Crude Oil to Petrol
From Crude Oil to Petrol
Lesson duration is: 1 minItems in this lesson
From Crude Oil to Petrol
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Learning goal
I can explain how crude oil is separated to make petrol and how hydrocarbons are cracked.
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What words do you link with crude oil and petrol?
Use this to surface prior ideas such as fuel, oil, cars, hydrocarbons, refinery, pollution, mixture, petrol, gas, and plastic. Pick a few entries to connect to the learning goal.
What is crude oil?
Crude oil is a mixture of hydrocarbons
Hydrocarbons are compounds made from hydrogen and carbon only
Crude oil formed from the remains of ancient living things
Different hydrocarbons in crude oil have different sizes
Students often think crude oil is one substance. Stress that it is a mixture, so it must be separated physically before useful products can be collected.
Why is crude oil useful?
Different hydrocarbons have different uses because their properties are different.
Some are used as fuels, while others are used to make chemicals and plastics.
This is why crude oil is separated into fractions instead of being used all at once.
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A mixture can be separated
Crude oil is separated by fractional distillation.
This works because the hydrocarbons in crude oil have different boiling points.
Each part of the mixture boils and condenses at a different temperature.
Link to prior knowledge: mixtures are separated by differences in physical properties. Here, the key property is boiling point.
Fractional distillation
Crude oil is heated until most of it vaporises
The vapour enters a fractionating column
The column is hot at the bottom and cooler at the top
Hydrocarbons condense at different levels
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How petrol is collected
Smaller hydrocarbons have lower boiling points.
They rise higher in the fractionating column before they condense.
Petrol is collected near the top of the column because it contains relatively small molecules.
You may mention that refinery diagrams vary, but petrol is shown in the upper part because it is a lighter fraction than diesel or fuel oil.
Why can crude oil be separated by fractional distillation?
Different boiling points
Different colours
Different masses
Different smells
Checks the key idea from slides 2 to 6: crude oil is a mixture, and the fractions are separated because the hydrocarbons have different boiling points.
Complete the process
Crude oil is a mixture of hydrocarbons. It is separated by fractional distillation because the hydrocarbons have different boiling points. Petrol contains relatively small molecules, so it is collected near the top of the column.
All four blanks come directly from the first subtopic. Emphasise that petrol is near the top because it has smaller molecules with lower boiling points.
Where in the fractionating column is petrol collected?
At the bottom
Near the top
In the middle
Outside the column
Distractors match common diagram-reading mistakes. The correct answer links position in the column to lower boiling point and smaller molecules.
Fractions and molecule size
Short-chain hydrocarbons: low boiling points, runny, burn easily
Long-chain hydrocarbons: high boiling points, thicker, less flammable
Properties change in a pattern as molecules get larger
Avoid overloading with every property. The key pattern is that molecule size links to boiling point and use.
Why cracking is needed
There is often more demand for small hydrocarbons such as petrol than for large hydrocarbons.
Large hydrocarbons are less useful as fuels for cars.
Cracking changes long-chain hydrocarbons into smaller, more useful molecules.
This is the economic reason for cracking. Keep the focus on supply and demand rather than cost details.
What cracking does
Cracking is the thermal decomposition of long-chain hydrocarbons.
This means large hydrocarbon molecules are broken into smaller molecules by heating.
The products include alkanes and alkenes.
Students may confuse cracking with burning. Emphasise that molecules are broken apart, not reacted with oxygen.
How hydrocarbons are cracked
Long-chain hydrocarbons are vaporised
The vapour is heated to a very high temperature
The vapour passes over a hot catalyst
The molecules split into smaller hydrocarbons
At Foundation level, keep conditions simple: high temperature and a catalyst such as silica or alumina.
Products of cracking
Cracking can produce:
short-chain alkanes, which are useful fuels
alkenes, which are used to make polymers
This makes cracking important for both fuels and the chemical industry.
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Why is cracking needed?
To cool the vapour
To sort fractions
To make smaller molecules
To remove carbon
Tests the purpose of cracking from slides 8 to 11. Students may confuse cracking with distillation, so one distractor targets that misconception.
Complete the cracking summary
Cracking is the thermal decomposition of long-chain hydrocarbons. The hydrocarbons are vaporised and heated to a high temperature. The vapour passes over a hot catalyst and forms smaller molecules, including alkanes and alkenes.
Reinforces the definition, conditions, and products of cracking. Keep the contrast clear: cracking changes molecules; distillation only separates them.
How is cracking different from distillation?
Expected answer elements: fractional distillation is a physical separation of a mixture using different boiling points; cracking is a chemical change that breaks long-chain hydrocarbons into smaller molecules. Encourage students to mention both process and outcome.
From crude oil to useful products
Crude oil → fractional distillation → fractions such as petrol
Long-chain hydrocarbons → cracking → short-chain alkanes + alkenes
Separation sorts the mixture. Cracking changes the molecules.
This summary contrast is the key landing point: distillation is a physical separation process, while cracking is a chemical change.
What is one thing you learned today?
Use as an exit ticket reflection. Look for one clear takeaway, such as petrol is collected near the top, crude oil is a mixture, or cracking makes smaller molecules.
What helped you understand this best?
Encourage reflection on the learning process, for example diagrams, the summary slide, the column idea, or comparing distillation with cracking.
Which part still feels hardest?
Useful for planning reteaching. Likely answers: boiling point, where petrol is collected, or the difference between cracking and distillation.
Glossary
crude oil - a mixture of many different hydrocarbons
hydrocarbon - a compound containing hydrogen and carbon only
fractional distillation - separating a mixture using different boiling points
fraction - a group of hydrocarbons with similar boiling points
boiling point - the temperature at which a liquid changes to a gas
petrol - a fuel fraction made from relatively small hydrocarbons
cracking - breaking long-chain hydrocarbons into smaller molecules
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Glossary
thermal decomposition - breaking down a substance by heating
alkane - a saturated hydrocarbon
alkene - an unsaturated hydrocarbon with a carbon-carbon double bond
catalyst - a substance that speeds up a reaction without being used up
vapour - a substance in the gas state
long-chain hydrocarbon - a hydrocarbon with a large molecule
short-chain hydrocarbon - a hydrocarbon with a smaller molecule
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