---
title: "Crude Oil, Fractional Distillation and Cracking"
description: "Crude oil contains a mixture of hydrocarbons. Fractional distillation separates fractions by boiling range, and cracking converts some surplus larger hydrocarbons into smaller fuel molecules and alken"
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: 2026-11-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# Crude Oil, Fractional Distillation and Cracking

Crude oil contains a mixture of hydrocarbons. Fractional distillation separates fractions by boiling range, and cracking converts some surplus larger hydrocarbons into smaller fuel molecules and alkenes.

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 the column's temperature gradient separates fractions by boiling range and relates those ranges to typical hydrocarbon chain lengths
- 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 use cracking by comparing production with demand

## Lesson: Sorting and splitting crude oil

Crude oil is a fossil fuel formed from ancient organic remains, including tiny marine organisms. Sediment buried the remains. Heat and pressure over millions of years helped turn the material into oil. Crude oil contains many hydrocarbons. A hydrocarbon contains only carbon and hydrogen. Its molecules may contain chains, branches or rings.

You have met vaporisation and condensation in distillation. Heating crude oil vaporises much of it. A fractionating column is hot below and cooler above. Higher-boiling hydrocarbons condense lower down. Lower-boiling ones stay as vapour until cooler levels; some gases leave the top. The heaviest material, including bitumen, remains as residue near the base. Each fraction contains hydrocarbons with a similar boiling range.

*(drawing: Petrol is collected at cooler upper levels. Higher-boiling fractions collect lower down; the heaviest residue stays near the base.)*

For similar hydrocarbons, longer chains generally have higher boiling points, greater viscosity and lower volatility. Viscosity means resistance to flow. Volatility means how readily a substance vaporises. Shorter-chain fractions usually flow and ignite more easily. These are trends, not exact tests: molecular structure and temperature also affect properties.

Catalytic cracking heats vaporised larger hydrocarbons and passes them over a suitable catalyst. Smaller molecules form, including alkanes and alkenes. Steam cracking instead uses high temperature and steam. Equations conserve each kind of atom. For example, C12H26 -> C10H22 + C2H4 has 12 carbon atoms and 26 hydrogen atoms on each side.

Distillation separates existing molecules; cracking changes them by breaking bonds. Refineries can have more heavy fractions than customers demand and greater demand for smaller fuel molecules. Cracking converts some surplus larger molecules into smaller products. It also makes alkenes for polymers. Heavy fractions still have uses, including fuels and bitumen for roads.

**Recap.** A temperature gradient separates boiling ranges. Cracking changes surplus larger hydrocarbons into smaller fuels and useful alkenes.

*Lesson, questions and flashcards written for this dataset.*

## 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)