{"term":{"id":"14656","title":"equilibrium","longtitle":"IUPAC Gold Book - equilibrium","doi":"10.1351\/goldbook.14656","code":"14656","status":"current","definitions":[{"id":1,"text":"State of a system in which the macroscopic properties of the system become uniform and independent of time. If the temperature is uniform throughout the system, a state of thermal equilibrium has been reached; if the pressure is uniform, a state of hydrostatic equilibrium has been reached; and if the chemical potential of each component is uniform, a state of chemical equilibrium has been reached. If all these quantities become uniform, the system is said to be in a state of complete thermodynamic equilibrium.","notes":{"1":"Complete thermodynamic equilibrium can be expressed in many ways, depending on which variables are of interest. For solubility purposes, the important variables are \\(T\\), \\(p\\) and the chemical potentials \\(\\mu_{i}\\) of the \\(C\\) components. Equilibrium conditions involving these variables may be obtained by the condition that the variation of the Gibbs energy, expressed in terms of \\(T\\), \\(p\\) and the amount of substance \\(n_{i}\\) of the \\(C\\) components, is zero, \\(\\updelta G(T,p,n_{i}) = 0\\), i.e., the Gibbs energy is a minimum. If the variation is negative, an irreversible change of the system can occur.","2":"Solubility equilibrium is an example of a state of complete thermodynamic equilibrium. For example, a saturated solution of a solid in a liquid at a fixed temperature and pressure is in a state of complete thermodynamic equilibrium. If the system is subjected to a small increase in temperature, a small portion of solid will dissolve to restore the equilibrium (if the solubility increases with temperature), while if there is a small decrease in temperature, a small portion of solid will precipitate. This is the basis for determining accurate values of solubility by approaching the equilibrium solubility from both supersaturation and undersaturation directions."},"sources":["PAC, 2009, 81, 829. 'Glossary of terms used in ecotoxicology (IUPAC Recommendations 2009)' on page 880 (https:\/\/doi.org\/10.1351\/PAC-REC-08-07-09)"]}],"altoutputs":{"html":"https:\/\/goldbook.iupac.org\/terms\/view\/14656\/html","xml":"https:\/\/goldbook.iupac.org\/terms\/view\/14656\/xml","plain":"https:\/\/goldbook.iupac.org\/terms\/view\/14656\/plain"},"citation":"Citation: 'equilibrium' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351\/goldbook.14656","license":"The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/) for individual terms.","collection":"If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org .","disclaimer":"The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.","accessed":"2026-08-03T11:24:54+00:00"}}