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What causes a disposable portable body warmer to heat up (become warm)?

Q. What causes a disposable portable body warmer to heat up (become warm)?

A. It uses the heat created by the rusting action of the iron powder in the packets.
Iron oxidization occurs when the iron rusts. Usually oxidization occurs at a rate that allows the energy that is created to dissipate and not actually produce heat.
Thus, a lot of initiative and research was done to hasten the speed of rusting to produce the heat necessary for the disposable portable body warmer.

These initiatives include:

  1. The iron is turned into powder form.
    By turning it into powder, the amount of iron exposed to the air is greater.
  2. Adding in salt and water.
    By adding salt and water, it hastens the iron's rusting time.
  3. Adding in charcoal.
    The charcoal acts to draw in more oxygen.

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What is iron oxide?

The combination of iron and oxygen resulting in the oxidation of the iron where oxygen combined with any compound (iron, copper, carbon, etcetera) produces an oxidative reaction.