What chemicals are used in hand warmers?

[whowhatwhy]

Got a free handwarmer given out in London the other day....it's a clear blue plastic sac containing a clear fluid with the consistency of water. In the plastic sac is a small (five pence coin) steel disc with tiny perforations in it which, when flexed causes the clear liquid to turn to an opaque crystalline solid giving of alarming amounts of heat (exothermic reaction...i know that much!). Then you simply boil the bag in water and it's good to go again.

What's inside and how does the small disc start the reaction ?

[Gef]

There are different types but I think you are describing this one. Quote from Wikipedia.

A second type generate heat through exothermic crystallisation of supersaturated solutions (usually sodium acetate) and are 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 points which initiate crystallisation. Heat is required to dissolve the salt in its own water of crystalisation and it is this heat that is released when crystalisation is initiated.

[whowhatwhy]

thanks for that.