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Instant Heat Packs

  1. Feb 29, 2008 #1
    How do instant, reusable heat packs work? An example.
    I know the throw-away kind use iron dust that oxidizes due to the action of some chemical, giving off heat.
     
  2. jcsd
  3. Feb 29, 2008 #2

    mrjeffy321

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    Inside the heat packs there is a supersaturated solution of Sodium Acetate. The supersaturated solution is metastable / unstable since too much Sodium Acetate is dissolve in solution for its temperature. Upon being triggered (by clicking the metallic button inside the pack), a nucleation site is provided for Sodium Acetate crystals to begin to form. As the crystallization process proceeds, Sodium Acetate Trihydrate is formed and the whole process is exothermic (will give off heat).

    The heat packs are reusable since you can easily re-dissolve the Sodium Acetate crystals again by heating the contents in boiling (or even just warm) water. The Sodium Acetate Trihydrate will “melt” / dissolve, then you allow the pack to cool back down to room temperature and the solution becomes supersaturated again. Then you can just repeat the process.

    A common chemistry demo preformed to demonstrate supersaturated solutions is sometimes called “Hot Ice”, in which a supersaturated solution of Sodium Acetate is triggered by touching it or dropping in a seed crystal. The solution, which initially appears to be water, seems to freeze…only the “ice” is warm to the touch.
    See this web page for additional details,
    http://www.amazingrust.com/Experiments/how_to/Hot-Ice.html
     
  4. Mar 2, 2008 #3
    Thank you.
     
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