Energy can play many different roles in chemical reactions, and it can be a product of a chemical reaction, a necessary component for a chemical reaction, or energy can have little part in a chemical reaction. Chemical reactions that require additional energy are called endothermic reactions, while those that produce energy are called exothermic reactions.
Endothermic reactions involve a temperature decrease during the process. Conversely, exothermic reactions may produce dangerous amounts of energy, such as occurs when a flame burns or an explosive reaction detonates. Energetically neutral reactions also occur in the natural world.
There are many examples of endothermic chemical reactions, including the melting of ice cubes, the evaporation of water, baking bread and the fission of a gas molecule. All of these chemical reactions require a net input of energy. By contrast, examples of exothermic reactions include the freezing of water, burning sugars or hydrocarbons and nuclear fission. These reactions all produce a net increase of energy.
Many chemical reactions are endothermic when performed in one direction, and are exothermic when performed in the other. For example, the process of forming ion pairs releases energy, while separating paired ions requires an input of energy. Condensation and evaporation exhibit a similar tendency, with condensation producing energy and evaporation requiring energy.