Crude oil is described as a finite resource and a mixture. Explain what is meant by both of these terms and describe how crude oil was formed.

AQA A-Level Chemistry (7405) — 3.3.2 Alkanes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A finite resource means it cannot be replaced and will eventually run out – crude oil is non‑renewable.
A mixture contains two or more substances that are not chemically combined; each component keeps its own chemical properties.
Crude oil is formed from the remains of ancient biomass, mainly plankton, which were buried in mud and subjected to heat and pressure over millions of years.

Examiner tips

  • Use the exact terms ‘finite resource’ and ‘mixture’ as defined in the mark scheme.
  • Show the link between ancient biomass and the formation process.
  • Keep the answer concise – 5 marks allow a short paragraph with key points.

Common mistakes

  • Confusing crude oil with petroleum products; failing to mention it is non‑renewable.
  • Describing crude oil as a single compound instead of a mixture.
  • Omitting the role of heat and pressure in the transformation of biomass.

Mark scheme (5 marks)

  1. Finite resource means it cannot be replaced / will run out / is non-renewable
  2. A mixture contains two or more substances that are not chemically combined
  3. Each substance in the mixture retains its own chemical properties
  4. Crude oil is the remains of an ancient biomass / mainly plankton
  5. The biomass / plankton was buried in mud over millions of years

Key terms in this question

finite resource · mixture · crude oil

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