State two pollutant gases produced when petrol burns incompletely in a car engine, and for each gas, describe one harmful effect on either human health or the environment.

Cambridge International IGCSE Chemistry (0620) — 10.3 Air quality and climate · State · 4 marks · View as Markdown

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

Model answer (4 marks)

Carbon monoxide (CO) – reduces the ability of haemoglobin to carry oxygen, leading to hypoxia and potentially death.
Nitrogen oxides (NOx) – contribute to acid rain, damaging ecosystems and buildings, and irritate the respiratory system, causing breathing problems.

Examiner tips

  • Use the exact names CO and NOx; include the key harmful effect for each.
  • Keep each point brief – one sentence per gas is enough for full marks.
  • Show clear link between gas and effect to demonstrate understanding.

Common mistakes

  • Writing CO as carbon dioxide (CO₂) instead of carbon monoxide.
  • Using vague effects like "bad for health" without specifying the mechanism or impact.
  • Omitting the second gas or giving a non‑pollutant gas such as sulphur dioxide.

Mark scheme (4 marks)

  1. Names carbon monoxide (CO) as a pollutant gas from incomplete combustion
  2. Describes a harmful effect of carbon monoxide — reduces ability of blood/haemoglobin to carry oxygen, leading to reduced oxygen supply to the body / can cause death
  3. Names nitrogen monoxide / nitrogen dioxide / nitrogen oxides (NOx) as a pollutant gas from incomplete combustion or high-temperature combustion
  4. Describes a harmful effect of nitrogen oxides — causes acid rain which damages ecosystems / buildings / kills aquatic life, OR irritates the respiratory system / causes breathing problems

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