How do re-usable hand warmers work?
On cold days, a hand warmer can offer comfort to the chilly-fingered. Yet the science behind them is surprisingly straightforward...
1. A hand warmer contains sodium acetate, dissolved in water. The solution is ‘super-saturated’, which means it has been heated to dissolve more sodium acetate. The solution crystallises readily.
2. When the internal metal strip is bent, tiny bits of metal are released, which offer ‘nucleation sites’ for crystals to form.
3. As the crystals spread, the stored heat energy of the solution is released, heating the hand warmer up to 54°C – an exothermic reaction.
4. The hand warmer can be reset by boiling it in a pan of water to liquefy the crystals.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.
Sponsored Deals
May Half Price Sale
- Save up to 52% when you subscribe to BBC Science Focus Magazine.
- Risk - free offer! Cancel at any time when you subscribe via Direct Debit.
- FREE UK delivery.
- Stay up to date with the latest developments in the worlds of science and technology.
