How does crystallization begin in a sodium acetate heat pack? I have a pocket warmer heat pack, which is basically a plastic pouch containing a solution of sodium acetate and a flexible metal disk.  When the solution is melted and super-cooled it doesn't crystalize until the metal disk is clicked.
Can anyone shed some light on how clicking the metal disk triggers crystallization?
 A: This was the subject of the paper The physics and the chemistry of the heat pad by B. Sandnes in American Journal of Physics 76, 546 (2008) - (https://doi.org/10.1119/1.2830533). The PDF can be downloaded here.
The claim made in the paper is that the metal disk is designed to trap tiny crystals between sheets of the metal, and these crystals do not melt when the pack is heated. When the metal is flexed these crystals are exposed and act as nuclei for crystallisation of the supercooled melt.
A: You can read up on this link: https://home.howstuffworks.com/question290.htm.
In the website, this scenario is compared to super-cooling water using a very clean glass and pure distilled water to remove any point where the water can crystallize to ice(i.e. freeze) when placed in a refrigerator.  Hence, this can result in you having a cup of water less than $0^o$C.  However, lightly tapping or even creating any motion would immediately cause the water to go to $0^o$C and start freezing.  
The same goes to the sodium acetate solution in the heat pack.  The sodium acetate solution has a freezing/melting point of $55^o$C to $60^o$C, well above room temperature.  However, it remains in liquid form, as it is apparently very stable.  Clicking the metal disk however, causes some molecules to transform into a solid state, and this starts of a chain reaction to crystallize the sodium acetate solution.  This is because it froms a nucleation centre, which crystallizes a few molecules of sodium acetate, and this/these will be the point of origination where the crystallization propagates.  This then causes the temperature to jump to about $60^o$C(melting point of sodium acetate solution) to heat up.  However, the heat pack has to be a "clean environment" for this to work.  That is why even a tiny hole can cause it to crystallize without pressing the metal disk, rendering it useless.
A: being an organic chemist, the clicking provides energy to release the crystals that are on the surface for the metal disk.   many times in the lab when trying to crystalize a solid that is in solution, some solid film is already on the walls of the container and rubbing the walls with a stirrer releases the very small solid particles into the saturated solution.  once released, viola, crystallization follows.  and solids are now available for elemental analysis as well as other types of analyses.
