How do dry chemical heatpacks work?

All liquids release heat when they freeze. For example, the water in an ice cube tray releases heat into the freezer compartment as the ice cubes solidify. The molecules relax into a nice, stable, hydrogen-bonded network once they lose that excess thermal energy (which makes them rather jittery!).

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 senese@antoine.frostburg.edu

Copyright © 1997-2001 by Fred Senese
Comments & questions to senese@antoine.frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101-hidden/consumer/faq/how-do-handwarmers-work.shtml