Explain why the incomplete combustion of hydrocarbon fuels in car engines is considered harmful to human health and the environment.

Cambridge International IGCSE Chemistry (0620) — 11.3 Fuels · Explain · 4 marks · View as Markdown

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

Car engines burn petrol, a mixture of hydrocarbons. When the supply of air to the engine is limited, the combustion of petrol is incomplete.

Model answer (4 marks)

Incomplete combustion of petrol in car engines produces carbon monoxide, a colourless, odourless gas that is highly toxic to humans.
Carbon monoxide binds to haemoglobin in the blood, forming carboxyhaemoglobin, which reduces the oxygen‑carrying capacity of the blood and can cause headaches, dizziness, nausea and, in severe cases, death.
The same incomplete combustion also releases fine carbon particles, or soot, into the atmosphere.
Soot particles are inhaled into the lungs, causing respiratory irritation and long‑term lung disease, and they also scatter and absorb sunlight, contributing to global dimming and climate change.

Examiner tips

  • Use the exact terms ‘carbon monoxide’ and ‘soot’ – the mark scheme recognises these words. Explain the health effect of CO (binding to haemoglobin) and the environmental effect of soot (respiratory problems or global dimming).
  • Keep each point short and separate – the examiner looks for clear, distinct statements.

Mark scheme (4 marks)

  1. Incomplete combustion produces carbon monoxide
  2. Carbon monoxide is toxic / poisonous to humans
  3. Incomplete combustion produces carbon particles / soot
  4. Carbon particles / soot cause respiratory problems OR contribute to global dimming / reduced sunlight

Key terms in this question

incomplete combustion · hydrocarbon

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