UKPSC Prelims 2025 General Studies Paper-I · Question 16 of 149

Correct — A, Acid rain.

The Taj is faced in white marble, which is crystalline calcium carbonate. Sulphur dioxide and oxides of nitrogen released by burning coal, coke and oil are oxidised in the air and dissolve in cloud and rain water as sulphuric and nitric acid — acid rain.

When that acid reaches the stone, the surface calcium carbonate is converted into calcium sulphate, which forms gypsum. Gypsum is softer, dull and slightly soluble, so the polished skin turns yellow-brown, crusts and flakes away.

Carry away the chain rather than the label: sulphur and nitrogen oxides, then acid in rainwater, then carbonate stone turned to gypsum, then loss of lustre. That chain is also why the remedy chosen at Agra was to change the fuel that nearby industry burns.

• (b)
  Ozone layer — The ozone layer is the band of stratospheric ozone, roughly fifteen to thirty-five kilometres up, that absorbs incoming ultraviolet radiation before it reaches the surface.

  The layer belongs to the topic of ultraviolet shielding, chlorofluorocarbons and the Montreal Protocol. The hazard here is sliding from the layer to ground-level ozone, which is a surface pollutant and a different topic; the option as printed names the layer.

• (c)
  Methane — Methane is a hydrocarbon and a potent greenhouse gas, released from flooded paddy fields, ruminant digestion, landfills and fossil-fuel systems. Its fate in the atmosphere is oxidation to carbon dioxide and water.

  That places it in greenhouse warming and its sources, including the comparison of its warming potential with carbon dioxide. Marble decay of the kind described needs an acid capable of attacking a carbonate, and that acid comes from sulphur and nitrogen oxides.

• (d)
  Fog — Fog is water vapour condensed into droplets near the ground, and Agra lies in the winter fog belt of the Indo-Gangetic plain, so it is the conspicuous thing in the air there for weeks at a time.

  Water by itself does little to a polished marble surface. Where fog and dew do harm stone, it is because acidic pollutants are dissolved in the droplets — which returns you to the acid named in option (a).

  Fog belongs to radiation fog, winter visibility and transport disruption over north India.

Acid deposition is the arrival at the ground of sulphuric and nitric acid formed when sulphur dioxide and nitrogen oxides, emitted mainly by combustion, are oxidised in the atmosphere and taken up by water. It reaches the surface as rain, snow, fog, dew or dry particles.

Stone built of limestone or marble is vulnerable because carbonate rock reacts with acid. Sulphuric acid on calcium carbonate gives calcium sulphate, water and carbon dioxide.

The calcium sulphate hydrates into gypsum, which is softer and more soluble than the original stone. Where rain washes over the surface, that gypsum is carried away and carved detail is lost.

In sheltered areas it stays put as a crust that traps soot, darkens and eventually blisters off. Conservators speak of this pattern of damage as stone decay, or on marble as marble cancer.

• Marble is metamorphosed limestone, essentially calcium carbonate, and reacts with acids.
• Sulphuric acid acting on calcium carbonate yields calcium sulphate, water and carbon dioxide; the calcium sulphate hydrates to gypsum.
• Gypsum is softer and more soluble than marble, so the attacked surface loses polish, discolours and flakes.
• Acid rain is chiefly the sulphuric and nitric acid formed from sulphur dioxide and nitrogen oxides given off by burning fossil fuels.
• Even unpolluted rain is mildly acidic, near pH 5.6, because dissolved carbon dioxide forms carbonic acid.
• The Taj Trapezium Zone is a protected area drawn around the monument within which polluting industrial fuel use is regulated.
• In M.C. Mehta v. Union of India, decided on 30 December 1996, the Supreme Court directed polluting industries in that zone to switch to natural gas or stop operating there.
• The same judgment barred the use of coal and coke as industrial fuel inside the Taj Trapezium Zone.
• The Taj Mahal, built by Shah Jahan as a mausoleum for Mumtaz Mahal beside the Yamuna at Agra, became a UNESCO World Heritage Site in 1983.

• Reading 'ozone layer' as though it said 'ground-level ozone'. The option names the stratospheric shield, which is a different thing from the surface pollutant.
• If fog attracts you because it is the visible thing hanging over Agra in winter, note that fog carries whatever is dissolved in the droplets; the corrosive part is the acid.
• If methane attracts you as a familiar pollutant, note that its atmospheric fate is oxidation to carbon dioxide and water, which is warming chemistry rather than acid attack on a carbonate.
• Naming acid rain without being able to say what it produces. The product to hold on to is calcium sulphate, which forms gypsum.
• Forgetting that marble is calcium carbonate. Without that link the chemistry cannot be reconstructed under time pressure.
• Mixing up the two acids: sulphuric acid comes from sulphur dioxide, nitric acid from oxides of nitrogen. Carbon dioxide gives only weak carbonic acid.

• practice — not a real PYQ

  Acid rain damages marble monuments chiefly because the sulphuric acid it carries converts the calcium carbonate of the stone into:

  • (a)Calcium sulphate
  • (b)Calcium silicate
  • (c)Calcium chloride
  • (d)Calcium phosphate
  Answera — Sulphuric acid reacting with calcium carbonate gives calcium sulphate along with water and carbon dioxide. The calcium sulphate hydrates to gypsum, a softer and more soluble solid that dulls the surface and flakes away.

  (b) fails because calcium silicate is a high-temperature rock-forming and cement compound, not a product of acid attack on marble. (c) would require hydrochloric acid and (d) phosphoric acid, and the stem names sulphuric acid as the agent.

• practice — not a real PYQ

  In the Taj Trapezium case decided by the Supreme Court on 30 December 1996, the polluting industries operating inside the zone were directed to:

  • (a)Switch over to natural gas as industrial fuel or stop operating within the zone
  • (b)Pay a conservation cess to the Archaeological Survey of India
  • (c)Raise their chimneys to a minimum height of one hundred metres
  • (d)Replace furnace oil with coke as their industrial fuel
  Answera — The Court required the polluting units in the Taj Trapezium Zone to move to natural gas within a fixed time schedule, failing which they had to cease operations inside the zone or relocate outside it.

  (d) is the reverse of the ruling, since the same judgment barred coal and coke as industrial fuel in the zone. (b) and (c) describe a levy and a stack-height rule, whereas the order worked directly on which fuel could be burnt.

• practice — not a real PYQ

  Acid rain is produced chiefly when rainwater takes up the atmospheric oxidation products of:

  • (a)Sulphur dioxide and oxides of nitrogen
  • (b)Methane and carbon monoxide
  • (c)Chlorofluorocarbons and halons
  • (d)Ozone and water vapour
  Answera — Sulphur dioxide is oxidised and dissolves as sulphuric acid, while oxides of nitrogen give nitric acid. These two strong acids are what the term acid rain refers to.

  (b) fails because methane and carbon monoxide oxidise to carbon dioxide, which yields only weak carbonic acid and leaves rain near its natural mild acidity. (c) names the ozone-depleting compounds controlled by the Montreal Protocol. (d) describes the reaction that generates hydroxyl radicals, the atmosphere's oxidant, rather than the acid load itself.