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2025 O Level Chem P1 Q39 — Air & Atmosphere / Pollutants Solution

Air & Atmosphere / Pollutants 2025 O Level Pure Chemistry · Paper 1 · Q39

Question

Four sources of atmospheric pollutants are listed. 1 lightning. 2 volcanoes. 3 combustion of fossil fuels. 4 release of chlorofluorocarbons (CFCs). Which sources increase the concentration of sulfur dioxide in the atmosphere?

A1 and 3
B1 and 4
C2 and 3
D2 and 4

Correct answer: C

Video Solution

Explanation

Answer: C. Sulfur dioxide enters the atmosphere from two main sources:

Source 2 (volcanoes): volcanic eruptions release large amounts of \(\text{SO}_2\) directly into the air. ✓

Source 3 (combustion of fossil fuels): coal, oil and natural gas contain sulfur impurities. When burned, the sulfur is oxidised: \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\). This is the largest human contribution to atmospheric SO₂. ✓

Source 1 (lightning) primarily produces \(\text{NO}_x\), not SO₂. Source 4 (CFCs) contains chlorine and fluorine, not sulfur — releases CFCs, not SO₂. Sources 2 and 3 only → option C.

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