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, Dissolved calcium and
magnesium in water are the most common causes of hard water.
The level of hardness increases as the magnesium and calcium
increases.
Below is British Standard 7593: 1992 as used in the UK and this
categorises
mains water supplies as follows: