MPSC 2025 Prelims Paper-I · Question 31 of 100

Correct — option (2), 'a – iv, b – iii, c – i, d – ii,' matches each pollutant to the effect it is actually known for in environmental science. Phosphate fertilisers in water (a) cause eutrophication (iv): phosphate run-off from farmland is a limiting nutrient for aquatic plants and algae, so its excess triggers explosive algal growth, and when that algal bloom later dies and decomposes, the decomposition consumes dissolved oxygen and can leave a water body effectively dead. Methane in air (b) causes global warming (iii): methane is a potent greenhouse gas — molecule for molecule, far more effective at trapping heat than carbon dioxide over a 20-year horizon — released from sources such as wetlands, rice paddies, livestock digestion and landfills, and it is one of the principal non-CO2 drivers of anthropogenic climate change. Synthetic detergents in water (c) raise biochemical oxygen demand, BOD (i): detergents (and other organic pollutants) provide food for microorganisms in water, and as those microbes multiply to break the pollutant down, they consume dissolved oxygen, so a rising BOD is the standard laboratory measure of exactly this oxygen-depleting organic pollution load. Nitrogen oxides in air (d) cause acid rain (ii): NOx released mainly from vehicle exhaust and combustion reacts with atmospheric moisture to form nitric acid, which falls as acid precipitation alongside the sulphur-dioxide-derived sulphuric acid from the same combustion sources. Each of these four pollutant-effect pairs is a standard, independently verifiable fact of environmental chemistry, and the question simply asks a candidate to hold all four correctly in memory at once rather than confuse a pollutant with an effect it merely sounds similar to.

• (1)a – ii, b – i, c – iv, d – iii — Every pairing here is wrong. It assigns acid rain to phosphate fertilisers (that effect belongs to nitrogen oxides), rising BOD to methane (that effect belongs to synthetic detergents), eutrophication to synthetic detergents (that effect belongs to phosphate fertilisers), and global warming to nitrogen oxides (that effect belongs to methane). It is best read as a scrambled reassignment of the correct set rather than a partially right answer.
• (3)a – iii, b – ii, c – iv, d – i — This option also scrambles all four pairings: it gives global warming to phosphate fertilisers (methane's actual effect), acid rain to methane (nitrogen oxides' actual effect), eutrophication to synthetic detergents (phosphate fertilisers' actual effect), and rising BOD to nitrogen oxides (synthetic detergents' actual effect). None of the four assignments survives a check against the underlying chemistry.
• (4)a – i, b – iii, c – ii, d – iv — This option gets methane-global warming (b-iii) right but the remaining three wrong: it assigns rising BOD to phosphate fertilisers (that is synthetic detergents' effect), acid rain to synthetic detergents (that is nitrogen oxides' effect), and eutrophication to nitrogen oxides (that is phosphate fertilisers' effect). Getting one of four pairings correct is exactly what makes this option a believable distractor for a candidate who is sure about methane but has not fixed the other three.

Four classic pollutant-effect pairs recur across environmental-science questions: nutrient pollutants such as phosphates trigger eutrophication (excess algal growth followed by oxygen-depleting decomposition); organic pollutants such as synthetic detergents raise Biochemical Oxygen Demand, the measured oxygen microbes consume while breaking the pollutant down; potent greenhouse gases such as methane drive global warming; and combustion-derived gases such as nitrogen oxides (alongside sulphur oxides) form acid rain. Distinguishing these requires separating 'what the pollutant does directly' (adds a nutrient, adds an oxidisable organic load, traps heat, or forms an acid) from the downstream ecological consequence each mechanism produces.

MPSC and UPSC both test this fact set heavily because environmental pollution is a compact, high-yield topic that rewards precise pairing rather than vague familiarity — a candidate who knows 'detergents are bad for water' but cannot say specifically that they raise BOD (rather than causing eutrophication, which is a nutrient-driven effect) will be caught exactly here. The examiner's trap is almost always a reshuffle of genuine pairings, as in three of this question's four options, rather than an invented pollutant or effect, since an invented term would be too easy to eliminate.

• Phosphate fertiliser run-off into water bodies causes eutrophication — excess algal growth followed by oxygen-depleting decomposition when the algae die.
• Methane is a potent greenhouse gas and a major driver of global warming, released from wetlands, rice paddies, livestock and landfills.
• Synthetic detergents (and other organic pollutants) in water raise Biochemical Oxygen Demand, since microbes consume dissolved oxygen while metabolising them.
• Nitrogen oxides from combustion and vehicle exhaust react with atmospheric moisture to form nitric acid, contributing to acid rain alongside sulphur-dioxide-derived sulphuric acid.
• BOD and eutrophication are related but distinct: BOD measures the oxygen demand from organic pollutant breakdown directly, while eutrophication is the broader ecological cascade triggered specifically by excess nutrients such as phosphates and nitrates.

• Confusing eutrophication (a nutrient-driven ecological cascade) with rising BOD (a direct measure of oxygen consumed breaking down organic pollutants) — both concern water and oxygen, but from different causal starting points
• Assuming any water pollutant automatically causes eutrophication, rather than checking whether it is specifically a nutrient (like phosphates) or an organic load (like detergents)
• Treating acid rain and global warming as interchangeable 'air pollution' effects, when they are driven by different pollutant classes (acidic gases like NOx/SO2 versus greenhouse gases like methane/CO2)

MPSC's Paper-I environment section regularly tests pollutant-effect matching across four or more pairs at once, since holding several pairings correctly and simultaneously is what actually separates a well-prepared candidate from one with only surface familiarity. Expect this exact four-pollutant set, or a close variant swapping in sulphur dioxide, carbon monoxide, or nitrates, to recur across editions in match-the-columns form.

No directly related past PYQ was found.

• practice — not a real PYQ

  Which of the following pollutants is primarily responsible for acid rain when released into the atmosphere ?

  • (a)Methane
  • (b)Nitrogen oxides
  • (c)Phosphates
  • (d)Synthetic detergents

  Answer(b) Nitrogen oxides — released mainly from combustion and vehicle exhaust, they react with atmospheric moisture to form nitric acid, contributing to acid rain alongside sulphur-dioxide-derived sulphuric acid.

• practice — not a real PYQ

  A rising Biochemical Oxygen Demand (BOD) in a water body most directly indicates which of the following ?

  • (a)Excess dissolved nutrients causing algal blooms
  • (b)Increased organic pollutant load being broken down by microorganisms, consuming dissolved oxygen
  • (c)Rising atmospheric methane concentration
  • (d)Formation of acid precipitation over the water body

  Answer(b) Increased organic pollutant load being broken down by microorganisms, consuming dissolved oxygen — BOD specifically measures the oxygen consumed by microbes metabolising organic pollutants such as synthetic detergents, distinct from the nutrient-driven eutrophication caused by pollutants like phosphates.