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Packing Heat
The Scientific Secret Behind Instant Winter Warmth
 Listen to Joe Palca's report.
 Video: The heat pack makes its rapid transformation from a cool liquid to a hot solid.
Video: Liane Hansen and Joe Palca warm up for the interview with a few turns around the National Gallery of Art's skating rink.
Dec. 30, 2001 ---
Pocket-sized instant heat packs can provide quick relief just as winter activities start to get a little too chilly. Weekend Edition Sunday host Liane Hansen and NPR Science Correspondent Joe Palca made a recent trip to the National Gallery of Art's ice skating rink to talk about the science behind the portable thermal packs.
Think of the thermal pack as a kind of rechargeable heat battery. In its cool state, the pack holds a liquid solution of sodium acetate -- a type of salt -- and water. A small metal disc about the size of a dime floats inside. Snap the disk and the pack changes within seconds from a clear, cool liquid to a hot, crystalline solid that can stay warm for hours. To use it again, boil the hard pack in water for about 10 minutes until it melts and let it cool.
What's going on? Palca explains by beginning at the end.
Boiling the hard pack melts the crystals and forces the sodium acetate back into a liquid. In this liquid state, the sodium acetate stores the heat from the boiling process.
Under normal circumstances, sodium acetate solution turns back into a solid when it cools. But it's possible to keep it in liquid form as long as it's kept in a smooth pouch with nothing inside to which the sodium acetate molecules can adhere. This prevents crystals from forming and changing the liquid into a solid.
Twisting the disk ignites a kind of chain reaction: a single crystal forms and then the rest of the sodium acetate rushes to crystallize. The chemical energy released as the crystals form is given off as heat. Essentially, the pack is a rechargeable battery: Boiling the pack recharges it by restoring heat to the solution.
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