Unpolluted
rain water is slightly acidic owing to the presence of carbon dioxide
in the air. Its pH is at 5.7.
Therefore, rain water with pH values lower than 5.7 is called acid rain. In some parts of the world,a pH as low as 2.5 in rainwater has been recorded.
The Cause
Sulphur dioxide
It is the primary cause of acid rain. Sulphur dioxide is released from burning of fossil fuels and industrial plants.
The sequence of changes from the emitted gas to acid is at
follows:
[Note: The actual pathways are more complex.]
The forming of sulphur trioxide from sulphur dioxide is influenced by the prevailing atmospheric conditions:
Besides the formation of sulphuric acid [sulphuric(VI) acid],
sulphurous acid [sulphuric(IV) acid] is also formed.
Nitrogen oxides
It is another cause of acid rain. Nitrogen oxides are released from vehicles and power plants.
The sequence of changes are as follows:
[Note: The actual pathways are more complex.]
Effects
Vegetation
Acid rain washes away esssential nutrients that the plants are in need. In adition, it makes the soil acidic and aids the release of aluminium and copper ions which are harmful to plants.
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| A woodland in Germany in 1972. | The same spot in 1983. Notice that acid rain has destroyed the forest. |
Aquatic life
When
pH is less than 4.5, calcium metabolism in fresh water fish will be affected,
leading to poor health and stunted growth. As a result, diversity and population
of some fish species will be reduced.
Building
materials
Acid rain will cause damage to common building materials
(such as limestone and marble), statues and monuments.
Many metals will become oxidised. Iron corrodes with the presence of
acid rain to form rust. The cost of maintainance of iron structures is
high in highly polluted areas.
Related Topics
Sulphur dioxide
Nitrogen oxides
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