CDS (I) 2022 General Knowledge · Question 27 of 120

Correct — A, Dissolution of sulphur and nitrogen oxides in rain. Rain becomes acidic when oxides of sulphur and nitrogen enter the atmosphere, dissolve in cloud droplets and are converted there into sulphuric and nitric acid. Sulphur dioxide comes mainly from burning coal and from smelting sulphide ores; the nitrogen oxides come from any combustion hot enough to make the nitrogen and oxygen of the air itself react, which is why vehicle engines and power-station furnaces are the main sources. Ordinary rain is already mildly acidic at about pH 5·6, because carbon dioxide dissolves in it to give carbonic acid, and rain is called acid rain when the sulphur and nitrogen acids push it below that. The consequences are acidified lakes and streams with fish dying out, damage to forest soils, and the visible eating away of limestone and marble on buildings and statues.

• (b)Dissolution of minerals in rain — Rain forms high in the atmosphere, where there are no minerals to dissolve. Mineral matter that does reach rainwater is usually alkaline and would raise the pH rather than lower it.
• (c)Dissolution of dust particles in rain — Dust does get washed out of the air, and in much of India it is rich in calcium compounds, which neutralise acidity. This is why measured rainfall acidity in parts of the country is lower than the emissions alone would suggest — dust works against acid rain, not for it.
• (d)Dissolution of soil solution in rain — Reverses the direction in which the two meet. Rain falls onto soil and then leaches it; soil solution does not rise into the clouds to be dissolved.

Sulphur and nitrogen are non-metals, so their oxides are acidic. Sulphur dioxide released into the air is oxidised and dissolves as sulphuric acid; nitrogen oxides give nitric acid. Both are carried by winds well beyond the source, which makes acid deposition an international problem — emissions in one country falling as acid rain in another. The damage runs along several paths: lowering the pH of lakes until fish eggs no longer hatch, leaching aluminium out of soil into water where it is toxic to roots and gills, stripping nutrients from forest soils, and dissolving the calcium carbonate of limestone and marble.

Only one option in this item names substances that actually turn into acids, which is why identifying the chemistry is the whole task. The reasoning worth holding is the general rule behind it: non-metal oxides are acidic, metal oxides are basic. Sulphur and nitrogen are non-metals, so their oxides acidify rain, whereas dust in India is often calcium-rich and does the opposite. The remedies follow the same logic — flue-gas desulphurisation scrubs sulphur dioxide out of power-station exhaust, catalytic converters cut the nitrogen oxides from vehicles, and lower-sulphur fuels reduce the input at source.

• Acid rain is caused by oxides of sulphur and nitrogen dissolving in atmospheric moisture to give sulphuric and nitric acid.
• Unpolluted rain is already about pH 5·6 because dissolved carbon dioxide forms carbonic acid.
• Sulphur dioxide comes largely from burning coal and smelting ores; nitrogen oxides form wherever combustion is hot enough.
• Acid deposition crosses national boundaries, since the oxides travel far on the wind before falling.
• It acidifies lakes, leaches aluminium and nutrients from soils, and corrodes limestone and marble.

• Attributing acid rain to carbon dioxide alone, which explains the natural pH of 5·6 but not acid rain.
• Adding carbon monoxide or ozone to the list of causes; neither forms an acid in rainwater.
• Assuming dust makes rain more acidic, when calcium-rich dust neutralises it.

As a straight cause question, or as a list of pollutants from which the acid-forming ones have to be picked out.

• practice — not a real PYQ

  Unpolluted rainwater has a pH of about 5·6 rather than 7. This is because

  • (a)it dissolves dust as it falls
  • (b)dissolved carbon dioxide forms carbonic acid in it
  • (c)it dissolves oxides of nitrogen from the upper atmosphere
  • (d)water itself is naturally acidic

  Answer(b) dissolved carbon dioxide forms carbonic acid in it — a weak acid, which is enough to bring the pH a little below neutral even in clean air.

• practice — not a real PYQ

  The corrosion and blackening of marble monuments in industrial cities is caused chiefly by

  • (a)carbon monoxide in vehicle exhaust
  • (b)acid rain formed from sulphur and nitrogen oxides
  • (c)ozone in the lower atmosphere
  • (d)ultraviolet radiation reaching the surface

  Answer(b) acid rain formed from sulphur and nitrogen oxides — marble is calcium carbonate, and acids dissolve it away.