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, and state one use of the fraction with the lowest boiling point.
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 a complex mixture of hydrocarbon molecules of different chain lengths.
Model answer (5 marks)
Crude oil is heated in a distillation unit so that the hydrocarbon molecules evaporate.
The vapours rise through a fractionating column that has a temperature gradient – hot at the bottom, cool at the top.
As the vapours ascend, each hydrocarbon condenses when it reaches the part of the column whose temperature equals its boiling point.
Because longer‑chain hydrocarbons have higher boiling points they condense lower in the column, whereas shorter‑chain hydrocarbons condense higher up.
The fraction that condenses at the top of the column has the lowest boiling point – it is petrol (gasoline), which is used as a fuel for cars.
The vapours rise through a fractionating column that has a temperature gradient – hot at the bottom, cool at the top.
As the vapours ascend, each hydrocarbon condenses when it reaches the part of the column whose temperature equals its boiling point.
Because longer‑chain hydrocarbons have higher boiling points they condense lower in the column, whereas shorter‑chain hydrocarbons condense higher up.
The fraction that condenses at the top of the column has the lowest boiling point – it is petrol (gasoline), which is used as a fuel for cars.
Examiner tips
- Use the word ‘evaporate’ or ‘vaporise’ for the first step. Mention the temperature gradient in the column. Explain that condensation occurs at the column section that matches the boiling point. State the lowest‑boiling fraction and give its common use.
Mark scheme (5 marks)
- Crude oil is heated and the hydrocarbons evaporate / vaporise
- The vapours rise up the fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
- Hydrocarbons condense when they reach the part of the column that is at their boiling point
- Longer hydrocarbon chains have higher boiling points so condense lower in the column / shorter chains have lower boiling points so rise higher and condense near the top
- The fraction with the lowest boiling point is petrol / gasoline / used as fuel for cars / used as a fuel
Key terms in this question
fractional distillation · hydrocarbon · fraction · boiling point
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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