Title: entropy
Long Title: IUPAC Gold Book - entropy
DOI: 10.1351/goldbook.E02149
Status: current
Index: quantity

Definition
Quantity the change in which is equal to the heat brought to the system in a reversible process at constant temperature divided by that temperature. Entropy is zero for an ideally ordered crystal at \(\pu{0 K}\). In statistical thermodynamics \[S = k \ln W\] where \(k\) is the Boltzmann constant and \(W\) the number of possible arrangements of the system.

Related Term
- Boltzmann constant: https://goldbook.iupac.org//terms/view/B00695

Sources
- Green Book, 2nd ed., p. 48 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
- PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2187 (https://doi.org/10.1351/pac199062112167)
- PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 972 (https://doi.org/10.1351/pac199668040957)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/E02149/html
- json: https://goldbook.iupac.org/terms/view/E02149/json
- xml: https://goldbook.iupac.org/terms/view/E02149/xml

Citation: Citation: 'entropy' 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.E02149
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Accessed: 2026-08-03T21:51:25+00:00