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,
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:
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 |