Many common chemicals undergo exothermic reactions. For example, simply dissolving sodium hydroxide (NaOH) in water produces enough heat that if this is not done carefully it could melt a plastic container! Our entry on oxidation has an example of an even more exothermic reaction.
The heat that a chemical reaction gives off can quickly heat the surrounding area (or rest of the chemicals in the container) to very high temperature. As temperature increases, the rate of chemical reactions generally increases as well. Thus, once an exothermic reaction begins, it can quickly "run away" -- accelerating its rate because of the heat produced. This can be especially dangerous if the material reaches its flash point or autoignition temperature, at which point a fire or explosion could occur.
Therefore, it is very important to know when a chemical reaction can generate excess heat and to take appropriate measures to deal with this. Examples include slow mixing, using a cooling bath or avoiding that reaction. Most mixtures of incompatible chemicals involve violently rapid exothermic reactions.
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