Crude oil is a mixture of alkanes. Describe how fractional distillation separates crude oil into fractions, and state one use of the fraction obtained at the lowest temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a finite resource formed over millions of years from the remains of ancient organisms. It is an important source of fuels and chemical feedstocks.
Model answer (4 marks)
Crude oil is heated until it vapourises and the vapours rise into a fractionating column.
The column has a temperature gradient – hot at the bottom and cool at the top.
As the vapours ascend, alkanes with higher boiling points condense first and are collected at lower levels, while those with lower boiling points remain vapourised and condense higher up.
The fraction that condenses at the lowest temperature (the lightest alkanes, e.g. LPG) is collected as a fuel – it is bottled for use as a cooking or camping gas.
The column has a temperature gradient – hot at the bottom and cool at the top.
As the vapours ascend, alkanes with higher boiling points condense first and are collected at lower levels, while those with lower boiling points remain vapourised and condense higher up.
The fraction that condenses at the lowest temperature (the lightest alkanes, e.g. LPG) is collected as a fuel – it is bottled for use as a cooking or camping gas.
Examiner tips
- Use the word "fractionating column" and mention the temperature gradient. Explain that higher‑boiling alkanes condense lower in the column. State the lowest‑temperature fraction and give a specific use such as LPG for cooking or camping gas.
- Common mistakes: • Forgetting to mention the temperature gradient in the column. • Mixing up the order of condensation (higher‑boiling alkanes condense lower). • Giving a use that is not a fuel (e.g. industrial solvent) for the lowest‑temperature fraction.
Mark scheme (4 marks)
- Crude oil is heated / vaporised and the vapours enter a fractionating column
- The column has a temperature gradient — hotter at the bottom, cooler at the top
- Fractions condense and are collected at different levels because they have different boiling points
- The fraction collected at the lowest temperature (refinery gas / LPG) is used as a fuel / bottled gas / camping gas
Key terms in this question
fractional distillation · fraction
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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