How do instant heat packs work? A chemist explains

Instant heat packs feel like magic—slip one into your glove and it slowly warms up over hours. The Conversation Australia, in a piece by chemistry professor Allan Blackman, explains the secret: it's actually straightforward chemistry. Most hand warmers use an exothermic reaction—a process that releases heat. Air-activated packs contain powdered iron that rusts when exposed to oxygen, along with salt and water to speed the reaction. Once all the iron oxidizes, the pack is spent. Reusable heat packs work differently: they contain sodium acetate in a special supersaturated solution. Click the metal piece inside and the disturbance triggers crystallization—another exothermic process—which heats the water and lasts for hours. To reuse it, heat the pack until the sodium acetate dissolves again. So next time you're warming your fingers, you're experiencing two very different but equally clever chemistry tricks.

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