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I'm a big fan of EZ Heat hand warmer packets. They're just plastic packets filled with a super-saturated solution of sodium acetate in water. You put them in boiling water to break down the crystals and "charge" the packet. The crystals can't reform with seeds, so energy from the boiling water is "stored" as potential. Once the packets have cooled, you can click the metal "activator disk" inside to provide a seeding site for new crystals. When the solution cystalizes, the stored heat is released, and the packet's temperature quickly goes up.
I like these things because they can be used over and over for no additional cost. I can charge them at home and then keep them in my pockets for whenever winter is getting me down.
So anyway, yesterday I charged up my packets and put them in the freezer. I took the first out after about an hour, when it was really cold but still liquid. Clicking the activator caused the solution to cystallize, and the packet quickly rose to room temperature. Interesting. All the stored energy was used up making he packet "not cold", so it never got hot. OK, I guess I should have anticipated that.
I just took the other one out this morning. It is frozen solid, so I can't click the activator. I can't be sure, however, if it's frozen solid, or if it spontaneoulsy re-crystalizes at some low temperature. That is, I can't tell if it's full of water ice crystals or sodium acetate crystals. I'll have to let it sit at room temperature and see if it thaws. If it melts as it thaws, that would be pretty interesting, having something you could use as an ice pack that turns into heat pack.
It would also be pretty interesting if it spontaneously crystallizes as it freezes. Does it warm up when the crystals form? That would delay the freezing, wouldn't it? It would be useful to have a heating packet that kicks in automatically if it gets too cold. If you're stuck in an emergency situation, you wouldn't have to worry about when to activate the packet if you know it will self-activate when it gets cold enough.
Anyway, I guess we will see with science.
2009.12.04 at 8:00am EST
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