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Supercooling is the process by which a liquid or gas is cooled past its freezing temperature without crystallizing into a solid. Under normal freezing conditions, such substances would freeze into crystals around a seed nucleus, a process known as heterogeneous nucleation. When a seed crystal or nucleus does not exist, the substance can remain a liquid until the point of homogeneous nucleation, which occurs at a much lower temperature.
Pure water freezes at 32°F (0°C) but can be supercooled to -43.6°F (-42°C). It must be distilled for supercooling to take place, because the presence of impurities would create nucleation points and enable ice crystals to form. Supercooled water transforms very rapidly into ice or slush when it encounters a substance on which it can form crystals. It also is possible to supercool water past the point of homogeneous nucleation, in which case it eventually solidifies into a type of glass.
Many different types of substances and solutions can be supercooled, making the process useful in a variety of applications. For example, supercooled metal alloys are used in the production of semiconductor nanostructures. Instant hand warmers, a popular commercial product, produce heat from the rapid crystallization of a supercooled sodium acetate solution.
The supersaturated sodium acetate solution used in hand warmers is created by heating water so that more sodium acetate than usual can be dissolved. This mixture is then supercooled to room temperature, remaining liquid when it would normally crystallize. When the hand warmer is squeezed, the disturbance lowers the energy barrier to crystallization, and heat is released as crystals form.
Supercooling can be widely observed in nature. It can happen to water beneath glaciers, resulting in changes in sediment transport and glacial dynamics. Supercooled water droplets often form in high-altitude clouds and crystallize into ice upon encountering a solid object. This phenomenon is responsible for the formation of ice on aircraft wings.
Many living organisms use this process as well. Species of trees and insects that live in cold climates rely on it to lower the freezing point of their internal fluids. This produces tolerance to freezing conditions and allows these organisms to survive in extremely cold temperatures.
Research has shown that supercooling takes place in cases where a substance’s structure consists of atoms in pentagonal clusters. Pentagons cannot be geometrically arranged in such a way as to completely fill a crystalline space, so crystallization does not occur. The heightened effect of this has been demonstrated during studies on the growth of nanowire structures on silicon.