A seed crystal can be dropped into a saturated solution of sodium acetate. The solution will suddenly crystallize and turn to solid. Alternately, the solution may be slowly poured out onto a seed ...

When the disc is bent, crystals of sodium acetate begin to form. This process of changing from a liquid to a solid produces heat. The third contains a type of magnesium sulfate, and a pouch of water.

Commercial heat packs (containing iron and water, or supersaturated sodium acetate) and cold packs (various ammonium salts) can be used to show exo- and endothermicity. Heat packs that contain iron ...