As a Christmas gift, my wife surprised me with some pretty neat reusable hand warmers for my gloves while running. Each pouch is filled with a clear liquid of sodium acetate (CH3COONa ) and water with a little round metal disc floating freely inside. When you bend the metal disk inside the pouch, a crystallization process is initiated throughout the liquid, taking it to 130 degrees Fahrenheit within minutes. Instant warm hands!
The whole idea is fascinating, not to say very practical, so I had to investigate just what was going on. What is happening seems strange, but not really. It's a process that can best be understood if you think about water freezing. Water freezes at 32 degrees Fahrenheit. If you measure the temperature of a cup of water put into the freezer, you find that the temperature of the water falls to 32 degrees F and then hangs there until all of the water is completely frozen solid. Then the temperature of the ice continues to fall to equal the temperature of the freezer.
But what if you could somehow super-cool the water? That is, get the water's temperature to 22 degrees, 10 degrees below the freezing point, without it crystallizing into a solid. This can be done using a very clean glass and distilled water so that there are no points for the water to begin crystallizing. In this condition, if you tap the glass the temperature of the water will then jump to 32 degrees F, and the water will solidify quickly.
It turns out that sodium acetate is very good at super-cooling. It "freezes" at 130 degrees F, but is happy to exist as a liquid at a much lower temperature, say room temperature or 15 degrees like it was outside when I first tried it, and is extremely stable. Bending the disk, however, has the ability to force a few molecules of the sodium acetate liquid to flip to the solid state, and the rest of the liquid then rushes to solidify as well. The temperature of the solidifying liquid jumps up to 130 degrees F in the process.
The pouches are great to carry as a contingency, and not used until the hands get really cold. Once they are initiated and their temperature hits 130 degrees F they take on the shape of your hands as they turn solid as ice before losing temperature back to that of the surrounding air in a half hour to forty minutes.
Once I finished my run I plopped the frozen pouches into water and heated them to a boil to reliqufy their contents for use the next time. When you boil the solid pouch, you melt it back to a liquid state. But you have to completely melt every crystal or the liquid will quickly re-solidify. You can repeat this cycle forever, theoretically, just as you can freeze and melt water as many times as you like. The pouch is likely to eventually wear out and leak, but that's okay since sodium acetate is a food additive and non-toxic. Pretty neat gift!