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Hard water is defined as the presence of divalent cations, Ca2+ and Mg2+. These ions do not affect the health of people who drink the water but does affect the volume of soap powder needed to get a froth. If the calcium and magnesium is in the form of bicarbonates it is simple to soften the water by boiling it. We see the effects of this if we look inside the kettle. The brown residue is due to calcium and magnesium "scale". This causes a problem in commercial boilers as the scale build up can clog pipes. The water has a temporary hardness that can be removed by boiling. Permanent hardness occurs when the calcium and magnesium salts are not bicarbonates but have another anion like sulfate. This hardness has to be removed by adding water softener.