Crude oil is described as a finite resource. Explain what is meant by the term 'finite resource' and describe the origin of crude oil, including the type of ancient biomass from which it formed.

AQA A-Level Chemistry (7405) — 3.3.1 Introduction to organic chemistry · 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 is one that cannot be replaced and will eventually run out because it is being used up faster than it is formed.

Crude oil is formed from the remains of ancient biomass – mainly dead plankton – that were buried in mud and sediment.

Under heat and pressure over millions of years, the organic material was transformed into hydrocarbons, which are molecules composed only of hydrogen and carbon.

Examiner tips

  • Use the exact phrase ‘finite resource’ and explain it in one sentence. Mention ‘dead plankton’ and ‘buried in mud’ to hit the key points. State that crude oil is mainly hydrocarbons (C and H).
  • Keep the answer concise – 5 marks, so one sentence per point.
  • Use UK spelling (e.g. ‘hydrocarbons’).

Common mistakes

  • Confusing finite resource with renewable resource. Forgetting to mention plankton as the primary biomass. Saying crude oil is ‘made of carbon and hydrogen’ without noting it is mainly hydrocarbons.”

Mark scheme (5 marks)

  1. Finite resource means it cannot be replaced / will eventually run out / is being used up faster than it is formed
  2. Crude oil is the remains of ancient biomass / dead organisms
  3. The biomass consisted mainly of plankton
  4. The plankton / biomass was buried in mud
  5. Most of the compounds in crude oil are hydrocarbons / molecules made up of hydrogen and carbon only

Key terms in this question

finite resource · crude oil · biomass

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