UPSC Prelims 2022 Q100: Environment & Ecology – Pollution & Waste Management
Consider the following:
1. Carbon monoxide
2. Nitrogen oxide
3. Ozone
4. Sulphur dioxide
Excess of which of the above in the environment is/are cause(s) of acid rain?
- (a) 1, 2 and 3
- (b) 2 and 4 only
- (c) 4 only
- (d) 1, 3 and 4
Answer: (b) 2 and 4 only
Explanation
Gases that cause acid rain
Acid rain forms when sulphur dioxide and nitrogen oxides react with water vapour to form acids.
| Gas | Forms | Causes acid rain? |
|---|---|---|
| Carbon monoxide (CO) | Does not form a strong acid | No |
| Nitrogen oxides (NOx) | Nitric acid (HNO₃) | Yes |
| Ozone (O₃) | An oxidant, not an acid former on its own | No |
| Sulphur dioxide (SO₂) | Sulphuric acid (H₂SO₄) | Yes |
Effects of acid rain:
| Target | Damage |
|---|---|
| Monuments | Marble corrodes; Taj Mahal yellowing ('marble cancer') |
| Lakes | Acidification kills fish |
| Forests | Leaches nutrients from soil |
Normal rain has a pH of about 5.6; acid rain is below 5.6.
Answer: (b) 2 and 4 only.
Why each option is right or wrong
- (a) Incorrect. Carbon monoxide and ozone do not form acids, and it leaves out SO₂.
- (b) CORRECT. NOx forms nitric acid and SO₂ forms sulphuric acid in rain.
- (c) Incorrect. Nitrogen oxides also cause acid rain alongside SO₂.
- (d) Incorrect. CO and ozone are not acid-rain gases, and NOx is missing.
Key point
Acid rain = SO₂ (sulphuric acid) + NOx (nitric acid); rain with pH below 5.6; damages marble (Taj Mahal), lakes and forests.
Remember
Acid rain gases: SO₂ and NOx.
Common mistakes
- Including carbon monoxide because it is a major pollutant; it does not form an acid.
- Adding ozone, which is a smog component, not an acid-rain gas.
- Forgetting that NOx contributes alongside SO₂.
Exam tip
Acid rain = S and N oxides. Think 'SOx and NOx'.
Sources
- NCERT Class XI — Chemistry, Ch 14 (Environmental Chemistry)
- Central Pollution Control Board — Acid rain and air quality