NDA (II) 2017 General Ability Test · Question 138 of 150

Correct — C, sulphur dioxide. Acid rain is produced when the oxides of sulphur and nitrogen released by burning coal and petroleum are oxidised in the atmosphere and dissolve in cloud droplets, giving sulphuric and nitric acid. Sulphur dioxide from coal-fired power stations, smelters and refineries is the single largest contributor, and it is the only gas among the four offered that yields a strong acid on dissolving. Normal rainwater is already mildly acidic, at about pH 5.6, because atmospheric carbon dioxide forms weak carbonic acid; rain is called acid rain only when the strong acids drive the pH below that.

• (a)Ozone — Ozone at ground level is a serious pollutant, but a different one. It is a secondary pollutant formed when sunlight acts on nitrogen oxides and volatile organic compounds, and it is the marker of photochemical smog. It is an oxidising agent, not an acid-forming oxide.
• (b)Ammonia — Ammonia is alkaline. Dissolved in water it gives ammonium hydroxide, and in the atmosphere it actually neutralises acids rather than creating them, which is why ammonium sulphate is found in polluted air. It contributes to soil acidification only indirectly, through nitrification long after deposition, and is never cited as a cause of acid rain.
• (d)Carbon monoxide — Carbon monoxide is dangerous because it binds to haemoglobin, but it is almost insoluble in water and forms no acid. It is carbon dioxide, not monoxide, that dissolves to give the weak carbonic acid responsible for the natural pH of 5.6, and even that is not classed as acid rain.

Sulphur dioxide is oxidised in the atmosphere to sulphur trioxide, which dissolves in water to give sulphuric acid; nitrogen oxides follow a parallel route to nitric acid. The result falls as acid rain, or is carried down as dry deposition on dust. Its effects are acidification of lakes and soils, leaching of nutrient bases and mobilisation of aluminium in soil, damage to forests, corrosion of metal and the eating away of limestone and marble in buildings and statues.

The clean way to answer any acid-rain item is to run each gas through the question: does it dissolve to give a strong acid? Sulphur dioxide and the nitrogen oxides do. Carbon monoxide does not dissolve, ammonia gives a base, and ozone is an oxidant rather than an acid precursor. The same pair of culprits recurs across boards and years, and the UPSC has set it four separate times, so the pairing of sulphur and nitrogen oxides is worth committing to memory as a unit.

• Acid rain is caused chiefly by sulphur dioxide and oxides of nitrogen, which give sulphuric and nitric acid.
• Unpolluted rain already has a pH of about 5.6 because of dissolved carbon dioxide.
• Coal combustion in power stations and smelting of sulphide ores are the main sources of sulphur dioxide.
• Acid deposition acidifies lakes and soils, harms forests and corrodes limestone and marble structures.
• Ground-level ozone is a secondary pollutant of photochemical smog, a separate problem from acid rain.

• Blaming carbon dioxide or carbon monoxide for acid rain; the strong acids come from sulphur and nitrogen.
• Treating ozone as an acid-forming gas rather than a photochemical oxidant.
• Forgetting that ammonia is basic and works against acidification in the air.

Acid rain is asked as a single-gas item like this one, as a multi-statement list of gases to be filtered, or as an assertion-reason pair about thermal power stations.

• practice — not a real PYQ

  The pH below which rainwater is normally described as acid rain is about

  • (a)7.0
  • (b)6.5
  • (c)5.6
  • (d)4.0

  Answer(c) 5.6 — the natural pH of rain in equilibrium with atmospheric carbon dioxide.

• practice — not a real PYQ

  Acid rain damages marble and limestone monuments because these are made largely of

  • (a)silicon dioxide
  • (b)calcium carbonate
  • (c)aluminium oxide
  • (d)iron oxide

  Answer(b) calcium carbonate — it reacts readily with sulphuric and nitric acid and is eaten away.