The activation energy for a(n) is quite large and usually takes extra energy from the environment, it is normally not a natural spontaneous process.

Endothermic Reaction. This reaction or process absorbs energy from its surroundings, mostly, but will not always be, in form of heat. A reaction that requires the input of energy that breaks a bond will always be endothermic. Best examples of endothermic reaction are: 1) melting cubes; 2) evaporating of liquid; 3) melting solid salts, etc.

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