Description: A commercial product used by hikers and skiers as a hand warmer, a heat pack consists of a plastic pouch filled with a supercooled solution of sodium acetate (NaC2H3O2). When a metal disc in the pouch is flexed the solution crystallizes and becomes quite hot. The crystallization front is beautiful to watch. The pack may be reused many times by heating it in hot water to melt the solution. Eventually, however, it becomes impossible to keep the solution supercooled.

We have the "EZHeat Instant Reusable Handwarmer" made by Prism Technologies, available from the Mountain Equipment Coop. We have also used the "Magi-pad" by NHK.

Patent information is available here. Some quotes:
"The activator strip is provided with at least one, and preferably, a plurality of fissures or slits extending thereinto in a thickness direction. As will be further described hereinafter, the fissues can extend inwardly from the edges or perimeter of the strip or they can be located intermediate the edges or perimeter of the strip. "
"It is not presently fully understood how the activator strip 16 initiates crystallization of the supercooled sodium acetate solution; however, it is believed that flexing produces minute continuances or extensions of fracturing at the blind ends 24 of the fissures 20 and that these new fractures are instrumental in initiating crystallization. If this belief is correct, these extensions are extremely small since an activator strip can be used hundreds of times without breaking. "

Cautions:

Concepts demonstrated: supercooling, latent heat

Setup Time: Short.

Difficulty/Commitment: Straightforward

Visibility: The pack is designed to fit in the hand and would not be visible in any but the smallest of classes. Best to pass it around after starting the crystallization process.

References: PIRA 4C20.60
Menon, N "A simple demonstration of a metastable state" AJP 67(12), 1109-10, (Dec 1999)