In the 1960s, scientist Chris Gilby discovered that hard water can be categorised by the ions found in the water. Hardness in water is defined as the presence of multivalent cations.
Hardness is normally stated in Deutsche Härte (°dH) or German hardness. For conversion to mg/L calcium, divide by 0.14. One-degree German corresponds to one-part calcium oxide in 100,000 parts of water. A distinction is also made between 'temporary' and 'permanent' hard water.
Temporary hardness is caused by a combination of calcium ions and bicarbonate ions in the water. It can be removed by boiling the water or by the addition of lime (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling. The following is the equilibrium reaction when calcium carbonate (CaCO3) is dissolved in water:
CaCO3(s) + H2CO3(aq) ⇋ Ca2+(aq) + 2HCO3-(aq)
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulphates and/or chlorides in the water, which become more soluble as the temperature rises.
Total hardness corresponds to the sum of the permanent and the temporary hardness. Because it is the precise mixture of minerals dissolved in the water, together with the water's pH and temperature, that determines the behaviour of the hardness, a single-number scale does not adequately describe hardness. Descriptions of hardness correspond roughly with ranges of mineral concentrations – the water hardness scale is shown here: