AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Fractional distillation and petrochemicals: The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
Crude oil is a mixture of many hydrocarbons, compounds containing only carbon and hydrogen.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Crude oil is a mixture of many hydrocarbons, compounds containing only carbon and hydrogen. Because these molecules have different numbers of carbon atoms, they have different boiling points. Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms. The crude oil is heated and enters a tall fractionating column as a vapour. The column is hotter at the bottom and cooler at the top. Hydrocarbons with small molecules, low boiling points and weak intermolecular forces rise far up the column before condensing. Larger molecules with higher boiling points condense lower down. Each fraction is collected and used as a fuel or as feedstock.
What earns the marks
- Crude oil is a mixture of many hydrocarbons, which are compounds of carbon and hydrogen only.
- Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms.
- The fractionating column has a temperature gradient: hot at the bottom and cooler towards the top.
- Hydrocarbons with smaller molecules have lower boiling points and condense higher up the column.
- Hydrocarbons with larger molecules have higher boiling points and condense lower down the column.
- Each fraction is collected separately and can be used as a fuel or as feedstock for the petrochemical industry.
Common mistakes and how to put them right
Thinking a fraction is a single pure compound.
Put it right by stating that a fraction is a mixture of hydrocarbons with a similar number of carbon atoms.
Believing that fractional distillation breaks large molecules into smaller ones.
Put it right by explaining that it is a physical separation by boiling point, not a chemical reaction.
Confusing the temperature gradient, for example saying the top is hottest.
Put it right by stating that the bottom is hottest and the top is coolest.
Exam tips
- Link boiling point to molecular size: more carbon atoms means stronger intermolecular forces and a higher boiling point.
- Describe the process in order: vaporisation, rising through the column, condensation at different heights, collection.
- Use the phrase similar number of carbon atoms when defining a fraction, rather than identical molecules.
Try the lesson
The start of the Fractional distillation and petrochemicals: The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation. lesson, made from checked teaching notes for this specification point.
Have a go first
In fractional distillation, crude oil is separated into fractions. What do the molecules in one fraction have in common?
Listen
Crude oil is a mixture of many hydrocarbons, compounds containing only carbon and hydrogen. Because these molecules have different numbers of carbon atoms, they have different boiling points. Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms. The crude oil is heated and enters a tall fractionating column as a vapour. The column is hotter at the bottom and cooler at the top. Hydrocarbons with small molecules, low boiling points and weak intermolecular forces rise far up the column before condensing. Larger molecules with higher boiling points condense lower down. Each fraction is collected and used as a fuel or as feedstock. A strong answer earns marks for these points. Crude oil is a mixture of many hydrocarbons, which are compounds of carbon and hydrogen only. Fractional distillation separates the mixture into fractions, each containing molecules with a similar number of carbon atoms. The fractionating column has a temperature gradient: hot at the bottom and cooler towards the top. Hydrocarbons with smaller molecules have lower boiling points and condense higher up the column. A common slip: Thinking a fraction is a single pure compound. Instead, stating that a fraction is a mixture of hydrocarbons with a similar number of carbon atoms. Exam tip: Link boiling point to molecular size: more carbon atoms means stronger intermolecular forces and a higher boiling point.
Key words
Say what each word means before you turn the card.
Fractional distillation
Try answering aloud or in your head before turning the card.
Fraction
Try answering aloud or in your head before turning the card.
Hydrocarbons
Try answering aloud or in your head before turning the card.
See every definition
- Fractional distillation
- Separates the mixture of hydrocarbons in crude oil into fractions, each containing molecules with a similar number of carbon atoms.
- Fraction
- A mixture of hydrocarbons with a similar number of carbon atoms, collected separately and used as a fuel or as feedstock.
- Hydrocarbons
- Compounds containing only carbon and hydrogen, of which crude oil is a mixture of many.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Fractional distillation and petrochemicals: The many hydrocarbons in crude oil may be separated into fractions, each of which contains molecules with a similar number of carbon atoms, by fractional distillation.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge