Chemistry Question #1849

Brad McCarthy, a 16 year old male from Bridgewater NS asks on January 19, 2004,

In reusable heat packs, the crystalization of sodium acetate is exothermic. This process can be 'recharged' by boiling it in water. Is the sodium acetate reacted with water like other hot packs, and how is the compound able to crystalize over and over? Also why does the sodium acetate not dissolve like most salts do in water?

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The answer

Barry Shell answered on January 26, 2004

It's not reacting with water, it's dissolving in the water when you heat it. Most soluble compounds can dissolve and crystallize over and over. It just depends on the temperature of the solvent. Warming up the pack dissolves the sodium acetate. When it cools it may still be in solution, but supercooled and ready to crystallize. You then do something that encourages it to crystallize (clicking a disk) and this releases the heat. The sodium acetate does dissolve, but at room temperature it crystallizes. Find out more by going to and typing in: how sodium acetate heat packs work.

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