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Is there an exothermic reaction which produces 1 to 2 minutes of heat at around 175 degrees Fahrenheit, no toxic gasses and is considered to be more or less safe?

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    $\begingroup$ Safe is a matter of scale. The reactions mentioned in the other answers are all "safe" on small scales, say less than 100 mL or 100 g. What are you planning on heating? How big is it? $\endgroup$
    – Ben Norris
    Oct 20 '12 at 12:36
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Crystallisation of supersaturated sodium acetate is a good one, as seen in my answer here.

Another is the reaction of water with calcium oxide, which is commonly seen in self-heating coffee and soup cans. This reaction can get very hot, though.

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Simplest thing that comes to my mind is dissolving sodium or potassium hydroxide in water, and/or neutralizing it. Both steps are exothermic, and also rather safe (given that you don't just blindly throw things together). Should become hot enough, I'd guess.

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Use a mix of ammonium chloride, sodium nitrite and acetic acid. It produces water, nitrogen and heat.

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  • $\begingroup$ Could you elaborate? Maybe add reaction equation $\endgroup$
    – Mithoron
    Jun 9 '16 at 23:15
  • $\begingroup$ "Ammonium chloride" and "sodium nitrite" are actually aqueous solutions of those salts. The reaction is $$NH_4Cl + NaNO_2 --> N_2 + 2H_2O + NaCl$$. $\endgroup$ Jun 10 '16 at 3:09

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