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Hard water contains calcium and/or magnesium ions. Hard water is formed when rainwater falls in areas containing calcium and/or magnesium minerals.
Temporary hard water contains the calcium and/or magnesium as the hydrogencarbonates. On boiling the water a reaction occurs which forms insoluble carbonates:
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Ca(HCO3)2(aq) ⇒ CaCO3(s) + H2O(l) + CO2(g) |
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This removes the calcium and/or magnesium ions, and so the water is no longer hard. Hence the name temporary hard water. However, this reaction can cause problems as it deposits the insoluble carbonate as scale. This is the reason for kettles scaling up. The same process occurring in a boiler can eventually seal up the pipes and cause the boiler to explode! This same reaction in Nature very slowly causes the formation of stalagmites and stalactites. |
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Permanent hard water contains the calcium and or magnesium as other salts like sulfates. These remain in solution on boiling, so the water remains hard, hence the name.
A major problem with hard water is that the ions cause the formation of an insoluble scum with soap. Soap is usually a sodium salt of a fatty acid. A common soap is sodium stearate (sodium octadecanoate), which we can represent as NaSt. Being a sodium salt it is soluble. However, on adding hard water a precipitate of calcium stearate or magnesium stearate forms:
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NaSt(aq) + Ca2+(aq) ⇒ CaSt2(s) + 2Na+(aq) |
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The soap will not lather until this precipitation has finished and some sodium stearate goes into solution. This is very wasteful of soap and also produces an unpleasant scum in basins and baths. Consequently, water softeners are often used in hard water areas to remove the calcium and magnesium ions and give soft water.
Hard water does have some advantages. It has been suggested that it is associated with reduced risk of heart disease and it does make good beer. Traditional brewing centres like Burton-on-Trent and Tadcaster have hard water bore holes.
