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Why do calcium and magnesium ions make water hard?
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ion-exchange reaction
Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked +. The whole structure is permeated by solvent molecules, usually water (not shown).
Water is classified as hard water when it contains dissolved salts of calcium and magnesium, typically in the form of bicarbonates, chlorides, and sulfates. These minerals cause hardness by reacting with other substances in ways that create problematic deposits or interfere with cleaning agents.12
The primary reasons calcium and magnesium ions are considered to cause hardness include:
- Interference with Soap: These ions react with the higher fatty acids found in soap to form an insoluble, gelatinous curd. This reaction prevents the soap from lathering effectively and results in a waste of the product.13
- Scale Formation: In boilers and hot water pipes, these ions form a hard, adherent scale on surfaces. Because this scale has poor heat conductivity, it increases fuel consumption and can lead to the rapid deterioration of equipment due to overheating.123
To mitigate these effects, water softening processes—such as ion exchange—are used to remove or replace these ions.3