What does exothermic mean?
Exothermic is an adjective used to describe a chemical reaction or physical process that releases energy in the form of heat to its surroundings. Combustion is the classic example: burning wood, gas, or fuel gives off warmth and light, which is why fires heat a room and engines power vehicles. Other familiar exothermic processes include rusting, the setting of concrete, and the chemical reaction inside disposable hand-warmers. In thermodynamic terms, an exothermic reaction has a negative change in enthalpy, meaning the products hold less energy than the reactants did. The word's opposite is endothermic, describing processes such as photosynthesis or melting that absorb heat instead. Derived from Greek roots meaning 'outside' and 'heat', exothermic entered scientific vocabulary in the nineteenth century and remains standard terminology in classrooms and laboratories worldwide. It carries no emotional connotation — it is a precise, neutral descriptor — yet it underpins everyday phenomena from cooking flames to the warmth of a compost heap, making it one of the more practical terms a student of science can master.
adjectiveRelating to or denoting a chemical reaction or process that releases heat (or light) to its surroundings. More generally, describing any process that gives off energy in the form of heat.
- (of a chemical reaction or physical process) releasing heat to the surroundings, typically with a negative enthalpy change.
"Combustion of natural gas is an exothermic reaction, releasing energy that heats the home."
"The neutralization of an acid with a base is a classic exothermic reaction, warming the solution noticeably."
"Concrete cures through an exothermic process, and engineers must monitor the internal temperature of large pours to prevent cracking."
Every candle flame, hand-warmer, and firework burst you've ever enjoyed was chemistry giving heat away — that's exothermic in action.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of exothermic
Exothermic derives from Greek elements: 'exo-', meaning 'outside' or 'external', and 'therm', from 'thermē' (or 'thermos'), meaning 'heat'. The suffix '-ic' forms adjectives. The term was coined in the context of nineteenth-century chemistry, when scientists were systematizing the study of heat changes in reactions. It shares its roots with a wide family of thermal vocabulary, including thermostat, thermodynamics, and isotherm, all built on the same Greek stem for heat.
Related word forms
How exothermic is actually used
Exothermic is a technical term used chiefly in chemistry, physics, and engineering; it is formal but widely understood thanks to school science curricula. It applies primarily to reactions and processes rather than objects — a furnace is hot, but the combustion inside it is exothermic. The standard comparative forms are 'more exothermic' and 'most exothermic'; in technical writing, reactions are often described as 'more exothermic' when they release greater amounts of heat per mole.
Easily confused with exothermic
Endothermic describes processes that absorb heat from their surroundings, whereas exothermic describes processes that release heat into them.