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UPSC Prelims 2022 · Environment · Pollution Files
Consider the following: Excess of which of the above in the environment is/are cause(s) of acid rain?
- Carbon monoxide
- Nitrogen oxide
- Ozone
- Sulphur dioxide
- (a) 1, 2 and 3
- (b) 2 and 4 only
- (c) 4 only
- (d) 1, 3 and 4
Show the answer and explanation
Answer: (b) 2 and 4 only
NOₓ and SO₂ form nitric and sulphuric acids.
Statement by statement
1. ✗ NO CO is toxic but doesn’t form an acid in rain.
2. ✓ YES NOₓ forms nitric acid.
3. ✗ NO Ozone is an oxidant in smog, not an acid.
4. ✓ YES SO₂ forms sulphuric acid.
The idea
A gas causes acid rain only if it reacts with water in the air to form a strong acid. Sulphur dioxide gives sulphuric acid and nitrogen oxides give nitric acid. Carbon monoxide does not dissolve to form an acid, and ozone is an oxidising gas, not an acid-former.
SO₂ → oxidised in air → sulphuric acid → NOₓ → oxidised in air → nitric acid → Acids dissolve in cloud droplets → Rain with pH below 5.6
The trap
CO and ozone are famous pollutants, so they get included. Acid rain needs acid-forming gases.
Also know
Effects: lakes turn acidic and fish die; acid washes nutrients out of soil and releases aluminium, which harms tree roots; stone and metal structures corrode. The pH scale is logarithmic: rain of pH 4.6 is ten times as acidic as rain of pH 5.6.
🔒 Acid rain = SO₂ (→ H₂SO₄) + NOₓ (→ HNO₃); pH below 5.6.