Fractional Distillation and Cracking

This lesson covers two key industrial processes in the AQA GCSE Combined Science Trilogy (8464) specification: fractional distillation of crude oil and cracking of long-chain hydrocarbons. You need to understand how crude oil is separated into useful fractions and why cracking is necessary.

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Fractional Distillation of Crude Oil

Fractional distillation is the process used to separate crude oil into useful fractions — groups of hydrocarbons with similar boiling points and chain lengths.

How It Works

1. Crude oil is heated in a furnace until most of it vaporises.
2. The vapour enters a fractionating column — a tall tower that is hot at the bottom and cool at the top.
3. Vapours rise up the column. Each hydrocarbon condenses when it reaches the level where the temperature equals its boiling point.
4. Short-chain hydrocarbons (low boiling points) rise to the top before condensing.
5. Long-chain hydrocarbons (high boiling points) condense near the bottom.
6. Fractions are collected at different levels.