Hardness is caused by divalent cations (ions with a positive charge of 2+). Usually the offending cations are calcium (Ca2+) and magnesium (Mg2+). These and similar cations react with compounds containing monovalent cations (usually sodium, Na+) to form insoluble products. Along with several lesser problems, these precipitants form encrustations and deposits in hot water pipes, heat exchangers, and boilers (insolubility and precipitation increase with temperature) and also form scum when using soap for cleaning.

The effect of soap added to water containing a calcium compound is most striking. Soap and many calcium compounds such as bicarbonate are normally soluble in water. When the monovalent sodium ion in soap is replaced by calcium, an insoluble end product is formed:

Soap Calcium bicarbonate

Insoluble scum Sodium bicarbonate

Two types of hardness exist: carbonate hardness and non-carbonate hardness. For the former, the cations are combined with either bicarbonate or carbonate. For noncar-bonate hardness, the cations are combined with chlorides, sulfates, and other anions.

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