What is the primary indicator of an exothermic reaction occurring?

The primary indicator of an exothermic reaction is the release of heat. In an exothermic reaction, energy is released to the surroundings, typically in the form of heat, as reactants change into products. This release of heat causes an increase in temperature in the surrounding environment. For example, combustion reactions, where fuels react with oxygen, are classic examples of exothermic processes.

While absorption of light, formation of a precipitate, and a change in color can occur during various chemical reactions, they are not definitive indicators of exothermic reactions. Heat release is a clear signature of energy transformation in these processes, distinguishing exothermic reactions from endothermic ones, which absorb heat and lead to a decrease in temperature in the surroundings. Understanding these principles is crucial in identifying and categorizing chemical reactions accurately.