Acid rain refers to wet and dry depositions in the atmosphere that contain very high amounts of nitric and sulphuric acids. Acid rains occur when gases like sulphur dioxides (SOx) and nitrogen oxides (NOx) react in the atmosphere with water and oxygen.
Together, they form chemical compounds that are acidic in nature. Although normal rainwater always has some small amount of acids because of the presence of carbon dioxide, but it is usually below dangerous levels. Normal rains are termed as acid rains when the pH level goes below an acceptable level.
The lower the pH value of a substance, the more acidic it is. Neutral water has a pH of 7. So, when the pH level goes below 7, it is termed as acidic. Normal rain with small amounts of carbon dioxide has a pH of about 5.6.
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While carbon dioxide mixes with rainwater to form carbonic acid, H2CO3, sulphur dioxide and nitrogen dioxide mix with rainwater to form more harmful acids, H2SO4 and HNO3, respectively. A small amount of hydrochloric acid (HCI) may also be present in rainwater.
Sulphur dioxides are formed when sulphur mixes with oxygen to form sulphur dioxide.
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Sulphur dioxide mixes with oxygen to form sulphur trioxide.
S02 + 02 -> 2S03
When nitrogen mixes with oxygen it forms nitrogen oxide.
N2 + 02 2NO
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Nitric oxide mixes with ozone to form nitrogen dioxide and oxygen.
NO + 03 N02 + Oz
Nitrogen dioxide mixes with ozone to form nitrogen trioxide and oxygen.
No2 + o3 -> No3 + o2
And nitrogen trioxide mixes with nitrogen dioxide to form nitrogen pentaoxide.
No3 + NO2 -> N7O5
Nitrogen and sulphur oxides react with rainfall to form nitric acid and sulphuric acid.
2SOz + 02 + H20 -> 2H2S04 (sulphuric acid)
N205 + H20 -> 2HN03 (nitric acid)
Causes of acid rain:
The major cause of acid rain is industrialization, which releases sulphur dioxide and nitrogen oxides into the atmosphere. Power plants that use coal as an input are known to release about 70 per cent of the world’s sulphur dioxide emissions. Vehicular pollution is known to release about 50 per cent of the world’s nitrogen oxide.
While there are some natural causes of acid rains, like volcanic eruptions, decomposition of bacteria and fires, they cause little harm to the environment compared to man-made causes. Sulphur dioxide and nitrogen dioxide can travel long distances and they mix with other chemicals in the air to form new, more harmful chemicals and substances. Acid rain can be formed as rain, snow, hail, fog, frost, or dew.