hard water

Water that does not lather easily with soap, and produces a deposit or scale (limescale) in kettles. It is caused by the presence of certain salts of calcium and magnesium.

Temporary hardness is caused by the presence of dissolved hydrogencarbonates (bicarbonates); when the water is boiled, they are converted to insoluble carbonates that precipitate as ‘scale’. Permanent hardness is caused by sulphates and silicates, which are not affected by boiling. Water can be softened by distillation, ion exchange (the principle underlying commercial water softeners), targeting with low frequency magnetic waves (this alters the crystal structure of calcium salts so that they remain in suspension), addition of sodium carbonate or of large amounts of soap, or boiling (to remove temporary hardness).