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K326 / K328 / 2027
Maintaining Air Quality

Lesson 2 of 3

Explain harm through a chemical cause

Link the pollutant to the process and then the effect.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

  • I can explain pollutant effects.Syllabus

    Syllabus K326 / K328, 12(d). Explain pollutant effects

Explain harm through a chemical cause

Link the pollutant to the process and then the effect.

Make a guess. You are not marked.

Rain falls through air polluted with sulfur dioxide from burning fuels. How does its pH compare with rain in clean air?

  1. The pH is higher, as the gas is a base
  2. The pH is the same, as rain is water
  3. The pH is lower, as acids form in the rain
Show the answer

The pH is lower, as acids form in the rain

Sulfur dioxide dissolves in the rain and reacts to form acids, so the rain has more hydrogen ions and a lower pH.

Carbon monoxide binds strongly to haemoglobin and reduces the blood's ability to carry oxygen. It is not the same hazard as carbon dioxide's role in climate change. Because incomplete combustion can produce CO, a fuel burning does not guarantee that its exhaust contains only carbon dioxide and water.

Nitrogen dioxide and sulfur dioxide react in the atmosphere with water and oxygen to produce acidic substances that enter rain. Acid rain can damage plants and aquatic ecosystems and attack buildings containing carbonate stone. These gases also irritate the respiratory system; health effects are not confined to rainwater.

For a limestone surface, carbonate reacts with acid: CaCO3(s) + 2H+(aq) -> Ca2+(aq) + H2O(l) + CO2(g). Solid material is gradually lost. This connects the visible damage to an acid-carbonate reaction rather than merely saying that pollution is bad.

Worked exampleUse source evidence

Two fuels release the same energy. One contains much more sulfur. What additional air-quality concern does it create?

Try it first. Then show the worked stepsHide the worked steps
  1. Combustion can convert its sulfur compounds into sulfur dioxide.
  2. Sulfur dioxide contributes to acid rain and respiratory irritation.
  3. Equal useful energy does not imply equal pollutant emissions.
Answer

The higher-sulfur fuel can create greater SO2 pollution; compare sulfur content and any treatment of the exhaust.

Check your understanding

A student says carbon monoxide causes acid rain. Which correction is right?

  1. Acid rain comes from sulfur dioxide and nitrogen oxides; CO is harmful as a toxic gas.
  2. Acid rain comes from carbon dioxide, since CO2 dissolves in rain to form carbonic acid.
  3. Acid rain comes from methane, which burns in the air and forms acidic droplets in clouds.
Show the answer

Acid rain comes from sulfur dioxide and nitrogen oxides; CO is harmful as a toxic gas.

SO2 and NO2 react with water and oxygen to form acids. Carbon monoxide is dangerous because it binds to haemoglobin and reduces oxygen transport.

Exam-style question

Sulfur dioxide is released when fuels containing sulfur are burned. Explain how this can damage buildings. [2]

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