It isn't as easy to make an ice cube as you might expect. If the water is very pure and air-free, you must cool it several degrees below its freezing point to get it to freeze. The ice crystals need some sort of nucleus to form around- a scratch, an impurity, a dust speck- or you get supercooled water. When supercooled liquids freeze, their temperature rises to the melting point as they freeze.
That's the basic idea behind most of the chemical handwarmers. A flexible plastic pillow pad is filled with a liquid (sodium acetate trihydrate) that is supercooled below its freezing point of 54°C.
When you flex a tiny metal disk embedded in the pad, rough surfaces on the disk are exposed that allow crystals to grow. The sodium acetate instantly begins to crystallize as the pad warms to a toasty 54°C.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/consumer/faq/print-how-do-handwarmers-work.shtml