Crude oil is a mixture of hydrocarbons. Explain how fractional distillation separates crude oil into fractions, and state one use of the fraction with the shortest hydrocarbon chains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is an important raw material obtained from beneath the Earth's surface. It contains hydrocarbons of different chain lengths, which can be separated and used as fuels and feedstocks.
Model answer (5 marks)
Crude oil is heated in a furnace so that the hydrocarbons evaporate. The vapours rise through a fractionating column that has a temperature gradient – hot at the bottom, cooler at the top. Because hydrocarbons of different chain lengths have different boiling points, each component condenses at the level of the column where the temperature equals its boiling point. The condensed liquids are collected as separate fractions.
The fraction containing the shortest hydrocarbon chains (e.g. methane, ethane, propane) is used as a fuel – for example as LPG for cooking or heating.
The fraction containing the shortest hydrocarbon chains (e.g. methane, ethane, propane) is used as a fuel – for example as LPG for cooking or heating.
Examiner tips
- Show the heating step and the temperature gradient in the column
- Explain that condensation occurs at the temperature matching the component’s boiling point
- Give a specific use of the lightest fraction (fuel, LPG)
Common mistakes
- Forgetting to mention the temperature gradient in the column
- Confusing the order of fractions (placing heavy fractions at the top)
- Giving a non‑fuel use for the lightest fraction
Mark scheme (5 marks)
- Crude oil is heated / vaporised so that the hydrocarbons evaporate
- Hydrocarbons have different boiling points (due to different chain lengths)
- The vapours rise up the fractionating column where there is a temperature gradient (cooler at the top)
- Hydrocarbons condense and are collected at different levels / fractions where the temperature matches their boiling point
- A correct use of the fraction with the shortest chains — e.g. refinery gas used as a fuel / LPG for cooking or heating
Key terms in this question
hydrocarbon · fractional distillation · fraction
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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