Reusable Heat Packs

Description:  This phenomenon uses a supersaturated solution of sodium acetate. Clicking the metal disc releases a small number of crystals of sodium acetate which act as nucleation sites for the crystallization of the sodium acetate into a hydrated salt. Energy is released from the crystal lattice. The heating pack can be placed in boiling water and the sodium acetate can be dissolved again. This phenomenon shows how bond energy can be released.  It also shows the importance of chemical engineering and could lead to a section where students design a device (or application) of their own.

Web Resource:  Chemical Heat Pack - Wikipedia, Snappy Heat - Amazon

Clicking the metal disk inside causes a few of liquid sodium acetate molecules to flip to a crystallized state causing a chain reaction. The solidifying liquid creates 130°F of exothermic heat in the process. You can boil the package to return the crystallized sodium acetate back to a liquid form for reuse.