Properties of Hydrocarbons

This lesson covers how the physical properties of hydrocarbons change with chain length, and why there is a mismatch between supply and demand for different fractions. This is key content for the Edexcel GCSE Chemistry specification (1CH0) and builds directly on what you learned about fractional distillation.

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How Properties Change with Chain Length

As the number of carbon atoms in a hydrocarbon chain increases, several physical properties change in a predictable way. These trends apply to alkanes and other homologous series.

The Key Trends

Property	As chain length increases…	Reason
Boiling point	Increases	Longer chains have stronger intermolecular forces — more energy is needed to separate the molecules
Viscosity	Increases (thicker, flows less easily)	Longer molecules become more tangled and experience greater intermolecular forces, making the liquid thicker
Flammability	Decreases (harder to ignite)	Longer molecules need more energy to vaporise before they can burn; shorter chains ignite more easily
Colour	Becomes darker	Short-chain fractions are colourless or pale yellow; long-chain fractions are dark brown to black