What is crude oil, how is it separated, and what are alkanes, alkenes and cracking?
Crude oil as a mixture of hydrocarbons separated by fractional distillation, how the properties of fractions change with chain length, alkanes and alkenes as saturated and unsaturated hydrocarbons, the bromine water test, and cracking.
A focused CCEA GCSE Double Award Science (Chemistry Unit C2) answer on crude oil and hydrocarbons, covering fractional distillation, how fraction properties change with chain length, alkanes and alkenes, the bromine water test for unsaturation, and cracking.
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What this dot point is asking
CCEA Double Award wants crude oil as a mixture separated by fractional distillation, how fraction properties change with chain length, the difference between alkanes and alkenes, the bromine water test, and cracking. The fractional distillation and the bromine water test are reliable marks.
Crude oil and hydrocarbons
Because it is a mixture, crude oil itself is not very useful. It is separated into more useful fractions that each contain hydrocarbons of similar chain length.
Fractional distillation
The oil is heated and the vapours rise up a fractionating column that is hot at the bottom and cooler at the top. Each fraction condenses at the height where the temperature matches its boiling point:
- Short chains (low boiling point) condense near the top - petrol and gases.
- Long chains (high boiling point) condense near the bottom - bitumen and fuel oil.
As the chain gets longer, the boiling point rises, the fraction becomes more viscous (thicker), less volatile and less flammable.
Alkanes and alkenes
The double bond makes alkenes more reactive than alkanes. The bromine water test tells them apart:
Cracking
For example, a long alkane can crack into a shorter alkane (used as fuel) plus ethene (used to make poly(ethene)). Cracking matches supply to demand, because there is more demand for short-chain fuels than the oil naturally provides. It is carried out by heating the long hydrocarbons to vaporise them and passing them over a hot catalyst, which breaks the long chains apart.
Examples in context
Example 1. Why bitumen is used for roads. Bitumen is a very long-chain fraction with a very high boiling point and high viscosity, so it is thick and tar-like - ideal for surfacing roads. Its properties follow directly from its long chain length.
Example 2. Making more petrol. There is far more demand for petrol (short chains) than crude oil supplies, and a surplus of long fractions. Cracking the long chains into shorter ones makes more petrol and also produces alkenes for plastics, which is why refineries crack.
Try this
Q1. What elements are in a hydrocarbon? [1 mark]
- Cue. Hydrogen and carbon only.
Q2. What does an alkene do to bromine water? [1 mark]
- Cue. It decolourises it (orange to colourless).
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-style4 marksDescribe how crude oil is separated into fractions, and explain why the fractions separate.Show worked answer →
Describe fractional distillation for four marks.
The crude oil is heated and the vapours rise up a fractionating column that is hot at the bottom and cooler at the top.
Each fraction condenses at a different height depending on its boiling point.
Fractions with short chains and low boiling points condense near the top; long chains with high boiling points condense near the bottom.
So the oil is separated by boiling point, which depends on chain length. Markers reward heating, the temperature gradient, and separation by boiling point.
CCEA-style3 marksDescribe the bromine water test to tell an alkane from an alkene.Show worked answer →
State the test and both results for three marks.
Add orange bromine water to each hydrocarbon and shake.
An alkene decolourises the bromine water (it turns from orange to colourless) because it is unsaturated.
An alkane does not decolourise it; the bromine water stays orange. Markers reward the colour change for the alkene and no change for the alkane.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)