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Q10 (IAS/2022) Environment & Ecology › Pollution & Conservation › Acid rain pollution Official Key

Consider the following : 1. Carbon monoxide 2. Nitrogen oxide 3. Ozone 4. Sulphur dioxide Excess of which of the above in the environment is/are cause(s) of acid rain ?

Result
Your answer: —  ·  Correct: B
Explanation

The correct answer is Option 2 (2 and 4 only). Acid rain is primarily caused by a chemical reaction that begins when compounds like Nitrogen oxides (NOx) and Sulphur dioxide (SO2) are released into the atmosphere.

The scientific reasoning is as follows:

  • Sulphur dioxide (4): When emitted, it reacts with water vapor and oxygen in the atmosphere to form Sulphuric acid (H2SO4).
  • Nitrogen oxides (2): These react with atmospheric moisture to form Nitric acid (HNO3).

Both acids decrease the pH of rainwater, leading to acid rain. In contrast, Carbon monoxide (1) is a criteria pollutant but does not contribute to acidification. Ozone (3) is a secondary pollutant and a greenhouse gas at the ground level, but it is not a direct precursor to acid rain. Therefore, only Nitrogen oxides and Sulphur dioxide are the primary causative agents.

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PROVENANCE & STUDY PATTERN
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Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.
Q. Consider the following : 1. Carbon monoxide 2. Nitrogen oxide 3. Ozone 4. Sulphur dioxide Excess of which of the above in the environment…
At a glance
Origin: From standard books Fairness: High fairness Books / CA: 10/10 · 0/10

This is a non-negotiable 'Sitter'. It comes directly from the 'Pollution' chapter of any standard Environment book (Shankar/NCERT). If you get this wrong, you are statistically out of the race because 95% of serious aspirants will mark this correctly.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Which of the following gases—carbon monoxide, nitrogen oxides, ozone, and sulphur dioxide—cause acid rain?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 90
Presence: 5/5
“• Acid rain occurs when gases such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rainwater to produce stronger acids and can lower the pH to less than 4. These acids can attack certain kinds of rocks in much the way carbonic acid does.”
Why this source?
  • Explicitly states acid rain occurs when sulphur dioxide and nitrogen oxides are present in the atmosphere
  • Notes these oxides react in rainwater to produce stronger acids and markedly lower pH
Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 8
Presence: 5/5
“motor vehicles inject large amounts of these chemicals into the atmosphere. In the atmosphere, the chemicals combine with water to form sulphuric acid and nitric acid. Tese droplets may be transported at great distances by wind before they precipitate to the ground (Fig. 6.1). Te main gases involved in the production of acid rains are nitrogen oxides—sulphur dioxide (SO2), nitrogen dioxide (NO2) and nitrous oxide (NO). Te major sources of these gases is due to burning of fossil fuel and industrial processes. Acid rain is a phrase that applies to a process that results in deposition of acid on the surface of the Earth.”
Why this source?
  • Identifies the main gases involved in production of acid rains as nitrogen oxides and sulphur dioxide
  • Explains these gases combine with water to form sulphuric and nitric acids
Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > 5.I5. ACID RAIN > p. 101
Presence: 5/5
“Acid rain is the rainfall that has been acidified. It is formed when oxides of sulfur and nitrogen react with the moisture in the atmosphere. It is rain with a pH of less than 5.6. Acid rain is particularly damaging to lakes, streams, and forests and the plants and animals that live in these ecosystems.”
Why this source?
  • Defines acid rain as formed when oxides of sulfur and nitrogen react with moisture in the atmosphere
  • Gives the operational pH threshold for acid rain (rainfall acidified, pH < 5.6)
Pattern takeaway: UPSC rewards clarity over depth here. The pattern is to list 4-5 common pollutants and ask you to map them to a specific phenomenon (Acid Rain, Smog, Global Warming). They test if your mental folders are distinct or mixed up.
How you should have studied
  1. [THE VERDICT]: Absolute Sitter. Direct lift from NCERT Class 11 Chemistry (Environmental Chemistry) or Shankar IAS Chapter 5.
  2. [THE CONCEPTUAL TRIGGER]: Major Air Pollutants and their specific pathological/ecological impacts (Acid Rain vs. Smog vs. Greenhouse Effect).
  3. [THE HORIZONTAL EXPANSION]: Memorize the distinctions: 1. **Photochemical Smog**: NOx + VOCs + Sunlight = Ozone + PAN. 2. **Greenhouse Gases**: CO2, CH4, N2O (Not NO/NO2), SF6. 3. **Stone Leprosy**: Reaction of H2SO4 with CaCO3 (Marble) -> CaSO4 (Gypsum). 4. **pH Threshold**: Normal rain is ~5.6 (due to Carbonic acid); Acid rain is < 5.6.
  4. [THE STRATEGIC METACOGNITION]: Do not confuse 'Greenhouse Gases' with 'Acid Rain precursors'. CO2 causes warming; SO2/NOx cause acidity. The preparation logic is to maintain strict mental folders for each phenomenon.
Concept hooks from this question
📌 Adjacent topic to master
S1
👉 Sulphur and nitrogen oxides as primary acid-rain precursors
💡 The insight

Sulphur dioxide and nitrogen oxides are the principal gases that react with atmospheric moisture to produce acid rain.

High-yield for pollution and environment questions: distinguishes key anthropogenic acids from weaker natural acidity and links to sources (fossil fuel combustion, industry). Helps answer causation, impact and mitigation questions in UPSC GS papers and environment optional.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 90
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 8
🔗 Anchor: "Which of the following gases—carbon monoxide, nitrogen oxides, ozone, and sulphu..."
📌 Adjacent topic to master
S1
👉 Formation of sulphuric and nitric acids in the atmosphere
💡 The insight

Oxidation of sulphur and nitrogen oxides in air leads to H2SO4 and HNO3, which are the chemical basis of acid rain.

Important for questions on atmospheric chemistry and environmental impacts; connects pollution sources to ecological damage and policy responses (emission controls, clean technology). Enables explanation-type answers on mechanisms and consequences.

📚 Reading List :
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > 5.15.4. Chemistry of Acid Rain > p. 103
  • Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 6: Environmental Degradation and Management > Acidification > p. 8
🔗 Anchor: "Which of the following gases—carbon monoxide, nitrogen oxides, ozone, and sulphu..."
📌 Adjacent topic to master
S1
👉 Carbon dioxide/monoxide and natural rainfall acidity
💡 The insight

Dissolution of CO2 (and CO-derived carbonic acid) makes rainfall naturally slightly acidic, distinct from strong acid rain caused by S and N oxides.

Useful to contrast background acidity (pH ~5.6) with anthropogenic acid rain in essays and short answers; aids in evaluating severity and sources of acidity in policy contexts.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 6: Geomorphic Movements > Anthropogenic Solution Weathering > p. 91
  • Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 5: Environmental Pollution > a) Sulphur > p. 102
🔗 Anchor: "Which of the following gases—carbon monoxide, nitrogen oxides, ozone, and sulphu..."
🌑 The Hidden Trap

Photochemical Smog precursors. While Acid Rain is SO2 + NOx, Photochemical Smog is NOx + VOCs + Sunlight (producing Ground-level Ozone). UPSC loves swapping these ingredients in options.

⚡ Elimination Cheat Code

Chemistry Common Sense: Carbon Monoxide (CO) is neutral/weakly acidic but doesn't form strong acids like H2SO4 or HNO3. If CO caused acid rain, every city with traffic would have melting infrastructure. Ozone (O3) is an oxidant, not an acid-former. Eliminating 1 and 3 leaves only Option B (2 and 4).

🔗 Mains Connection

Mains GS-3 (Environment) & GS-2 (IR): Acid Rain is a classic example of 'Transboundary Pollution'. Emissions in one country (e.g., industrial belts) cause acid rain in neighbors, leading to diplomatic friction and treaties like the 1979 Convention on Long-Range Transboundary Air Pollution.

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