Crude oil is separated into fractions by fractional distillation. Explain why the different fractions are collected at different heights in the fractionating column, and state one use of the fraction collected near the top of the column.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a mixture of hydrocarbons. It is separated into useful fractions by fractional distillation in an oil refinery.
Model answer (4 marks)
The fractions have different boiling points because they contain hydrocarbons of different chain lengths and molecular sizes. The fractionating column has a temperature gradient – it is hot at the bottom and cools towards the top. Hydrocarbons with lower boiling points therefore rise higher in the column before they condense, so they are collected at higher positions. The fraction collected near the top contains short‑chain, low‑boiling hydrocarbons, which are used as petrol (fuel for cars).
Examiner tips
- Mention the temperature gradient and boiling point difference; link lower boiling point to higher collection point; give a specific use such as petrol or LPG.
- Use clear, concise sentences and the exact terminology (boiling point, fractionating column, short‑chain hydrocarbons).
Common mistakes
- Confusing the direction of the temperature gradient (hot at top, cool at bottom).
- Failing to state that lower boiling point hydrocarbons rise higher before condensing.
- Giving a use that is not typical for the top fraction, e.g. diesel instead of petrol.
Mark scheme (4 marks)
- The fractions have different boiling points (due to different chain lengths / molecular sizes)
- The column has a temperature gradient — hotter at the bottom and cooler at the top
- Hydrocarbons with lower boiling points rise higher before condensing / shorter chain hydrocarbons condense higher up
- Fractions collected near the top have short chains / low boiling points — named use e.g. petrol (fuel for cars), or used as fuel / as a feedstock for making chemicals / LPG for camping gas
Key terms in this question
fractional distillation · fraction
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Crude oil, hydrocarbons and alkanes (Chem only) questions
- Explain why alkanes are described as saturated hydrocarbons.
- Explain why shorter-chain alkane fractions obtained from crude oil are more usef…
- Methane, pentane and decane are all alkanes found in crude oil. Describe how the…
- Explain why crude oil is described as a finite resource and outline why this is …