freezing, change of a substance from the liquid to the solid state. The temperature at which freezing occurs for a pure crystalline solid is called the freezing point and is a characteristic of the particular substance. The reverse process, the change of a solid to a liquid, is called melting. See melting point; refrigeration.

Temperature at which a liquid becomes a solid. When the pressure surrounding the liquid is increased, the freezing point is raised. The addition of some solids can lower the freezing point of a liquid, a principle used when salt is applied to melt ice on frozen surfaces. For pure substances, the freezing point is the same as the melting point. In mixtures and certain organic compounds, the early solid formation changes the composition of the remaining liquid, usually steadily lowering its freezing point, a principle that is applied in mixture separation. The freezing point of pure water at standard atmospheric pressure is 32°F (0°C). To change a liquid at its freezing point to a solid at the same temperature, the heat of fusion (see latent heat) must be removed.

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Method of food preservation in which low temperatures (0 °F [−18 °C] or lower) inhibit the growth of microorganisms. Used for centuries in cold regions, it was not until the advent of mechanical refrigeration in the mid 19th century that the process became widely applicable commercially. In the 20th century, quick (or flash) freezing was developed by Clarence Birdseye. Except for beef and venison, which benefit from an aging process, meat is frozen as promptly as possible after slaughter. Fruits and vegetables are often frozen in a syrup or vacuum-sealed to exclude air and prevent oxidation and desiccation.

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For freezing as a method of food preservation, see frozen food. For a TV series of the same name, see Freezing (TV series). For freezing in terms of computers, see hang (computing)

In physics and chemistry, freezing is the process whereby a liquid turns to a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing. All known liquids undergo freezing when the temperature is lowered with the sole exception of helium, which remains fluid at absolute zero and can only be solidified under pressure. For most substances, the melting and freezing points are the same temperature, however, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as thermal hysteresis.

Crystallization

Most liquids freeze by crystallization, formation of crystalline solid from the uniform liquid. This is a first-order thermodynamic phase transition, which means that as long as solid and liquid coexist, the equilibrium temperature of the system remains constant and equal to the melting point. Crystallization consists of two major events, nucleation and crystal growth. Nucleation is the step where the molecules start to gather into clusters, on the nanometer scale, arranging in a defined and periodic manner that defines the crystal structure. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size no no no

Supercooling

In spite of the second law of thermodynamics, crystallization of pure liquids usually begins at lower temperature than the melting point, due to high activation energy of homogeneous nucleation. The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is expended to form this interface, based on the surface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. Freezing does not start until the temperature is low enough to provide enough energy to form stable nuclei. In presence of irregularities on the surface of the containing vessel, solid or gaseous impurities, pre-formed solid crystals, or other nucleators, heterogeneous nucleation may occur, where some energy is released by the partial destruction of the previous interface, rising the supercooling point to be near or equal to the melting point. The melting point of water at 1 atmosphere of pressure is very close to 0 °C (32 °F, 273.15 K), and in the presence of nucleating substances the freezing point of water is close to the melting point, but in the absence of nucleators water can super cool to −42 °C (−43.6 °F, 231 K) before freezing. Under high pressure (2,000 atmospheres) water will super cool to as low as −70°C (−94°F, 203 K) before freezing.

Vitrification

Certain materials, such as glass or glycerol, may harden without crystallizing; these are called amorphous solids. Amorphous materials as well as some polymers do not have a true freezing point as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in their viscoelastic properties over a range of temperatures. Such materials are characterized by a glass transition temperature which may be roughly defined as the "knee" point of the material's density vs. temperature graph.

Freezing of biological fluids

Most living organisms accumulate cryoprotectants such as anti-nucleating proteins, polyols, and glucose to protect themselves against frost damage by sharp ice crystals. Most plants, in particular, can safely reach temperatures of −4°C to −12°C. Certain bacteria, notably Pseudomonas syringae, produce specialized proteins that serve as potent ice nucleators, which they use to force ice formation on the surface of various fruits and plants at about −2°C. The freezing causes injuries in the epithelia and makes the nutrients in the underlying plant tissues available to the bacteria.

Food preservation

Freezing is a common method of food preservation which slows both food decay and the growth of micro-organisms. Besides the effect of lower temperatures on reaction rates, freezing makes water less available for bacterial growth.

References

See also

• Flash freezing
• Frost
• Melting point, for a more detailed description of the physical process.
• Mpemba effect
• Nucleation
• Phase diagram
• Phase transition
• Supercooling

External links

• Video of an intermetallic compound solidifying/freezing

From	To
	Solid	Liquid	Gas	Plasma
Solid	Solid-Solid Transformation	Melting	Sublimation	-
Liquid	Freezing	N/A	Boiling/Evaporation	-
Gas	Deposition	Condensation	N/A	Ionization
Plasma	-	-	Recombination/Deionization	N/A