What is crude oil, and how is it separated into useful fuels?
Crude oil as a mixture of hydrocarbons, separation by fractional distillation, and the properties and uses of the fractions.
A focused answer to WJEC GCSE Chemistry topic 2.5 on crude oil, covering crude oil as a finite mixture of hydrocarbons, how fractional distillation separates it by boiling point, and how the properties of the fractions change down the column.
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What this topic is asking
WJEC wants you to describe what crude oil is and explain how fractional distillation separates it into useful fractions. This is part of topic 2.5 Crude oil, fuels and organic chemistry in Unit 2 of WJEC GCSE Chemistry (3430).
What crude oil is
Most hydrocarbons in crude oil are alkanes, chains of carbon atoms each with their full set of hydrogen atoms.
Fractional distillation
Trends down the column
Try this
Q1. State what is meant by a hydrocarbon. [1 mark]
- Cue. A compound made of hydrogen and carbon only.
Q2. State how the boiling point of the fractions changes as you go down the column. [1 mark]
- Cue. The boiling point increases down the column.
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 20194 marksExplain how crude oil is separated into fractions by fractional distillation.Show worked answer →
A topic 2.5 Explain question. The crude oil is heated until most of it evaporates and the vapours pass into a fractionating column that is hot at the bottom and cooler at the top (1 mark). As the vapours rise they cool; each fraction condenses when it reaches a temperature equal to its boiling point (1 mark). Fractions with high boiling points (large molecules) condense low down; fractions with low boiling points (small molecules) rise higher before condensing (1 mark). The separated fractions are collected at different heights (1 mark). Markers reward the temperature gradient, condensing at the boiling point and the link to molecule size. A common error is to call it a chemical reaction.
WJEC 20223 marksDescribe how the properties of the hydrocarbon fractions change as you go down the fractionating column.Show worked answer →
A topic 2.5 structured question. Going down the column, the molecules get larger (longer chains) (1 mark), the boiling points increase and the fractions become more viscous (thicker) and less flammable (1 mark). They are also less volatile and burn with a smokier flame (1 mark). Markers reward the trends in molecule size, boiling point/viscosity and flammability. A common error is to reverse the trend or to confuse viscosity with density.
Related dot points
- Alkanes and alkenes as homologous series, the test for unsaturation, combustion of hydrocarbons, and cracking to make smaller alkanes and alkenes.
A focused answer to WJEC GCSE Chemistry topic 2.5 on alkanes and alkenes, covering their general formulae, the bromine water test for alkenes, complete and incomplete combustion, and how cracking converts large hydrocarbons into smaller alkanes and useful alkenes.
- Alcohols as a homologous series, the functional group, and the production of ethanol by fermentation and by hydration of ethene.
A focused answer to WJEC GCSE Chemistry topic 2.5 on alcohols, covering the alcohol homologous series and the -OH functional group, the uses of ethanol, and how ethanol is produced by fermentation of sugars and by the hydration of ethene.
- Addition polymerisation of alkenes, drawing the repeat unit, the uses of common polymers, and the problems of polymer waste and disposal.
A focused answer to WJEC GCSE Chemistry topic 2.5 on polymers, covering how alkene monomers join by addition polymerisation, how to draw the repeat unit, the uses of common plastics, and the environmental problems of polymer waste and its disposal.
Sources & how we know this
- WJEC GCSE Chemistry specification (3430) from 2016 — WJEC (2016)