In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning “inside” and the Greek suffix –thermic, meaning “to heat”. The opposite of an endothermic process is an exothermic process, one that releases energy in the form of heat. The term “endothermic” was coined by Marcellin Berthelot.
The concept is frequently applied in physical sciences to e.g. chemical reactions, where thermal energy (heat) is converted to chemical bond energy.
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Endothermic, also incorrectly known as endergonic, refers to a transformation in which a system receives heat from the surroundings:
When the transformation occurs at constant pressure:
and constant volume:
If the surroundings do not supply heat (e.g., when the system is adiabatic), an endothermic transformation leads to a decrease in the temperature of the system.
Some examples of endothermic processes are:
Chemical endothermic reactions need heat to be performed. In a thermochemical reaction that is endothermic, the heat is placed on the reactants side (heat is necessary for and absorbed during the reaction).
Endothermic substances, both natural, e.g. gypsum, and synthetic, e.g. resin-based intumescents, are popular for use in heat shielding, ablation, materials in space physics, fireproofing, e.g. fire-resistive coatings for LPG vessels, and compartmentalisation of fire in buildings, which is the cornerstone of passive fire protection. Typically, the technological basis is the conversion of hydrates, or chemically-bound water into vapour, or steam.