Crude oil is separated by fractional distillation. Describe how fractional distillation works and explain why the different hydrocarbons in crude oil can be separated by this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Crude oil is a finite resource formed from the remains of ancient marine organisms. It is a mixture of many different hydrocarbon molecules that must be separated before they can be used as fuels.
Model answer (4 marks)
Crude oil is heated until it vapourises.
The vapours rise through a fractionating column that has a temperature gradient – hot at the bottom, cooler at the top.
Because hydrocarbons have different boiling points, each component vapourises at a different temperature.
As the vapours travel up the column they cool; when the temperature falls to a component’s boiling point it condenses and is collected at that level of the column.
The vapours rise through a fractionating column that has a temperature gradient – hot at the bottom, cooler at the top.
Because hydrocarbons have different boiling points, each component vapourises at a different temperature.
As the vapours travel up the column they cool; when the temperature falls to a component’s boiling point it condenses and is collected at that level of the column.
Examiner tips
- Use the word ‘vapourises’ and ‘condenses’ to show the phase change. Mention the temperature gradient in the column. Explain that each hydrocarbon condenses where its boiling point is reached. Keep the answer concise – 4 marks only.
Common mistakes
- Forgetting to state that the column has a temperature gradient. Using vague terms like ‘separated by heat’ without mentioning boiling points. Including extra information about the composition of crude oil that is not required.
Mark scheme (4 marks)
- The crude oil mixture is heated / vaporised
- The vapours rise up a fractionating column where there is a temperature gradient (hotter at the bottom, cooler at the top)
- Different hydrocarbons have different boiling points
- Each hydrocarbon condenses and is collected at the part of the column that matches its boiling point / where it cools enough to condense
Key terms in this question
fractional distillation · hydrocarbons · crude oil
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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