Freezing

In physics and chemistry, freezing is the process whereby a liquid turns to a solid. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is the opposite of freezing. For most substances, the melting and freezing points are the same temperature.

Freezing of water
Substances not having a freezing point at the same temperature as the melting point (such as pure water) are said to display thermal hysteresis. The melting point of water is 0°C (32°F, 273 K). The freezing point for water is only the same temperature as the melting point when nucleators are present to prevent supercooling. Rain water and tap water will normally freeze at close to the melting point of water (as high as &minus;2°C) because of the presence of nucleating bacteria in the environment, notably Pseudomonas syringae. Water never freezes at 0°C except when in equilibrium with ice in ice water. In the absence of nucleators the freezing point of pure water is not much below &minus;40°C (&minus;40°F, 2 K). Under high pressure (2,000 atmospheres) water will supercool to as low as &minus;70°C (&minus;94°F, 233 K) before freezing.

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.

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 absolute 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 &minus;42 °C (&minus;43.6 °F, 231 K) before freezing. Under high pressure (2,000 atmospheres) water will super cool to as low as &minus;70°C (&minus;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.