How do liquid hand-warmers work?

06 November 2005

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Question

I have recently bought these cool hand warmers that are filled with a gel and a metal disc. When you bend the metal disc, the gel goes hard and warm. What's the chemistry behind them?

Answer

I'm pretty sure that what they work with is sodium acetate. What you've got is a super-saturated solution of sodium acetate, and as this begins to crystallise, it gives off a constant heat of about 55 degrees centigrade, which is a nice comfortable temperature for something like a hand warmer. Now how do you get this to work? Well as you bend the metal, I think you create surfaces that are like the crystals that could be formed from the solution. Once you've done this, you're really seeding the crystals, and so one forms and then another and another until it spreads throughout the bulk of the hand warmer. All the time it is giving off this heat. Eventually, the heat will stop because you've crystallised the whole thing. You can repeat the whole process by putting the used hand warmer in something like and oven, until the crystal is all melted again.

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