Beginning of Essay
, temperature at which a liquid congeals into the solid state at a given pressure (see PRESSURE; TEMPERATURE). The 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 temperatu ....

Middle of Essay
.... 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 ....