Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separate crude oil into fractions. Explain how fractional distillation separates crude oil into fractions with different boiling points.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a fossil fuel formed over millions of years from the remains of marine organisms. It contains hydrocarbons of different chain lengths.
Model answer (4 marks)
Crude oil is heated in a distillation column until the hydrocarbons vaporise. The vapour rises through the column, which has a temperature gradient – hot at the bottom and cool at the top. Hydrocarbons with shorter carbon chains have lower boiling points and therefore evaporate first; they rise higher in the column. As the vapour moves upward it encounters cooler zones; when the temperature falls to a hydrocarbon’s boiling point it condenses back into liquid. Each condensed liquid is collected at the corresponding height, giving separate fractions with different boiling points.
Examiner tips
- Use the word ‘fractional’ to show you understand the column’s temperature gradient.
- Mention that shorter chains boil lower and rise higher.
- Explain that condensation occurs at the boiling point of each component.
- Show the sequence: heating → vapour rise → temperature gradient → condensation → collection.
Common mistakes
- Confusing boiling point with melting point.
- Saying all hydrocarbons condense at the same temperature.
- Omitting the temperature gradient in the column.
Mark scheme (4 marks)
- The crude oil is heated and the hydrocarbons vaporise / turn to gas
- Hydrocarbons with shorter carbon chains / smaller molecules have lower boiling points and rise higher up the column
- There is a temperature gradient in the column — it is cooler at the top and hotter at the bottom
- Hydrocarbons condense / turn back to liquid when they reach the part of the column that is at their boiling point, and are collected as separate fractions
Key terms in this question
hydrocarbon · fractional distillation · boiling point · fraction
Related
- All Eduqas GCSE Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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