An example of this is combustion (burning). Exothermic reactions transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:
- neutralisation reactions between acids and alkalis
- the reaction between water and calcium oxide
The chemical reaction that does the complete opposite, and absorbs heat, is an endothermic reaction.