How does it work?
If you attempt to dissolve most solutes in water, you reach a point where you cannot dissolve any more sugar. This is called a saturated solution. However, if you heat the solution, more solvent will sometimes dissolve. (This is true of the sodium acetate we are using, but not true of something like sodium chloride. Raising the temperature of a sodium chloride solution does not really allow much more solute to be added.) When our solution is cooled, the sodium acetate will remain in solution. This is called a supersaturated solution, which is very unstable and will crystallize easily. The process of crystallization gives off heat. It’s said to be exothermic. That’s why the solution is used in the old-school type of hand warmers.
How do Hand Warmers Work?
Commercially available hand warmers use a supersaturated solution of sodium acetate. These products consist of a concentrated aqueous salt solution together with a flexible metallic steel activator strip in a sealed, flexible container. Sodium acetate and calcium nitrate are examples of suitable salts. These salts are much more soluble in hot water than in cold water. The flexible metal strip is bent back and forth a few times, whereupon a white cloud of crystals begins to precipitate. Within seconds, the entire pack is filled up with solid crystalline needles of sodium acetate without any solution left, and the temperature rises to 130°F for about 30 minutes.
Supercooled liquids can be cooled below their normal freezing point without turning solid. Then, at the flick of button, the supercooled liquid is triggered to solidify (crystallize) and at the same time release large amounts of heat. Salt solutions that have been processed in such a way that their temperature can be lowered well below their solidification (or melting) temperature and still remain in liquid are defined as supercooled or metastable liquids.
The triggering device initiates the rapid solidification of the solution. The solidification process is a rapid crystallization that releases a large amount of heat at the salt solution's normal melting temperature (common table salt sodium chloride does not do this).
The activator is a thin metal piece with ridges and a specially roughened surface. The flexing causes metal-to-metal contact that releases one or more very tiny particles of metal from the roughened surface. This acts as a nesting site for one crystal deposited from the solution and BAM! all of the crystals fall out instantly. These heat packs are reusable because, by reheating the pack in boiling water for a few minutes, the salt re-dissolves and the pack again contains a clear solution. Best of all, the activator strip can be reused dozens of times!