This Liquid Freezes Solid and Gets Hot When You Click It - Video

Click a small metal disc and a clear liquid turns solid in seconds, and it gets hot. Here's the physics in 20 seconds.

This is a reusable hand warmer: a pouch of sodium acetate solution that stays liquid even below its freezing point. Clicking the metal disc gives crystals a place to start, and they spread through the liquid. Freezing releases the heat that was stored in the liquid, so the pouch warms up to about 54 °C. Boil the pouch to melt the crystals and it resets.

Visuals are a 3D visualization, not live footage. Follow the product's instructions: hot and boiling water cause burns.

Which surprised you more? 1️⃣ One click turns liquid to solid 2️⃣ Freezing makes it hot. Reply 1 or 2 ????

Sources:
- BBC Science Focus, "How do re-usable hand warmers work?"
- Simon Fraser University Physics Demos, "4C20.60 Heat Pack"
- N. Menon, American Journal of Physics 67, 1109 (1999), on the thermodynamics of the sodium acetate heat pack
- Institute of Making, "Sodium Acetate Hand Warmers" (Materials Library)

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