BBC Earth (Asia) - - Q&a -

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. 4. The hand warmer can be re­set by boil­ing it in a pan of wa­ter to liq­uefy the crys­tals. 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.

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