For acid rain, the pH of rain water should be less than
- (a)7.0
- (b)6.6
- (c)5.6
- (d)8.0
Correct — C, 5.6. Even clean rain is mildly acidic because atmospheric carbon dioxide dissolves in it to form weak carbonic acid, giving a natural pH of about 5.6. Rain is therefore called acid rain only when its pH drops below this 5.6 baseline, which happens when sulphur dioxide and nitrogen oxides add much stronger sulphuric and nitric acids.
- (a)7.0 — A pH of 7 is neutral, but ordinary unpolluted rain is already slightly acidic (about 5.6) from dissolved CO2. Using 7 as the threshold would wrongly brand normal rain as acid rain.
- (b)6.6 — 6.6 is below neutral but still well above the natural rainwater value; rain at 6.6 is not classed as acid rain. The accepted cut-off is the natural baseline of 5.6, not 6.6.
- (d)8.0 — A pH of 8 is basic (above neutral), the opposite of acidic. Acid rain must be more acidic than natural rain, so the threshold cannot be above 7.
Acid rain is precipitation made unusually acidic by air pollution. The reference point is natural rainwater, whose pH is around 5.6 because dissolved carbon dioxide forms weak carbonic acid. Sulphur dioxide and nitrogen oxides from burning fossil fuels form sulphuric and nitric acids that push the pH below this value.
The trap is to assume anything below neutral pH 7 counts as acid rain. The correct benchmark is the natural CO2-driven value of 5.6; only rain more acidic than that is 'acid rain'. So the threshold is 'less than 5.6', option (c).
- Normal rainwater has a pH of about 5.6 due to dissolved CO2 forming carbonic acid.
- Rain is termed acid rain when its pH falls below 5.6.
- The main culprits are SO2 and NOx, which form sulphuric and nitric acids.
- Acid rain damages soils, lakes, forests and limestone/marble monuments.
- Taking neutral pH 7 as the acid-rain threshold.
- Forgetting that clean rain is already slightly acidic.
- Confusing the gases (SO2/NOx) with CO2, which only sets the 5.6 baseline.
NDA/UPSC ask for the pH threshold of acid rain or the gases that cause it — the number to remember is a pH below 5.6.
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
The companion fact to this NDA question. It asks which pollutants cause acid rain (nitrogen oxides and sulphur dioxide), while the NDA item asks the pH threshold (below 5.6) that those same acids produce.
Consider the following: 1. Oxides of Hydrogen 2. Oxides of Nitrogen 3. Oxides of Sulphur. Which of the above causes/cause acid rain?
- (a) 1 and 2 only
- (b) 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer(c) 2 and 3 only
Same acid-rain chemistry — oxides of nitrogen and sulphur (not hydrogen) form the acids that drop rainwater below the pH 5.6 threshold this NDA question asks for.
- practice — not a real PYQ
The two main gases chiefly responsible for acid rain are
- (a)CO2 and CO
- (b)SO2 and NOx
- (c)O2 and N2
- (d)CH4 and CO2
Answer(b) SO2 and NOx — they form sulphuric and nitric acids in rain.
- practice — not a real PYQ
Normal (unpolluted) rain water is slightly acidic mainly because it dissolves
- (a)Sulphur dioxide
- (b)Carbon dioxide
- (c)Ammonia
- (d)Ozone
Answer(b) Carbon dioxide — dissolved CO2 forms weak carbonic acid, giving pH about 5.6.