God, I love the internet!!!

FROM: http://imartinez.etsin.upm.es/bk3/c07/Hot%20pads%20and%20cold%20pads.htm

The usual type of hot pad is a sodium-acetate solution aqueous solution (half and half salt and water in weight), in metastable equilibrium at ambient temperatures (say 0..40 ºC) and pressure (adding a small crystal would trigger the phase change in a few seconds). A typical heat pad is a thin, flexible, vinyl plastic pouch say 0.1 m in side, and some 0.01 m in total thickness. Similar behaviour to sodium acetate is shown by other salts, as sodium sulphate (Na2SO4) and sodium thiosulphate (Na2S2O3), but they are not used.

Pure anhydrous sodium acetate (NaC2H3O2, or CH3COONa, or NaA for short) is a water-soluble food-grade white powder salt, with M=0.082 kg/mol, r=1530 kg/m3, melting at Tm=324 ºC (hSL=250 kJ/kg, cS=2200 J/(kg∙K), cL=3600 J/(kg∙K)), decomposing at Tb>400 ºC, pH=8..10, water solubility 470 g/L at 0 ºC, is obtained from acetic acid and sodium hydroxide by thermal dehydration. It is a white or transparent hygroscopic crystalline powder when solid, transparent when molten, sometimes used as road de-icing (less harmful than NaCl, but more expensive) and as mordant in dyeing.

Sodium acetate forms a hydrate compound with water: sodium acetate trihydrate (NaC2H3O2×3H2O(s), NaAT for short) with M=0.136 kg/mol, r=1450 kg/m3, water solubility 760 g/L at 0 ºC, pH=9, thawing at Ts=54 ºC, melting at Ts=120 ºC and decomposing at Ts>300 ºC. Both, NaA and NaAT are partially soluble in water, yielding sodium and acetate ions, with a small mixing enthalpy (the heat of solution of NaAT is 19.7 kJ/mole, i.e. endothermic, contrary to the crystallization of NaAT that is highly exothermic).

The flexible plastic pouch contains nearly equal amounts of dehydrated salt and water. Well above the crystallisation point of NaAT (e.g. at >60 ºC), there is a transparent liquid inside the plastic pouch: a NaA and NaAT solution. But when temperature falls below NaAT-solidification point (e.g. at 0..40 ºC), a subcooled-liquid remains, supersaturated with respect to NaAT (but unsaturated with respect to NaA). This kind of metastable equilibrium is common in viscous materials with complex molecules; it is difficult for these particles to organize themselves and they do not simply freeze at a sharp temperature like water, but form subcooled liquids, gels or glasses. Nucleation is not easy, small perturbations are not effective, but a crystal seed or a pressure-pulse from a small metallic clicker inside can trigger it (causes small cavitation bubbles), and the solidification of NaAT starts (it lasts some seconds), releasing the solidification enthalpy:

NaC2H3O2·3H2O(aq) ® NaC2H3O2·3H2O(s) + 150 kJ/kg

When solid-NaAT is heated well above its crystallisation point, it releases its water of hydration and begins to dissolve in that water (it completely dissolved when the mixture reaches 79 ºC, the temperature at which a saturated solution of anhydrous sodium acetate has the same composition as the trihydrate.

It is better to store pouches in subcooled liquid form, since water loss through the vinyl enclosure is larger when the salt is crystallised (what might give rise to some snowflake-like white-crystals in the supersaturated liquid. This evaporation can be reversed by immersing the pad in simmering water for extended periods and leaving it in the water to cool.