A liquid below its standard freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs. The homogeneous nucleation can occur above the glass transition where the system is an amorphous (non-crystalline) solid.
Pure water normally freezes at 273.15 K (0 °C or 32 °F) but it can also be "supercooled" at standard pressure down to its crystal homogeneous nucleation at almost 231 K (−42 °C/−43.6 °F). If cooled at a rate on the order of 106 K/s, the crystal nucleation can be avoided and water becomes a glass. Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108 °C/−162.4 °F). Glassy water can be heated up to approximately 150 K (−123 °C/−189.4 °F). In the range of temperatures between 231 K (−42 °C/−43.6 °F) and 150 K (−123 °C/−189.4 °F) experiments find only crystal ice.
Droplets of supercooled water often exist in stratiform and cumulus clouds. Aircraft flying through these clouds seed an abrupt crystallization of these droplets, which can lead to a reduction of lift unless the aircraft are equipped with an appropriate de-icing system. Freezing rain is also caused by supercooled droplets.
The process opposite to supercooling, the melting of a solid below the freezing point, is much more difficult, and a solid will almost always melt at the same temperature for a given pressure. For this reason, it is the melting point which is usually identified, using melting point apparatus; even when the subject of a paper is "freezing-point determination", the actual methodology is "the principle of observing the disappearance rather than the formation of ice". It is, however, possible, at a given pressure to superheat a liquid above its boiling point without it becoming gaseous.
Supercooling is often confused with freezing-point depression. Supercooling is the cooling of a liquid below its freezing point without it becoming solid. Freezing point depression is when a solution can be cooled below the freezing point of the corresponding pure liquid due to the presence of the solute; an example of this is the freezing point depression that occurs when sodium chloride is added to pure water.
Constitutional supercooling occurs during solidification, is due to compositional changes, and results in cooling a liquid below the freezing point ahead of the solid-liquid interface. When solidifying a liquid, the interface is often unstable, and the velocity of the solid-liquid interface must be small in order to avoid constitutional supercooling.
Supercooled zones are observed when the liquidus temperature gradient at the interface is larger than the temperature gradient.
The slope of the liquidus phase boundary on the phase diagram is
The concentration gradient is related to points, CLS and CSL, on the phase diagram:
The minimum thermal gradient necessary to create a stable solid front is as expressed below.
Some plants are able to supercool the fluid in their cells cytosol and vacuole and thereby survive temperatures down to −40 °C. This is partly achieved through the synthesis of antifreeze proteins that prevent ice nucleation.
One commercial application of supercooling is in refrigeration. For example, there are freezers that cool drink to a supercooled level so that when it is opened it slushes over. Another example is a product that can supercool the beverage in a conventional freezer. Coca-Cola also briefly marketed supercooled Sprite in the UK, which required special vending machines to store the bottles in a supercooled state so that they would turn to slush upon opening.
Other uses of supercooling include commercial hand warmers. The hand warmers contain sodium acetate in a supercooled and supersaturated state in a sealed plastic pouch. There is a disk in the pouch that is clicked to stimulate the liquid to turn into a solid by creating a crystallisation nucleus. Crystallisation is an exothermic process, so when the liquid turns into a solid, it releases enough energy to heat the hands. To supercool the liquid again, it must first be heated to the melting temperature again and allowed to cool.