What is crude oil, and how is it separated into useful fractions?
Crude oil as a mixture of hydrocarbons, its separation by fractional distillation into fractions, and how the properties of fractions change with chain length.
A CCEA GCSE Chemistry answer on crude oil, covering it as a mixture of hydrocarbons, how fractional distillation separates it into fractions by boiling point, the main fractions and their uses, and how viscosity, boiling point and flammability change with chain length.
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What this dot point is asking
CCEA wants you to describe crude oil as a mixture of hydrocarbons, explain how fractional distillation separates it into fractions by boiling point, name the main fractions and their uses, and state how the properties of the fractions change with chain length.
Crude oil and hydrocarbons
Because crude oil is a mixture, the substances in it are not chemically bonded and can be separated by a physical method, using the different boiling points of molecules of different sizes.
Fractional distillation
So short-chain hydrocarbons, with low boiling points, reach the cool top before condensing, while long-chain hydrocarbons condense low down near the hot base. The column thus sorts the mixture by boiling point, which depends on molecule size.
The fractions and their uses
How properties change with chain length
This is why short-chain fractions such as petrol make good, easily ignited fuels, while long-chain bitumen is a thick, high-boiling solid used to surface roads.
Worked example
Examples in context
Example 1. Why aircraft use kerosene. Kerosene sits between petrol and diesel in chain length, giving it the right balance of volatility and energy content for jet engines. Its place in the fractionating column reflects the properties that make it suitable for aviation.
Example 2. A finite resource. Because crude oil takes millions of years to form, it is non-renewable, which drives the search for alternative fuels and the careful use of each fraction. The chemistry of separation underlies major economic and environmental decisions about energy.
Try this
Q1. State what is meant by a hydrocarbon. [1 mark]
- Cue. A compound containing only hydrogen and carbon.
Q2. State how viscosity changes as the hydrocarbon chains get longer. [1 mark]
- Cue. Viscosity increases (they get thicker).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksExplain how crude oil is separated into fractions by fractional distillation in a fractionating column.Show worked answer →
Markers want the heating, the temperature gradient and the collection of fractions.
The crude oil is heated until it vapourises and the vapour enters the bottom of a fractionating column that is hot at the bottom and cooler at the top.
As the vapour rises it cools. Each fraction condenses when it reaches the part of the column at its boiling point: fractions with high boiling points (long chains) condense low down, and fractions with low boiling points (short chains) condense near the top.
The separated fractions are tapped off at different heights, so the mixture is separated by boiling point.
Markers reward heating to vapourise, a temperature gradient up the column, and fractions condensing and being collected at their boiling points.
CCEA 20213 marksDescribe how the boiling point, viscosity and flammability of the fractions change as the hydrocarbon chains get longer.Show worked answer →
The marks are for the trend in each property with chain length.
As the hydrocarbon chains get longer:
- the boiling point increases, because larger molecules have stronger intermolecular forces;
- the viscosity increases (they get thicker and flow less easily);
- the flammability decreases (longer chains are harder to ignite and burn).
So short-chain fractions are runny, low-boiling and easy to ignite (good fuels), while long-chain fractions are thick, high-boiling and harder to burn (such as bitumen).
Markers reward boiling point up, viscosity up, flammability down as chains get longer.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)