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According to this page: http://burnanenergyjournal.com/forms-of-energy-motion-heat-light-sound-2/

Potential Energy is any type of stored energy. It can be chemical, nuclear, gravitational, or mechanical.

I may be mistaken, but it seems to me that a super-cooled liquid stores potential energy. This is used in sodium acetate heat packs which release this potential energy as heat when the gel they contain transitions from liquid to solid.

This definitely isn't nuclear or gravitational.

I consider mechanical as being the energy stored in a spring when it's (e.g.) compressed, and I consider chemical to be the energy that's release when (e.g.) sodium metal reacts directly with water.

I don't believe it's either of these, chemical is close, but there is no chemical change in a heat pack: it's sodium acetate and water going in, and sodium acetate and water coming out.

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  • $\begingroup$ It's still chemical energy; but is far more complex than a standard chemical reaction. I don't know enough about it to go into detail; but they seem to abuse nucleation properties to delay a phase change in a supersaturated mixture until it is desired. Someone hopefully can provide a little more insight on this form of energy. $\endgroup$ – JMac Jul 20 '17 at 21:05
  • $\begingroup$ @JMac your analysis of the process is correct. It's a supersaturated solution, supercooled, in a very clean pack that has no point that allows crystallization to start. There's a small spring "coin" in there that can be snapped to start crystallization, at which point it spreads across the whole pack within 5 to 10 seconds. If it genuinely is chemical, please post as an answer, and I'll accept. $\endgroup$ – dgnuff Jul 20 '17 at 21:10

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