How does those heat packs with a tiny metal disc inside work?

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How does those heat packs with a tiny metal disc inside work?

In: Chemistry
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The packs contain a liquid (actually a gel, usually made from a solution of sodium acetate) which has the property that it releases heat when crystals form in it.

When you flex the disc it simply causes a sudden sharp point to occur within the substance. This is enough to start the crystallisation process which then rapidly spreads throughout the entire gel. It therefore heats itself up and then gradually emits that heat over the next hour or so as it slowly cools down.

A sharp point starts the reaction only made me ask how does a sharp point start a reaction so I went looking: “When the activator is clicked, the friction causes a few molecules to react, creating a chain reaction.” Thank you Thermafreeze.com. Friction starting a reaction makes sense to me.

The sodium acetate inside has a melting / freezing point of 58 C (138 F), and is super-cooled because it’s very pure. When you click the disk, it sets off a freezing reaction. When it freezes, it releases all that “stress” of being super-cooled as heat and returns to its freezing point temperature. Sodium acetate is used because it is non-toxic (it’s the stuff used for salt and vinegar flavor chips), easy to make, can be super-cooled easily, and has a nice freezing point that’s warm but not too hot for us to handle.

You can even recreate this safely at home:

Get some vinegar concentrate (or just pure vinegar), then add baking soda until it becomes about pH-neutral; this roughly means it tastes neither soapy nor too acidic when you put a small amount on your tongue (the resulting chemicals are by the way safe unless consumed in exorbitant amounts).

Then heat the mixture to cook away most, but not all, of the water; the exact amount requires a bit of experimentation: let it slowly cool down. If it crystalizes immediately on its own, you have to add a bit of water and reheat; if it does not crystallize even if you stir it strongly, there is still too much water. The right amount means that it stays liquid until you start stirring, which will cause it to crystallize quite fast. But beware, it gets hot!