Freezing

In spite of the second law of thermodynamics, crystallization of pure liquids usually begins at a lower temperature than the melting point, due to high activation energy of homogeneous nucleation.

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.

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, raising the supercooling point to be near or equal to the melting point.

[6] Freezing is almost always an exothermic process, meaning that as liquid changes into solid, heat and pressure are released.

Amorphous materials, as well as some polymers, do not have a freezing point, as there is no abrupt phase change at any specific temperature.

Because vitrification is a non-equilibrium process, it does not qualify as freezing, which requires an equilibrium between the crystalline and liquid state.

Many living organisms are able to tolerate prolonged periods of time at temperatures below the freezing point of water.

Most living organisms accumulate cryoprotectants such as anti-nucleating proteins, polyols, and glucose to protect themselves against frost damage by sharp ice crystals.

[10] Three species of bacteria, Carnobacterium pleistocenium, as well as Chryseobacterium greenlandensis and Herminiimonas glaciei, have reportedly been revived after surviving for thousands of years frozen in ice.

[citation needed] Many plants undergo a process called hardening, which allows them to survive temperatures below 0 °C for weeks to months.