how do they work exactly?

Ive got one of those handwarmer things that is liquid in a small pouch and has a little metal disc inside. You click the disc and the liquid starts to solidify and gives off lots of heat in the process. When it is solid you can boil it in a pan of water and the whole thing goes back to liquid again and the process can start again.

Think you just explained it 🙂

If I remember rightly its a very very strong salt solution its super saturation if I remember rightly.

clicking the disc produces crystallisation which then goes onto produce an endothermic reaction.

Some kind of chemical reaction, which is reversed when the thing is boiled?

I believe, as I had it explained to me, that the molecules of the chemical (and it is one chemical) are stable in two states or shapes, think of a spring caught on a latch – its the same spring but if you put in a bit of energy to move the latch you get a lot of energy and the same molecule in a differnt shape. In this case the latch is the physical form of the molecule and the energy put in to release the latch is the click of the disc (it produces a sudden shockwave of high energy). The is shockwave triggers the latch of the nearest spring, the triggering of which triggers the nearby latches in a chain reaction.

I've probably oversimplified it vastly but that's my understanding of it. Anyone else expand/correct?

Its not a chemical reaction in that its not changing substances, its a reversible energy reaction. The boiling "compresses" the spring again and resets the latch.

Kind of the same trick as a beer being super-cooled and freezing instantly when being tapped.

does clicking the disc release something or expose something on the disc? Or is it acoustic? Tese are all theories we dreamt up over tea and cake on a wet afternoon…

Damn – neither quick enough OR knowledgable!

Ahh, thought it might be a wax (well, randomly guessed). Having to boil for a while to ensure dissolving makes more sense than boiling for a while to simply melt.

Wiki says

Supersaturated solution (crystallization-type)

A second type generate heat through exothermic crystallisation of supersaturated solutions and are usually reusable. These can be recharged by boiling the warmers and allowing them to cool. Heating of these pads is triggered by snapping a small metal device buried in the pad which generates nucleation centers which initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.[3]

This type typically has a shorter heat duration of 20 minutes to 2 hours.[4]

It's sodium acetate. It freezes at 53°
So you're looking at a super cooled liquid. Any sudden click will be enough to start it freezing and expelling all the heat in the process.

http://www.howstuffworks.com/question290.htm

clicking the disc produces crystallisation which then goes onto produce an endothermic reaction.

EXOthermic – it gives off heat.
It's heat (or strictly speaking enthalpy) of crystallisation of a super saturated salt solution – apply a localised force to it, in this case the tiny shock from the metal disc clicking, and it crashes out giving off a fair bit of heat in the process.