What is crude oil, and how is it separated into useful fractions?
Crude oil as a mixture of hydrocarbons, fractional distillation, and the properties and uses of the fractions.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on crude oil, covering crude oil as a mixture of hydrocarbons, how fractional distillation separates it, and the properties and uses of the fractions.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe crude oil as a mixture of hydrocarbons, explain fractional distillation, and describe the properties and uses of the fractions.
What crude oil is
Because it is a mixture, its parts are not chemically bonded and can be separated by physical means based on their different boiling points.
Fractional distillation
The fractions and their uses
From the top (smallest molecules) to the bottom (largest):
- Gases: used for domestic heating and cooking.
- Petrol: fuel for cars.
- Kerosene: fuel for aircraft.
- Diesel: fuel for larger vehicles.
- Fuel oil: fuel for ships and power stations.
- Bitumen: used to surface roads and roofs.
How properties change down the column
This trend is why the small, very flammable fractions (like petrol) make good fuels, and the large, thick fractions (like bitumen) are used for surfacing.
Why the boiling point increases with size
The trend in boiling point can be explained by the forces between molecules. As hydrocarbon molecules get larger, there are stronger forces between them (more points of contact), so more energy is needed to separate them, giving a higher boiling point. This is the same reason larger molecules are more viscous, because the molecules cling together more strongly. Linking the boiling point trend to the strength of the forces between molecules is a higher-mark explanation that examiners reward.
Crude oil as a finite resource
Crude oil is a finite (non-renewable) resource: it formed over millions of years and is being used far faster than it forms, so it will eventually run out. As well as fuels, crude oil provides the raw materials for plastics, medicines and many chemicals, so it is valuable beyond just energy. This is one reason for developing renewable energy and for recycling plastics, to make the remaining oil last longer. Recognising that crude oil is finite and is a feedstock for the chemical industry, not just a fuel, is a common discussion point.
Try this
Q1. What two elements are in a hydrocarbon? [1 mark]
- Cue. Hydrogen and carbon only.
Q2. Do small or large hydrocarbon molecules have lower boiling points? [1 mark]
- Cue. Small molecules have lower boiling points.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how crude oil is separated into fractions by fractional distillation.Show worked answer →
A Unit 5 describe question worth 4 marks. Reward: the crude oil is heated and evaporated (1); the vapours rise up a fractionating column that is hot at the bottom and cooler at the top (1); each fraction condenses at a different height, where the temperature matches its boiling point (1); fractions with smaller molecules and lower boiling points condense near the top, larger ones near the bottom (1). Markers credit heating/evaporating, the temperature gradient, and condensing at different heights. A common error is to say the oil is separated by reacting it.
WJEC style3 marksDescribe how the properties of the hydrocarbon fractions change as the molecules get larger.Show worked answer →
A Unit 5 trend question. Reward: as the molecules get larger, the boiling point increases (1); the fractions become more viscous (thicker) and less flammable (less easy to ignite) (1); they are also darker and less volatile (1). Markers credit higher boiling point, more viscous, and less flammable as the molecules get bigger. A common error is to reverse the trend.
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