14730 IUPAC Gold Book - hard water 10.1351/goldbook.14730 14730 current 1 Water that contains mineral salts of divalent cations, commonly calcium and magnesium and sometimes iron(II) (ferrous) ions, principally as hydrogencarbonates, chlorides, and sulfates. Hardness caused by calcium hydrogencarbonate is known as temporary, because boiling converts the hydrogencarbonate to calcium carbonate (\(\ce{CaCO3}\)), which has a very low solubility; hardness from the other salts is called permanent. PAC, 2009, 81, 829. 'Glossary of terms used in ecotoxicology (IUPAC Recommendations 2009)' on page 893 (https://doi.org/10.1351/PAC-REC-08-07-09) https://goldbook.iupac.org/terms/view/14730/html https://goldbook.iupac.org/terms/view/14730/json https://goldbook.iupac.org/terms/view/14730/plain Citation: 'hard water' 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.14730 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. If you are interested in licensing the Gold Book for commercial use, please contact the IUPAC Executive Director at executivedirector@iupac.org . 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. 2026-06-29T08:41:15+00:00