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Heating in boiling water for  hour melts all crystals (white) in the pouch. After the pouch dries and cools. it is a clear liquid. When the silver-colored metal disk is clicked, the super cooled liquid again freezes to a crystal. It is interesting to watch the freezing, as it progresses across the pouch. The latent heat of freezing warms the pouch quite a bit. The material is sodium acetate.

Latent Heat