Methane is the main hydrocarbon in natural gas and is used as a fuel in domestic boilers. State the products formed when methane undergoes complete combustion and explain why complete combustion is preferred over incomplete combustion in a domestic boiler.

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

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

Model answer (4 marks)

Complete combustion of methane produces carbon dioxide and water.

Complete combustion is preferred because it avoids the formation of toxic carbon monoxide, which is produced in incomplete combustion and is poisonous to humans. In addition, complete combustion releases more energy, making the boiler more efficient.

Examiner tips

  • Use the exact terms "carbon dioxide" and "water" for the products. Explain both safety (no CO) and efficiency (more energy released).
  • Show the link between complete combustion and higher energy output.

Common mistakes

  • Writing "carbon monoxide" as a product of complete combustion. Omitting the safety or efficiency reasons in the explanation.

Mark scheme (4 marks)

  1. Complete combustion of methane produces carbon dioxide
  2. Complete combustion of methane produces water
  3. Incomplete combustion produces carbon monoxide, which is toxic / poisonous to humans
  4. Complete combustion releases more energy than incomplete combustion / complete combustion is more efficient

Key terms in this question

complete combustion · incomplete combustion · hydrocarbon

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