Water is commonly classified as hard or soft depending on the type and amount of naturally occurring minerals and salts dissolved in it. The mineral content usually comprises the metal ions of calcium and magnesium (yes these are both metals!) in the form of their carbonates, calcium carbonate and magnesium carbonate but may include several other metals as well as sulphates and bicarbonates. When water has a relatively high content of dissolved minerals (solids) it is described as hard, whereas soft water has a low dissolved content.
Two common types of hardness in water are temporary hardness and permanent hardness: Temporary Hardness
The Temporary hardness of water can be eradicated the addition of lime (calcium hydroxide) or by boiling. It occurs because of dissolved calcium bicarbonate in the water. Calcium carbonate will not dissolve as easily in hot water as it does in cold water; As a result, boiling (which engenders carbonate) precipitates calcium carbonate from the solution, leaving a water that is less hard. This is what gives your kettle that lime scale coating!Permanent Hardness.
It is impossible to remove the permanent hardness of water by boiling. It occurs because of the concentration of calcium and magnesium sulphates and/or chlorides in the water. These become more soluble when there is a rise in temperature Degree of Measure.
The hardness of water is measured as milligrams per litre of calcium carbonate, e.g. 200mg/litre CaCO 3. Milligrams per litre is the same as parts per million (ppm). You may also see hardness expressed as mg/litre of calcium. These can be converted into mg/litre of CaCO3 by multiplying the reading by 2.5.