You may have seen reusable hand warmers in the shops around winter time. If you’ve ever used one, you’ll know that they are small, sealed, liquid filled bags with a little metal disc inside. To make them work, all you have to do it click the metal disc and the liquid turns into a solid and gets really warm. But how do they work?

The liquid inside the bags is actually a supersaturated solution of sodium acetate. A saturated solution is one where no more material can be dissolved at room temperature. A supersaturated solution is one where the solution contains more of the dissolved material than would be possible at room temperature. For a supersaturated solution of sodium acetate to be made you heat up some water and add sodium acetate to it until no more can dissolve.

When you click the metal disc inside the bag, you shock the liquid and this shock starts what is called a crystallisation reaction. A crystallisation reaction means that crystals (like crystals of table salt) are formed. It is these crystals that turn the liquid into a solid.

This crystallisation reaction gives off heat. Reactions that give off heat are called exothermic reactions. Reactions which take in heat are called endothermic reactions. It is this exothermic reaction that helps the hand warmer to keep your hands warm on a cold day.

By reheating the pack in boiling water for a few minutes, you are dissolving the salt again. This makes the hand warmer reusable.

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