# Fractional distillation

A way to separate crude oil into fractions, groups of hydrocarbons with similar boiling points. The oil is heated, and the vapours rise up a column and condense at different heights.

Chemistry (school) · Part 08: Carbon chemistry and the Earth · Card 3 of 10 · Last updated 2026-10-19

## Analogy

Like sorting runners by how far they get up a hill. Short chains reach the top.

## Example

**Fractions, top to bottom**

Gases, petrol, kerosene,
diesel, fuel oil, bitumen
Boiling point rises downwards.

## In practice

Small molecules (gases, petrol) come out near the top. Large molecules (bitumen) stay at the bottom.

## Practice

### Flashcards

- **What does fractional distillation separate?** Crude oil into fractions with different boiling points.

### Interview questions

**Describe how fractional distillation separates crude oil and explain why the fractions come out at different heights.**

- Oil is heated and vapours enter the column
- The column is hotter at the bottom and cooler at the top
- Molecules condense where the temperature is below their boiling point
- Smaller molecules with low boiling points rise higher

## Questions people ask

### Why do different fractions have different boiling points?

The longer a hydrocarbon chain, the stronger the forces between its molecules. More energy is needed to separate them, so the boiling point is higher. Short chains boil at low temperatures and rise to the top. Long chains condense lower down, where it is hotter.

### What is each fraction used for?

Gases are used for heating and cooking. Petrol fuels cars, kerosene fuels aircraft and diesel fuels lorries and trains. Fuel oil runs ships and heating systems. Bitumen is used to surface roads and roofs. Each is a mixture of hydrocarbons with similar chain lengths.

## Related concepts

- [Crude oil](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-1)
- [Hydrocarbon](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-2)
- [Cracking](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-6)

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