Pocket hand warmer

[Loremar]

Hi all...Can anyone help with this query.
I've been given a pocket hand warmer and was wondering if anyone could tell me how it works.It's a little plastic sachet filled with liquid and a small metal disc.When the disc is pressed it clicks and starts a reaction where the liquid begins to crystalise and the sachet begins to heat up.This lasts for around 1 hr then begins to cool.To re-use the sachet needs to be put in boiling water for 10 min or so and this causes the crystals to liquify and once it's cooled can be activated again....Can someone explain to me what liquid is inside the sachet and how by clicking the metal disc causes a reaction making the liquid to crytalise.
Thanks for any input

[Teddio]

I'm not sure what the liquid is, it could be a concentrated solution of sodium thiosulphate or some other low toxicity salt.
For crystals to form, a nucleation centre is needed. Clicking the metal disc stimulates the formation of crystals from the supersaturated solution. The phase change, from liquid to solid releases heat energy. This is the latent heat of crystallisation.

[Mortartube]

Possibly Sodium acetate. A ;ovely video here on making it and how it works/

https://www.youtube.com/watch?v=aC-KOYQsIvU

[Loremar]

Thanks for the replies T and M. .....Amazing video.....Cheers