how do those pocket hand-warmers (that you rub to generate heat) work?
In: Chemistry
The disposable sort are a packet filled with basically iron filings and a salt. The iron filings combined with the salt, moisture in the air, and oxygen from the air, rust (the salt helps this process). The process of iron rusting is an exothermic reaction (it gives off heat), which is what makes the hand-warmers warm.
If you seal the packet away from air (oxygen), it will stop rusting, and stop giving off heat. That’s why they sometimes seem to stop working if you put them in your boot or too airtight a glove or pocket. You could also arrest the reaction by sealing them in an airtight plastic bag until you wanted to use them again.
They have a limited lifespan, and stop giving off heat when all the iron filings inside have basically rusted completely. It’s pretty brilliant.
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