Supercooling

Supercooling is the process of chilling a liquid below its freezing point, without it becoming solid.

Related Topics:
Liquid - Freezing point - Solid

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A liquid below its melting 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 glass transition occurs, and the liquid solidifies into an amorphous ? that is, non-crystalline ? solid.

Related Topics:
Phase - Glass transition - Amorphous

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Water has a melting point of 273 K (0°C). Its glass transition temperature is much colder and harder to determine, but studies estimate it at about 165 K (−108°C).{{ref|water}}

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Droplets of supercooled water often exist in stratiform and cumulus clouds. They form into ice when they are struck by the wings of passing airplanes and abruptly crystallize.

Related Topics:
Cumulus - Cloud - Ice - Airplane

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An equivalent to supercooling for the process of melting solids does not exist: a solid will always melt at the same temperature for a given pressure. It is, however, possible to superheat a liquid above its boiling point without it becoming gaseous.

Related Topics:
Temperature - Pressure - Superheat - Boiling point

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An application of supercooling is the heat pad ("HotBag", "Thermo-Pad", etc.) containing sodium acetate (CH3COONa, also used as food additive E 262): the heat required for melting is retained when the temperature drops and the liquid gets below the melting temperature. The heat is released on solidification, which is triggered by flexing a (patentedhttp://www.sfu.ca/physics/ugrad/courses/teaching_resources/demoindex/thermal/th4c/patentheatpack.html) small flat disc of notched ferrous metal which releases very tiny adhered crystals of sodium acetate http://www3.interscience.wiley.com/cgi-bin/abstract/108065684/ABSTRACT into the solution which then act as nucleation sites for the recrystallization of the remainder of the salt solution.

Related Topics:
Heat pad - HotBag - Thermo-Pad - Sodium acetate - Food additive - E - Ferrous - Nucleation

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