Methane, propane and octane are all alkanes. Explain why alkanes are described as saturated hydrocarbons and explain why shorter-chain alkanes, such as methane, are more useful as fuels than longer-chain alkanes, such as octane.

Pearson Edexcel International GCSE Chemistry (4CH1) — 4.3 Alkanes · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Alkanes contain only carbon and hydrogen atoms, so they are hydrocarbons.
Alkanes contain only single bonds between carbon atoms, so they are saturated.
Shorter‑chain alkanes have lower boiling points and are more volatile, so they are easier to ignite and burn more readily.
Shorter‑chain alkanes flow and pour more easily (lower viscosity), making them easier to transport and use as fuels.

Examiner tips

  • Use the exact terms ‘hydrocarbons’ and ‘saturated’ as the mark scheme expects.
  • Show the link between volatility/boiling point and ease of ignition.
  • Mention lower viscosity for transport.
  • Keep the answer concise to fit the 4 marks.

Common mistakes

  • Confusing alkanes with alkenes or alkynes.
  • Using vague phrases like ‘more energy’ instead of ‘lower boiling point’. Not mentioning viscosity or transport ease.

Mark scheme (4 marks)

  1. Alkanes contain only carbon and hydrogen atoms, so they are hydrocarbons
  2. Alkanes contain only single bonds between carbon atoms, so they are saturated
  3. Shorter-chain alkanes have lower boiling points / are more volatile, so they are easier to ignite / burn more readily
  4. Shorter-chain alkanes flow / pour more easily (lower viscosity) making them easier to transport and use as fuels

Key terms in this question

saturated · hydrocarbon · alkane

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