Crude oil contains hydrocarbons with sulfur impurities. Explain why the incomplete combustion of these hydrocarbons in a car engine is harmful to human health and to the environment.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.1 Introduction · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Incomplete combustion occurs when there is insufficient oxygen, producing carbon monoxide (CO) which is toxic to humans.
Carbon particulates (soot) are also formed, causing respiratory and cardiovascular problems and contributing to global dimming.
Sulfur impurities in the oil burn to give sulfur dioxide (SO₂), which reacts with water vapour in the atmosphere to form acid rain.

Examiner tips

  • Use the word "incomplete combustion" early. Mention CO, particulates and SO₂ explicitly. Show the link to health and environmental effects.
  • Use concise, exam‑style sentences, avoid extra words.

Common mistakes

  • Forgetting to mention CO or particulates. Using vague terms like "pollution" instead of specific compounds. Mixing up complete and incomplete combustion.”

Mark scheme (4 marks)

  1. Incomplete combustion occurs when there is insufficient oxygen
  2. Carbon monoxide is produced, which is toxic / harmful to humans
  3. Carbon particulates are produced, which cause health problems / global dimming
  4. Sulfur impurities burn to produce sulfur dioxide, which reacts with water vapour to form acid rain

Key terms in this question

incomplete combustion · sulfur impurities

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