Crude oil is separated by fractional distillation. Explain how the properties of hydrocarbon molecules affect their separation into fractions and their usefulness as fuels.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Crude oil is a mixture of hydrocarbons with different chain lengths. Short‑chain hydrocarbons have lower boiling points, so they evaporate first and rise up the fractionating column. As they rise they condense when the temperature falls to their boiling point, allowing them to be collected in separate fractions. The shorter chains are more flammable and ignite more easily, making them useful as fuels. They are also less viscous and flow more easily than longer‑chain hydrocarbons.
Examiner tips
- Use the word "boiling point" to link chain length to separation.
- Mention that lower boiling point = higher in column and easier to ignite.
- Show that viscosity decreases with chain length to explain fuel flow.
- Keep answer concise and use UK spelling.
Common mistakes
- Confusing boiling point with melting point.
- Saying all hydrocarbons are equally flammable.
- Forgetting to mention viscosity or flow properties.
Mark scheme (5 marks)
- Crude oil is a mixture of hydrocarbons with different (chain) lengths / different numbers of carbon atoms
- Hydrocarbons with shorter chains / smaller molecules have lower boiling points and condense / are collected higher up the column
- Hydrocarbons evaporate and rise up the fractionating column, condensing when they reach the level at the temperature equal to their boiling point, allowing separation into fractions
- Shorter hydrocarbon molecules are more flammable / ignite more easily, making them more useful as fuels
- Shorter hydrocarbon molecules are less viscous / flow more easily (than longer chain molecules)
Key terms in this question
fractional distillation · fraction
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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