Since the process of crystallization is exothermic, when the crystals form in the hot pads they release heat which your hands feel for the time that it takes the solid to form. To regenerate the supersaturated solution, all you need to do is boil the heat pack (endothermic process!) and you can cause the recrystallization whenever you need that little burst of heat.
I'm sure many of my friends in New England are thinking these might be useful these days, as the weather is so cold!
In the name of SCIENCE!!! I have a few demo's to pass along if you'd like to experiment with the idea of the hot ice yourself. DISCLAIMER: While the science in these syntheses is sound, I have not tried these exact procedures myself and assume no responsibility for the exact nature of the directions I will include herein. Always use your best judgement when working with chemicals and wear proper safety gear- this may just be foodstuffs but you never know when something might go wrong!
(from http://www.instructables.com/id/Crystallization-of-homemade-sodium-acetate/)
This first procedure details the formation of a crude sodium acetate, mostly for playing around with but notes that this impure product will not act correctly under the supersaturated conditions because of the impurities still present. However, a link provided in the first procedure includes a more complete procedure to purify the crude product, allowing you to crystallize the sodium acetate and work with this circular process. So next time it's a chilly night out and you want to warm up your hand with a fun show, try out this process!
http://www.instructables.com/id/Crystallization-of-homemade-sodium-acetate/
http://www.instructables.com/id/Sodium-Acetate/
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