freezing

Freezing is the phase transition in which a liquid becomes a solid as it loses thermal energy. It occurs when the temperature falls to or below the substance's freezing point at a given pressure, and the process releases latent heat of fusion as molecules arrange into an organized crystal lattice.

The freezing point of a liquid is influenced by pressure and impurities. Impurities lower the freezing point, a phenomenon known as freezing point depression. Some liquids can be supercooled, remaining liquid below their normal freezing point until nucleation begins.

Solidification typically proceeds by nucleation, the formation of small stable clusters, followed by crystal growth. The resulting solid has a regular lattice structure; in water, the common crystalline form is ice Ih, with variability in crystal size and grain structure depending on cooling conditions.

Freezing has wide applications. It is used to preserve food by inhibiting biological activity and enzymatic decay, and in biology and medicine for cryopreservation of cells and tissues. In materials science, controlled freezing affects metal casting and glass formation. Weather phenomena include frost and ice formation.

Rapid freezing produces many small crystals, while slow freezing yields larger ones and can cause cracks due to ice expansion. Other related processes include frost, frost heave, and frost formation; freezing rain occurs when supercooled droplets freeze on contact with surfaces.