Supersaturated solution. Crystals of sodium acetate forming from solution in a flask. Sodium acetate trihydrate dissolves in its own water of crystallisation when heated above 58C. If this is allowed to cool to room temperature, the solution becomes supersaturated. If this is in contact with a small crystal of the acetate, it instantly starts to recrystallise. This is highly exothermic and is used in hand warmers and similar heating pads.