Introduction to Hard Water

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
Temporary hardness of water can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by dissolved calcium bicarbonate in the water. Calcium carbonate is less soluble in hot water than in cold water; therefore, boiling (which promotes the formation of carbonate) precipitates calcium carbonate out of solution, leaving water that is less hard on cooling. Unfortunately this is what causes your kettle to become coated with limescale!

Permanent Hardness
Permanent hardness of water cannot be removed by boiling. It is usually caused by the concentration of calcium and magnesium sulphates and/or chlorides in the water, which are more soluble as the temperature rises.

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.

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