What Makes Water Hard?

Acid in rain
Most people have heard of acid rain produced by absorption of sulphur and nitrogen oxides by rainfall; however, the presence of pollutants like these gases is not the only source of acidity. Carbon dioxide constitutes 0.3% of the Earth's atmosphere, and is readily absorbed by water to form carbonic acid - the acid found in carbonated beverages. Therefore, all rain is acidic to some extent, and has a strong tendency to dissolve minerals and rocks with which it comes into contact.

Water is a good solvent and gathers impurities easily. Pure water, which is tasteless, colorless, and odourless, is often called the universal solvent. As rain falls it becomes slightly acidic as described above. As the water travels through soil and rock, it dissolves small amounts of minerals and holds them in solution. As already mentioned, calcium and magnesium dissolved in water are the two most common minerals that make water 'hard.' The degree of hardness becomes greater as the magnesium and calcium content increases, and is related to the concentration of multivalent cations dissolved in the water.

Below is British Standard 7593: 1992 as used in the UK and this categorises
mains water supplies as follows:

Designation	Hardness Concentration mg/Litre (as CaCo3)
Soft	0-50
Moderately Soft	50-100
Slightly Hard	100-150
Moderately Hard	150-200
Hard	200-300
Very Hard	Over 300

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