AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Fractional distillation and petrochemicals: Students should be able to explain how fractional distillation works in terms of evaporation and condensation.
Crude oil is a mixture of hydrocarbons with different boiling points.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crude oil is a mixture of hydrocarbons with different boiling points. Fractional distillation separates them in a fractionating column that is hot at the bottom and cooler at the top. The crude oil is heated until most of it evaporates. Vapour rises up the column and cools as it goes. A hydrocarbon condenses when the temperature falls below its boiling point, so it leaves the column as a liquid at that level. Hydrocarbons with low boiling points, such as short-chain alkanes, stay as gases and rise to the top, where they are collected. Hydrocarbons with high boiling points condense near the bottom. Each collected group is a fraction, not a pure substance, and the fractions have different uses such as petrol, diesel and bitumen.
What earns the marks
- Crude oil is heated so that the hydrocarbons evaporate and rise up the fractionating column.
- The column has a temperature gradient, being hotter at the bottom and cooler at the top.
- As vapour rises it cools; a hydrocarbon condenses when the temperature drops below its boiling point.
- Hydrocarbons with low boiling points travel further up the column and are collected near the top, often as gases.
- Hydrocarbons with high boiling points condense lower down the column and are collected near the bottom.
- Each fraction is a mixture of hydrocarbons with a similar boiling point range, not a single pure compound.
Common mistakes and how to put them right
Saying hydrocarbons melt in the column.
Put it right: The change is condensation from vapour to liquid, not melting from solid to liquid.
Thinking all fractions have the same boiling point.
Put it right: A fraction is a mixture with a boiling point range, and different fractions have different ranges.
Reversing the temperature trend.
Put it right: The column is hottest at the bottom and coolest at the top, so low-boiling hydrocarbons rise furthest.
Exam tips
- Use the words evaporate, rise, cool and condense in the correct order when describing the process.
- Link boiling point to position in the column: lower boiling point means collected higher up.
- State that a fraction is a mixture, and give one use for a named fraction to show understanding.
Try the lesson
The start of the Fractional distillation and petrochemicals: Students should be able to explain how fractional distillation works in terms of evaporation and condensation. lesson, made from checked teaching notes for this specification point.
Have a go first
In fractional distillation, what happens to the liquid mixture when it is heated before it enters the fractionating column?
Listen
Crude oil is a mixture of hydrocarbons with different boiling points. Fractional distillation separates them in a fractionating column that is hot at the bottom and cooler at the top. The crude oil is heated until most of it evaporates. Vapour rises up the column and cools as it goes. A hydrocarbon condenses when the temperature falls below its boiling point, so it leaves the column as a liquid at that level. Hydrocarbons with low boiling points, such as short-chain alkanes, stay as gases and rise to the top, where they are collected. Hydrocarbons with high boiling points condense near the bottom. Each collected group is a fraction, not a pure substance, and the fractions have different uses such as petrol, diesel and bitumen. A strong answer earns marks for these points. Crude oil is heated so that the hydrocarbons evaporate and rise up the fractionating column. The column has a temperature gradient, being hotter at the bottom and cooler at the top. As vapour rises it cools; a hydrocarbon condenses when the temperature drops below its boiling point. Hydrocarbons with low boiling points travel further up the column and are collected near the top, often as gases. Exam tip: Use the words evaporate, rise, cool and condense in the correct order when describing the process.
Key words
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Fractional distillation
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Evaporation and condensation
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Industrial uses of crude oil fractions
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See every definition
- Fractional distillation
- Separates the hydrocarbons in crude oil in a fractionating column that is hot at the bottom and cooler at the top.
- Evaporation and condensation
- Crude oil is heated until most of it evaporates, and a hydrocarbon condenses when the temperature falls below its boiling point.
- Industrial uses of crude oil fractions
- Each collected group is a fraction, not a pure substance, and the fractions have different uses such as petrol, diesel and bitumen.
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