Thermodynamics > Changes of State
 DCS# 4C20.60
SUPERCOOLED HAND WARMER
APPARATUS
heat packs
 202-11-E1
video camera or document camera
 202-09-D
DESCRIPTION

The plastic pack contains a supercooled solution of sodium acetate and a metal disk.  When the disk is flexed, it exposes some trapped seed crystals, and the rest of the solution crystallizes outward from the disk.  The latent heat of fusion is released and the temperature rises to the freezing point of the substance at about 58 deg C.

There are slits in the metal disk that trap some of the crystals, clamped between the sides of the slits.  When the packs are heated, the trapped crystals, which are under pressure and thus have a higher melting point, remain while the rest of the material melts.  At room temperature, the melt is a supercooled liquid.  Flexing the disk exposes the trapped crystals to the liquid and seed the crystal growth.

NOTES
The deep fryer (101-12-A) is a good size for reheating the packs.  Place a cloth in the bottom to prevent the packs from making contact with the fryer and reheat for ten minutes in boiling water.  Then allow the packs to cool to room temperature.

The article below recommends that you "Flex once or twice before melting to ensure that the trigger is fully charged with crystal seeds."

Am. J. Phys. 76, 546 (2008)