopy.

In mixed substances and alloys, the freezing point of the mixture may be much lower than the freezing points of any of its individual components.

The freezing point of most substances is increased by increase of pressure. In substances, however, that expand on freezing (for example, water) pressure lowers the freezing point. An example of this effect can be observed if a heavy object is placed on a block of ice. The area immediately underneath the object will begin to turn to liquid and will refreeze, without any change in temperature, when the object is removed. This process is known as regelation.

See also Cryogenics; Crystal.