Glossary
exothermic
An exothermic reaction releases energy as heat (and often light); an endothermic reaction takes energy in and cools its surroundings.
An exothermic reaction releases energy to its surroundings, usually as heat and sometimes as light — burning is the classic example, and it leaves the surroundings warmer. An endothermic reaction does the opposite: it takes energy in, so its surroundings cool. The words describe where the energy goes, not what you have to do to start the reaction: lighting a fire supplies the activation energy, but the burning that follows releases far more, so combustion is exothermic. The common error is to judge by how hard the reaction was to start. Ask instead whether the surroundings end up warmer (exothermic) or cooler (endothermic).
Taught in
Related terms
- catalyst — A catalyst speeds up a reaction by lowering the energy it needs, and it is not used up by the reaction.
- chemical change — A chemical change makes one or more new substances; the evidence is a gas, a precipitate, a color change, or heat or light.
Source: OpenStax Chemistry 1e and Chemistry: Atoms First 1e, first editions (CC BY 4.0). Each definition restates the Chemistry guide in this exam, which carries the per-source citations on its own page. (OpenStax 1e accessed 2026-10-08)