Fractional Distillation
Recap Fractional Distillation for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Crude oil is separated into useful fractions by fractional distillation, using the different boiling points of the hydrocarbons.
Free downloads
These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Crude oil is separated into fractions by fractional distillation, because the different hydrocarbons have different boiling points.
The oil is heated until it evaporates, and the vapours enter 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, so hydrocarbons with high boiling points condense near the bottom and those with low boiling points near the top.
Each fraction contains molecules with a similar number of carbon atoms. The fractions are processed into fuels and feedstock for the petrochemical industry, including petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases.
Revision notes
The process
Crude oil is heated until it evaporates, and the vapours enter a fractionating column.
The column is hot at the bottom and cooler at the top. Each fraction condenses where the temperature matches its boiling point.
Where each fraction condenses
Hydrocarbons with the highest boiling points — the longest molecules — condense near the bottom.
Those with the lowest boiling points — the shortest molecules — rise highest before condensing near the top. Each fraction contains molecules with a similar number of carbon atoms.
Uses of the fractions
Fuels: petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases.
Feedstock for the petrochemical industry, used to make solvents, lubricants, polymers and detergents. The vast array of natural and synthetic carbon compounds arises from the ability of carbon atoms to bond to each other.
Key points
- Fractional distillation separates crude oil.
- It works because hydrocarbons have different boiling points.
- The column is hot at the bottom and cool at the top.
- Long molecules condense near the bottom.
- Short molecules condense near the top.
- Each fraction has a similar number of carbon atoms.
Worked examples
Example 1
Explain why crude oil can be separated by fractional distillation. [2 marks]
Model answer
Crude oil is a mixture of hydrocarbons with different boiling points (1 mark), so each fraction condenses at a different height in the column where the temperature matches its boiling point (1 mark).
Example 2
Explain why hydrocarbons with long molecules condense near the bottom of the column. [2 marks]
Model answer
Long molecules have stronger intermolecular forces and therefore higher boiling points (1 mark), so they condense first at the hotter bottom of the column (1 mark).
Example 3
Name three fractions obtained from crude oil and give a use for one of them. [3 marks]
Model answer
Petrol, diesel oil and kerosene (2 marks). Petrol is used as a fuel for cars (1 mark).
Common mistakes
Saying the column is cool at the bottom.
It is hot at the bottom and cooler at the top.
Saying fractions are pure substances.
Each fraction is still a mixture, of molecules with similar carbon numbers.
Confusing boiling point with molecule size incorrectly.
Longer molecules have higher boiling points, so they condense lower down.
Saying the fractions are separated by chemical reaction.
Fractional distillation is a physical process.
Exam tips
- Link boiling point to molecule length and position in the column.
- Say each fraction contains molecules of similar size.
- Learn the main fractions and one use for each.
- Remember it is a physical separation, not a reaction.
Key terms
- Fraction
- A group of hydrocarbons with similar boiling points.
- Fractionating column
- The column in which crude oil is separated.
- Feedstock
- Raw material for the petrochemical industry.
- Petrochemical
- A chemical made from crude oil.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.