# Butane and octane are both alkanes obtained from crude oil. When octane undergoes complete combustion, sulfur dioxide is also produced as a by-product. Explain why sulfur dioxide is produced during the combustion of octane and describe one environmental consequence of sulfur dioxide being released into the atmosphere.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 4.3 Alkanes · Explain · 4 marks

> Crude oil contains hydrocarbons as well as small amounts of sulfur impurities. Octane (C₈H₁₈) is a hydrocarbon fraction obtained from crude oil by fractional distillation.

## Mark scheme (4 marks)

1. Octane contains sulfur impurities (not part of the hydrocarbon itself)
2. During combustion, the sulfur impurities are oxidised / react with oxygen to produce sulfur dioxide
3. Sulfur dioxide reacts with water vapour in clouds / atmosphere to form acid rain
4. Acid rain causes damage to ecosystems / buildings / lakes / trees / kills aquatic life

## Key terms

- [combustion](https://www.gradenine.co.uk/glossary/combustion)
- [sulfur dioxide](https://www.gradenine.co.uk/glossary/sulfur-dioxide)

## Related

- [Revision notes for Pearson Edexcel International GCSE Chemistry (4CH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/butane-and-octane-are-both-alkanes-30a6d899) · Published by Druglandscape Ltd.