Exothermic refers to a chemical process that releases energy as a byproduct. This energy is usually in the form of heat, but can also be in the form of light, electricity, or sound. The release occurs when the bonds between the chemicals used in the reactions are converted into thermal energy.
First used by French chemist Marcellin Pierre Eugène Berthelot, the term comes from the Greek prefix "ex-" and the word thermein. Thermein refers to the processes of heating, while the prefix ex- means simply outside, indicating that this is the direction the heat is going when produced. Berthelot believed that all chemical processes could be measured in some way, such as by the heat released from exothermic reactions.
Exothermic reactions take place in conditions where the chemical reactants are under constant pressure and at a constant volume. In the cases of reactions which release heat, this is seen in an increase in the temperature. This heat can come from a prior release of another byproduct such as light. In this case, the light is absorbed into the molecules of the chemical, making the molecules vibrate. This in turn generates the heat.
Sometimes the heat is so minor that scientific instruments are needed to measure the change in temperature. Other reactions are felt without the aid of scientific equipment. Exothermic reactions are common, and occur when water condenses from vapor and forms rain or when snow forms in the clouds. Lighting a candle results in a chemical, exothermic reaction felt in the heat of the flame, just as the burning of wood or coal in a stove produces enough heat to warm an entire home.
Nuclear fission is another example of an exothermic reaction. In nuclear fission, the atoms of certain isotopes are split, releasing heat energy. When an atom is split with a neutron, it releases not only heat but an extra neutron that goes on to split another atom, starting a chain reaction. It is this chain reaction that powers a nuclear reactor, and the release of heat energy that makes it such a valuable energy source. The reactions are kept in check in a reactor, however; when left unchecked, the reaction turns into that which drives a nuclear bomb.
The opposite of an exothermic reaction is an endothermic one. In this type of chemical reaction, heat energy is absorbed in the process; this is seen in reactions like photosynthesis and evaporation. These chemical processes cannot take place without the existence of heat, and because exothermic reactions produce heat instead of needing it as a component of the reaction, exothermic processes are more spontaneously occurring.