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 (United Kingdom) 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 bicarbonate ions.
Because soft water has few calcium ions, there is no inhibition of the lathering 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 isn't soft is referred to as hard water.
In the UK, water is regarded as soft if the hardness is less than 50 mg/l of calcium carbonate. Water containing more than 50 mg/l of calcium carbonate is termed hard water. In the United States soft water is classified as having less than 60 mg/l of calcium carbonate.
In the USA, due to ancient sea beds with high limestone (calcium carbonate) concentrations, as much as 85% of water is hard, and many need water softening treatment. Waters in Eastern and Southern Florida, North-western Texas, and the entirety of South Dakota are considered extremely hard; the metropolitan cities with hardest water include Indianapolis, Las Vegas, Minneapolis, Phoenix, San Antonio and Tampa.
Sources of hardness
Hardness of water is determined by the concentration of multivalent cations. Common cations for hardness include: Ca2+ and Mg2+. 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 (contains calcium). Rainwater and distilled water are soft, as they contain fewer ions.
Calcium and magnesium ions are required for normal metabolism in many organisms including mammals. Reduced number of ions give rise to concerns about the health impacts of drinking soft water, including sudden cardiac death.
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