Sodium Acetate Hand Warmers

This slightly medical-looking liquid-filled pouch, containing a floating metal disc, is full of unreleased potential energy. The liquid it contains is a highly concentrated or ‘supersaturated’ solution of sodium acetate, a colourless salt dissolved in distilled water. The solution is simple and could be made at home with distilled vinegar and bicarbonate of soda, but when triggered by clicking the metal disc, an extraordinary chemical reaction takes place. The shockwaves caused by the click generates nucleation, or seeding of an exothermic reaction, causing the liquid to instantly crystallise and the forming solid to give off heat. On a cold day, if you flex and click the metal disc, the liquid in the pouch crystallises and hardens, releasing a lot of heat for up to 30 minutes. The pouch can then be used to keep your hands warm in your pockets, or to ease aching muscles. Once cooled, the solid sodium acetate crystals can be melted by boiling the pouch in a pan of water, thus the hand-warmer can then be re-used again and again.