What may seem like a mundane hand warmer is actually a fascinating chemical reaction of supersaturated sodium acetate.
Sodium acetate, CH₃COONa, is an organic salt solution that, when heated, can hold more of the dissolved material than it typically could. This is a unique condition called supersaturation.
Supersaturated solutions are said to be “metastable”, which means it just needs a slight nudge to release the excess solute and return to equilibrium. When the metal plate in the solution is bent, it snaps and creates nucleation sites where salt crystals can easily form. Once a single sodium salt crystal is formed, a rapid chain reaction takes place causing all of the solution to crystallize.
The cascade of crystallization is an exothermic reaction, meaning it releases excess energy as heat, about 280kJ/kg, which keeps your hands nice and warm. To reuse the hand warmer, boil it in water to melt the crystals back into a supersaturated solution. This might sound like a lot of work, but it’s just like charging a battery, storing potential energy that you can release at a later time when needed.
This process can be reversed hundreds of times.
Price is for 2 hand warmers.