Soft water is surface water that contains low concentrations of ions and in particular is low in ions of calcium and magnesium. Soft water naturally occurs where rainfall and the drainage basin of rivers are formed of hard, impervious and calcium poor rocks. Examples in the UK include Snowdonia in Wales and the Western Highlands in Scotland.
The term may also be used to describe water that has been produced by a water softening process although such water is more correctly termed softened water. In these cases the water may also contain elevated levels of sodium and / or bicarbonate ions .
Because soft water has few calcium ions, there is no inhibition of the action of soaps and no soap scum is formed in normal washing. Similarly, soft water produces no calcium deposits in water heating systems. Water that is not soft is referred to as hard water.
In the UK, water is regarded as soft if the hardness is less than 100 mg/l of calcium carbonate. Water containing more than 100 mg/l of calcium carbonate is termed hard water. In the USA soft water is classified as having less than 60 mg/l of calcium carbonate.
Sources of hardness
The hardness of water is determined by the concentration of multivalent cations in the water. Common cations found in hard water include Ca2+ and Mg2+. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
Calcium and magnesium ions are required for normal metabolism in many organisms including mammals. The lack of these ions in soft water have given rise to concerns about the possible health impacts of drinking soft water.
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- "Water hardness". US Geological Service. 8 April 2014.
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