Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : CO2, SO2 and NO2 gases get dissolved in rain water and produce acid rain.
Reason (R) : Atm

Question

Given below are two statements, one is labelled as Assertion (A) and the other as Reason (R).
Assertion (A) : CO2, SO2 and NO2 gases get dissolved in rain water and produce acid rain.
Reason (R) : Atmospheric air is polluted due to high concentration of gases like carbon dioxide, sulphur dioxide and oxides of nitrogen.
Select the correct answer from the codes given below :

UnlockIAS · Accepted Answer

Correct — A, both (A) and (R) are true and (R) is the correct explanation of (A). Taken as written, the assertion describes the standard mechanism of acid deposition: acidic oxides in the air dissolve in atmospheric moisture and come down as acidified precipitation. Sulphur dioxide dissolves to sulphurous acid and is oxidised to sulphuric acid; nitrogen dioxide gives nitrous and nitric acid; carbon dioxide dissolves to carbonic acid. The reason then supplies the cause the assertion presupposes — rain turns acidic only because the air carries these oxides in raised concentrations, chiefly from coal-fired power stations and smelters (SO2) and from high-temperature combustion in engines and furnaces (NOx). Because the pollutant loading is precisely what makes the dissolution happen on a scale that acidifies rainfall, (R) explains (A) rather than merely sitting alongside it, and the key marks (a). One refinement a serious student should carry, because UPSC has examined exactly this point: of the three gases named, only sulphur and nitrogen oxides yield the strong acids that define acid rain. Carbon dioxide dissolves to give carbonic acid, which is weak and is already present in clean air, so it sets the natural baseline — unpolluted rain has a pH of about 5.6 — and 'acid rain' is conventionally defined as precipitation more acidic than that baseline. In UPSC Prelims 2003 an assertion–reason item on this very subject was keyed (b) on the ground that oxides of carbon are not the principal acid-rain precursors. UPPSC's item is worded differently: its reason does not single out carbon, it names all three oxides as the pollution load, so the pairing holds together and (a) stands.

Why the other options are wrong — (b) Rejected because the assertion is not false. Each of the three named gases genuinely dissolves in water to yield an acid — SO2 to sulphurous and then sulphuric acid, NO2 to nitrous and nitric acid, CO2 to carbonic acid — and the resulting acidified precipitation is what is called acid rain. One could argue the assertion is imprecise for putting carbon dioxide on the same footing as the sulphur and nitrogen oxides, but imprecision is not falsity, and the key treats the statement as true.  (c) This is the trap option, and it fails on the logic rather than on the facts. Both statements are indeed true, but they are not independent observations: the assertion describes the effect and the reason states the cause. Acidic oxides can only dissolve into rain in acidifying amounts if they are present in the air in raised concentrations, which is exactly what (R) asserts. Where a reason supplies the necessary condition for the assertion's mechanism, it is the correct explanation, so (c) understates the link.  (d) The reason is not false. Elevated atmospheric concentrations of carbon dioxide, sulphur dioxide and nitrogen oxides are the defining signature of anthropogenic air pollution — SO2 overwhelmingly from burning sulphur-bearing coal and from ore smelting, NOx from combustion at high temperature in vehicles and boilers, CO2 from all fossil-fuel burning. All three are listed pollutants under India's National Ambient Air Quality Standards, so (R) is a plain statement of fact.

Concept: Acid rain is precipitation — rain, snow, fog or dry deposited particles — made abnormally acidic by pollutant gases. Even clean rain is mildly acidic, because atmospheric carbon dioxide dissolves in it to form carbonic acid, giving a pH of roughly 5.6; rainfall measurably below that figure is called acid rain. The extra acidity comes from sulphur dioxide and oxides of nitrogen, which are oxidised in the atmosphere and combine with water to give sulphuric and nitric acid — strong acids, unlike carbonic acid. Because the reaction happens while the air mass is moving, the damage often falls hundreds of kilometres downwind of the chimney that caused it, which is why acid rain became the classic example of transboundary pollution.

State PCS 2024 Q108 — Environment & Ecology PYQ with Answer

State PCS 2024 Q108 — Environment & Ecology PYQ with Answer