UPSC Prelims 2013 Q30: Environment & Ecology – Pollution & Waste Management
Acid rain is caused by the pollution of environment by
- (a) carbon dioxide and nitrogen
- (b) carbon monoxide and carbon dioxide
- (c) ozone and carbon dioxide
- (d) nitrous oxide and sulphur dioxide
Answer: (d) nitrous oxide and sulphur dioxide
Explanation
Cause of acid rain
Acid rain forms when oxides of sulphur and nitrogen in the air react with water vapour.
| Gas | Becomes | Main sources |
|---|---|---|
| Sulphur dioxide | Sulphuric acid | Burning of coal; smelters; thermal power plants |
| Oxides of nitrogen | Nitric acid | Vehicle exhaust; high-temperature burning |
Key facts:
| Feature | Detail |
|---|---|
| Normal rain | pH about 5.6, slightly acidic because of dissolved carbon dioxide |
| Acid rain | pH below 5.6 |
| Damage | Lakes turn acidic; forests die; marble corrodes (the Taj Mahal, 'marble cancer') |
The options checked: carbon dioxide gives only weak carbonic acid; carbon monoxide, nitrogen gas and ozone do not cause acid rain.
Answer: (d) nitrous oxide and sulphur dioxide.
Why each option is right or wrong
- (a) Incorrect. Carbon dioxide and nitrogen do not cause acid rain.
- (b) Incorrect. Carbon monoxide and carbon dioxide are not the cause.
- (c) Incorrect. Ozone and carbon dioxide are not the cause.
- (d) CORRECT. Acid rain is caused by oxides of nitrogen and sulphur dioxide.
Key point
Acid rain (pH below 5.6): sulphur dioxide gives sulphuric acid, nitrogen oxides give nitric acid; damages lakes, forests and marble.
Remember
Acid rain = sulphur dioxide + nitrogen oxides.
Common mistakes
- Blaming carbon dioxide, which only makes rain slightly acidic.
- Forgetting the pH 5.6 line between normal and acid rain.
- Not linking acid rain to damage at the Taj Mahal.
Exam tip
S and N make acid rain: sulphur and nitrogen oxides.
Sources
- NCERT Class XI — Chemistry (Environmental Chemistry: acid rain)
- NCERT Class XII — Biology (Environmental Issues)