human nervous system exothermic crystallization from supersaturated solutions provides safe heat although a variety of solutions are available sodium acetate solution is the best mode phase change and heating chamber liquid nach 3 coo aq gt nach 3 coo s 3h 2 o sodium acetate can be super cooled and kept in liquid state at room temperature a user heats the sodium acetate in a container holding the liquid and allows the liquid to cool to a stable state the liquid can freeze and release heat equal to the phase change heat the phase change heat increases the temperature of the sodium acetate liquid to a freezing point and subsequently maintains the temperature at that freezing point which is warm and above room temperature a triggering mechanism such as a metal disk having a plurality of slits cut into the disk has been used for triggering and initiating sodium acetate phase change from liquid to solid a semi rigid plastic bag of miniature size preferably contains a small amount of sodium acetate and no air the sodium acetate is provided in liquid state and can be triggered by a triggering device for phase change to solid state the plastic bag is preferably flexible and contains at least.