how do sodium acitate "reusable" hand warmers work?
Saturday, April 3rd, 2010 at
2:54 pm
I am wondering how the reusable handwarmers that look like a bag of water with a metal disk in side work.How does flexing the little disk start the thermal chain reaction (crystallization)and how does boiling it "reset" it?
Tagged with: chain reaction • crystallization • metal disk
Filed under: Skiing & Snowboarding
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The sodium acitate is ’supersaturated.’ There is more salt in the water than the water can hold at room temperature.
If you took a pot of water with table salt and boilded it, and kept adding salt to it until no more salt would go in, then it would be saturated. Once it started to cool, though, it would be supersaturated. All it would need is a single crystal for all of the salt to crystalize.
The sodium acitate works the same way. When it’s in its crystal form, it is thermodynamically efficient (it takes less energy to be in the crystal form than in the solution – this is also how the ‘cold packs’ work – a small amount of the crystal is in a bag with a pouch of water – you break the water pouch and the crystals absorb energy (heat) as the disolve into the solution). So, in solution, the sodium acitate has a lot of bottled up energy. You pop that little disc and a crack in it opens up, which allows a crystal to form, which will spread throughout the bag, releasing the heat.