4C20.60 Heat Pack
Supercooling, latent heat
A heat pack is a commercial product used by hikers and skiers as a hand warmer. It consists of a plastic pouch filled with a supercooled solution of sodium acetate (NaCOOCH3). When a metal disc in the pouch is flexed, the sodium acetate nucleates on the disc, eventually crystallizing the entire pack and becoming hot.
-  Heat pack
- The pack is designed to fit in the hand and would not be visible without a document camera in any but the smallest of classes. It's best to pass it around after starting the crystallization process.
- Show the class the room-temperature heat pack.
- Click the metal disc. The solution will crystallize and the pack will become hot.
- Pass the pack around the room to allow the students to see the crystallization and feel the heat.
- We use the "EZHeat Instant Reusable Handwarmer" made by Prism Technologies and the "Toasterz Reusable Heat Pack," available from Mountain Equipment Coop. We have also used the "Magi-pad" by NHK.
- The pack may be reused many times by heating it in hot water to melt the solution. Eventually, however, it becomes impossible to keep the solution supercooled.
- Put the packs in very hot or boiling water for a few minutes to reset them to liquid state. Some of our packs have deteriorated so much that when cooled, some material comes out of solution. Strangely, the sodium acetate doesn't nucleate on gel-like material.
- Store the heat packs in liquid form to prolong their useful lifetimes. Water probably slowly leaks out of the packs when they're stored in crystallized form.
If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.