those gelbag handwarmers with a piece of metal inside that you snap, and can reuse after boiling the bag

Question

those gelbag handwarmers with a piece of metal inside that you snap, and can reuse after boiling the bag

In: Chemistry

Answer 1

Yes, they contain a supersaturated solution of sodium acetate. Supersaturated means that it has more than the maximum possible amount of sodium acetate dissolved in it. This is pretty unstable in a sense because the sodium acetate would rather be a crystal than be dissolved at that point so it will take any opportunity to crystallize. That is what the metal is for, bending it creates an impurity which the crystals can start growing on which will cause all of the sodium acetate to crystalize all at once. This process is what you would call “exothermic”, which means that it produces heat. That is what makes these handwarmers work.