Exothermic reactions

An exothermic reaction is a reaction where the substances react chemically to release energy in the form of heat. 
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: 
The chemical reaction that does the complete opposite, and absorbs heat, is an endothermic reaction.