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Fractional distillation separates crude oil because different hydrocarbons have different boiling points. Free notes with key terms and 9 questions answered.
Separating Crude Oil by Fractional Distillation is part of Carbon compounds as fuels and feedstock in GCSE AQA Higher Chemistry.
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Fractional distillation separates crude oil because different hydrocarbons have different boiling points. Free notes with key terms and 9 questions answered.
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Use this lesson to stay inside Carbon compounds as fuels and feedstock while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Separating Crude Oil by Fractional Distillation before testing yourself from memory.
Crude oil straight from the ground is useless. It's a thick, black mixture of hundreds of different hydrocarbons all jumbled together. You can't put it in your car or use it to make plastics. The key scientific idea happens when we separate this mixture into useful fractions.
Think of crude oil like a fruit salad. The individual fruits (hydrocarbons) are valuable, but you need to separate them first. Fractional distillation is our method for sorting this molecular fruit salad.
Here's the crucial pattern: small hydrocarbon molecules have low boiling points, whilst large molecules have high boiling points. A molecule with 4 carbon atoms boils at around 0°C, but one with 20 carbon atoms needs over 300°C.
This difference in boiling points is what makes separation possible. When we heat crude oil, the small molecules evaporate first, leaving the large ones behind.
Picture a tall tower called a fractionating column. It's hottest at the bottom (around 400°C) and coolest at the top (around 40°C). This temperature gradient is the secret to separation.
Worked example: The Fractional Distillation Process
How does crude oil get separated into different fractions?
Answer: Crude oil is separated by heating it and allowing vapours to condense at different temperatures in a tall column
The brilliant part is that each fraction condenses at a different level. Small molecules like those in petrol reach the top before condensing. Medium molecules condense halfway up. Large molecules never make it far and condense near the bottom.
Fractional distillation is really just evaporation and condensation happening in a controlled way. When we heat crude oil, we're giving molecules energy to break free from the liquid and become gas - that's evaporation.
As these gas molecules rise up the column, they lose energy and cool down. When they cool enough, they turn back into liquid - that's condensation. The temperature where this happens is the boiling point.
The fractions collected at different levels have very different properties. This isn't random - it's all about molecular size. Small molecules make runny liquids that catch fire easily. Large molecules make thick, viscous liquids that are hard to ignite.
At the top of the column, you collect gases and light liquids like petrol. These have small molecules (4-12 carbon atoms). At the bottom, you get thick liquids and solids like bitumen, with huge molecules (over 50 carbon atoms).
Exam questions on fractional distillation often ask you to explain the process. The key is to mention both evaporation AND condensation, and link them to temperature differences.
The biggest mistake students make is forgetting to explain WHY the separation works. Don't just describe what happens - explain that it works because different molecules have different boiling points.
Exam precision note: In Separating Crude Oil by Fractional Distillation - Fractional distillation and petrochemicals - Carbon compounds as fuels and feedstock, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Fractional distillation separates crude oil because different hydrocarbons have different boiling points. The fractionating column is hottest at the bottom and cooler at the top, so longer-chain fractions condense lower down while shorter-chain fractions rise further before condensing.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Bitumen is the fraction of crude oil collected at the bottom of the fractionating column. It has huge molecules with over 50 carbon atoms, so it is a thick liquid or solid with a very high boiling point that is hard to ignite.
In fractional distillation, crude oil is heated to about 400°C so most of it evaporates, and the vapours rise up a fractionating column that is hot at the bottom and cool at the top. Each fraction condenses when it cools to its boiling point and is collected at a different height.
Fractions with small molecules, collected at the top of the column, are runny liquids or gases that catch fire easily, such as petrol with 4-12 carbon atoms. Fractions with large molecules, collected at the bottom, are thick, viscous and hard to ignite, such as bitumen with over 50 carbon atoms.
Fractional distillation separates crude oil because different hydrocarbons have different boiling points. Small molecules have low boiling points and rise to the top before condensing, while large molecules have high boiling points and condense near the bottom.
A fractionating column is hottest at the bottom, around 400°C, and coolest at the top, around 40°C. This temperature gradient lets each fraction condense at a different level.
Evaporation is the change from liquid to gas when molecules gain enough energy to escape the liquid, and condensation is the change from gas to liquid when molecules lose energy. In fractional distillation, crude oil evaporates when heated and each fraction condenses as it rises and cools.
Gases and light liquids such as petrol are collected at the top of the fractionating column, where it is coolest. They have small molecules, with 4-12 carbon atoms, so they have low boiling points and rise all the way up before condensing.
Crude oil is separated because, straight from the ground, it is a thick, black mixture of hundreds of different hydrocarbons that can't be used in a car or to make plastics. Separating it gives useful fractions, like sorting a fruit salad into the individual fruits.
A fraction is a part of crude oil containing hydrocarbons with similar numbers of carbon atoms, and so similar boiling points. Each fraction condenses at a different level of the fractionating column, such as petrol near the top and bitumen at the bottom.