After a few weeks of absence, I turned up at my fencing club for a few bouts a couple of nights ago. Today, I woke up feeling sore and rediscovered muscles in places I had forgotten there were any muscles. So out came the heating pads.
I have reusable heat pads. The ones that come in a plastic bag and have a disc to click that kicks off a crystallisation process giving off heat. And when done, I place the bag in a saucepan of hot boiling water to dissolve the crystals. Once the crystals are dissolved, the bag is allowed to cool ready for use for next time.
The liquid in the heat pad is a supersaturated solution of sodium acetate trihydrate. The solution cools without forming crystals which is where the disc comes in. The metal disc in the heating pad when clicked, creates a nucleation site, a place for crystals to form.
Sodium acetate is also used in a chemistry demonstration where a liquid is poured from a beaker or flask and instead of liquid splashing everywhere, a pillar of white salt forms.