# Propane (C₃H₈) is a simple molecular substance that is a gas at room temperature. Octane (C₈H₁₈) is also a simple molecular substance but is a liquid at room temperature. Both substances contain only carbon–hydrogen bonds. Explain why octane has a higher boiling point than propane.

> Edexcel A-Level Chemistry (9CH0) — 2.4 Forces between molecules · Explain · 5 marks

> Propane (C₃H₈) is a gas at room temperature with a boiling point of −42 °C. Octane (C₈H₁₈) is a liquid at room temperature with a boiling point of 126 °C. Both are simple molecular substances containing only carbon and hydrogen.

## Mark scheme (5 marks)

1. Both substances have intermolecular forces (between molecules) / van der Waals forces / London dispersion forces
2. Octane has more electrons / is a larger molecule / has a greater relative molecular mass than propane
3. Octane has stronger intermolecular forces than propane
4. More energy is needed to overcome / break the intermolecular forces in octane
5. Therefore octane has a higher boiling point / a higher temperature is needed to boil octane

## Key terms

- [simple molecular substance](https://www.gradenine.co.uk/glossary/simple-molecular-substance)
- [boiling point](https://www.gradenine.co.uk/glossary/boiling-point)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/propane-c-h-is-a-simple-molecular-7152347c) · Published by Druglandscape Ltd.