Explain why heavy fuel oil fractions from crude oil are less useful as fuels than petrol fractions.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Heavy fuel oil contains long‑chain hydrocarbons, so its molecules are larger than those in petrol.
It is more viscous and does not flow easily, making pumping and atomisation difficult.
Its higher boiling point means it is harder to vapourise, reducing the amount of fuel that can be supplied to the engine.
Because of the long chains and high viscosity, heavy fuel oil is less flammable and ignites more slowly than petrol.
It is more viscous and does not flow easily, making pumping and atomisation difficult.
Its higher boiling point means it is harder to vapourise, reducing the amount of fuel that can be supplied to the engine.
Because of the long chains and high viscosity, heavy fuel oil is less flammable and ignites more slowly than petrol.
Examiner tips
- Use the exact terms: ‘long‑chain hydrocarbons’, ‘viscous’, ‘higher boiling point’, ‘less flammable’.
- Show the logical order: size → viscosity → boiling point → flammability.
- Keep each point brief and separate with a line break or bullet.
- Avoid unnecessary explanation or examples.
Mark scheme (4 marks)
- Heavy fuel oil has longer hydrocarbon molecules / larger molecules than petrol
- Heavy fuel oil is more viscous / does not flow as easily as petrol
- Heavy fuel oil has a higher boiling point / is harder to vaporise than petrol
- Heavy fuel oil is less flammable / ignites less easily than petrol
Key terms in this question
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →