Chemical reactions that produce heat are called exothermic reactions. Exothermic reactions involve physical or chemical changes that generate heat in various quantities, which is then dispersed into the surrounding environment. Exothermic reactions come in several forms, and require more energy to occur than they produce.
Exothermic reactions take place throughout the world and may occur in the water, land, and in the atmosphere. Examples of exothermic reactions include combustion reactions of fuels, the addition of concentrated acid to water (also called neutralization), the burning of certain substances, and the addition of water to anhydrous copper sulfate. Most exothermic reactions take place when triggered by an external catalyst, although some come from internally produced sources of heat, such as the eruption of volcanoes.
The reaction that occurs when metals oxidize and corrode is another type of an exothermic reaction, as respiration and even the decomposition of decaying plant and food matter into compost or humus. Some exothermic reactions, such as the combustion of fuels in car and plane engines, take place through mechanical means. Others, such as the decomposition of vegetable matter to waste, occur solely through natural forces. Exothermic reactions are the opposite of endothermic reactions, which result in systems absorbing heat energy from their surroundings.