Page 203 - Chemistry ICSE Class IX

Atmospheric Pollution 191
Acid Rain

What is meant by acid rain
All forms of precipitation, such as rain, snowfall, fog, and dew, which are
more acidic than normal water are called acid rain.
The normal rain water has a pH of about 5.6 due to the dissolution of
carbon dioxide.
+
CO (g) + H O(l) o H CO (aq) U H + HCO 3 –
2
3
2
2
Therefore, all forms of precipitation, such as rain, snowfall, fog and
dew, which have pH below 5.6 (commonly between 3.5 and 2) are called
acid rain.
What causes acid rain
Acid rain is caused by the dissolution of oxides of sulphur (SO and SO )
3
2
and oxides of nitrogen (mainly NO ) in atmospheric moisture or rain water.
2
Sulphur oxides/Nitrogen oxides + Water vapour o Acid rain
Thus, acid rain consists of mineral acids, such as sulphuric acid,
sulphurous acid and nitric acid.

Formation of sulphuric acid and sulphurous acid
Sulphur oxides are released into the atmosphere by the following processes:
ƒ Burning of fossil fuels
ƒ Smelting of sulphide ores of copper, lead, zinc
ƒ Petroleum re ning
ƒ Manufacturing of sulphuric acid, superphosphate of lime, etc.
When coal and petroleum products are burnt, a small quantity of
sulphur dioxide and sulphur trioxide are evolved.
burning
S + O (g) o SO (g)
2
2
burning
2S + 3O (g) o 2SO (g)
2
3
A part of SO gets oxidised to SO on soot/dust,
3
2
burning
2SO (g) + O (g) o 2SO (g)
2
3
2
SO dissolves in water to form sulphurous acid (H SO ).
3
2
2
SO (g) + H O(l) o H SO (aq)
2
2
2
3
Sulphurous acid
SO dissolves in water to form sulphuric acid (H SO ).
2
4
3
SO (g) + H O(l) o H SO (aq)
3
2
2
4
Sulphuric acid
Formation of nitric acid
Nitrogen oxides (mainly NO) are released into the atmosphere during the
following processes:
ƒ Automobile exhausts
ƒ Electric discharge during lightning and thunder
In both the processes, nitrogen and oxygen (in air) react to form nitric
oxide (NO).
high temp.
N (g) + O (g) o 2NO(g)
2
2
electric discharge
From air