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How do instant hot/cold packs work? Like when you break and shake the heat/cold pack, how does it instantly heat up/cool down?
Answer 1:

From what I read, it sounds like they are using the combining of iron with oxygen in the air to make rust, which gives off heat. In order for the reaction to happen quickly enough for you to notice the heat given off, the iron is broken up into tiny fragments, or perhaps even just dissolved in something.



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