---
title: "Crude Oil, Fractional Distillation and Cracking"
description: "Crude oil is a mixture of hydrocarbons separated by boiling point in a fractionating column, and the long fractions nobody wants are cracked into shorter, more useful molecules."
canonical: https://lightmysky.com/learn/science/crude-oil-fractional-distillation-and-cracking-mt_Jg3O4EQ9Vz
source: https://lightmysky.com/learn/science/crude-oil-fractional-distillation-and-cracking-mt_Jg3O4EQ9Vz.md
retrieved: 2026-09-12
---

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# Crude Oil, Fractional Distillation and Cracking

Crude oil is a mixture of hydrocarbons separated by boiling point in a fractionating column, and the long fractions nobody wants are cracked into shorter, more useful molecules.

Subject: Science · Area: Matter & Materials · Ages 15 to 16
Page: https://lightmysky.com/learn/science/crude-oil-fractional-distillation-and-cracking-mt_Jg3O4EQ9Vz

## Ready when they can

- Explains how a column separates fractions by chain length and boiling point
- Links chain length to viscosity, how easily a fraction evaporates, and how easily it burns
- Describes the conditions for cracking and writes an equation showing a long chain becoming an alkane plus an alkene
- Explains why refineries crack at all, in terms of what is produced against what is wanted

## Lesson: Sorting and splitting crude oil

Crude oil is a fossil fuel and a mixture of hydrocarbons, molecules built from carbon and hydrogen. It formed from tiny sea plants and animals buried under sand and rock. Over millions of years, heat and pressure cooked the remains into the thick liquid called petroleum, which means rock oil.

A fractionating column sorts the mixture by boiling point. Hot vapours rise, cool, and condense at different levels. Short chains have low boiling points, so gases and petrol come off near the top. Long chains have high boiling points, so bitumen stays near the bottom.

Chain length sets behaviour. Short chains flow easily, evaporate fast and catch light fast. Long chains are thick and sticky, slow to evaporate and stubborn to burn. The number of carbons in the chain decides where each fraction lands and how it acts.

**Example.** Refineries crack the surplus long chains with heat and a catalyst. One long molecule splits into a shorter alkane plus an alkene, such as decane C10H22 -> octane C8H18 + ethene C2H4. They do it on purpose: drivers want far more short petrol than long bitumen, so cracking matches supply to demand.

**Recap.** Old seas made the mixture, boiling points sort it, and cracking cuts the surplus long chains down.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Alkanes and Alkenes: The First Homologous Series](https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL)
- [Choosing How to Separate Mixtures](https://lightmysky.com/learn/science/choosing-how-to-separate-mixtures-mt_U_tJvy3cbB)

## Opens up

- [Pollutants from Burning Fuels](https://lightmysky.com/learn/science/pollutants-from-burning-fuels-mt_DDiyDBy6YW)