Fuels and Crude Oil | O-Level Chemistry

Last updated: 9 October 2026

Diagnose → Learn → Master: Every study-note section includes 15 free MCQs — start with 5 Diagnostic MCQs to identify gaps in your understanding, then complete 10 Mastery MCQs near the end to check what you have learned.

Petrol for cars, fuels used for heating and many raw materials used by the chemical industry can all be traced back to resources found beneath the ground. Two especially important examples are natural gas and crude oil.

The Chemistry becomes more interesting when we realise that crude oil is not simply something that we burn. It is a mixture containing many useful molecules. By separating those molecules into groups, we can choose whether to use them for energy or as starting materials for making other chemicals.

At the same time, fossil fuels are finite and their use releases carbon dioxide. This is why biofuels such as bioethanol are also important: they help us think about how energy can be supplied more sustainably.

Diagnostic Quiz

How Much Do You Already Know?

Try these five questions before reading the chapter. They test the ideas you will need when learning about fuels, crude oil and fractional distillation.

5 quick questions
Instant feedback
Explanations for every option

Question 1 of 5

Score: 0

Which statement best describes a non-renewable energy resource?

It can never be found naturally.

Non-renewable resources occur naturally. The problem is that they are used much faster than they can be replaced.

It cannot be replaced as quickly as it is being used.

Correct. Fossil fuels take extremely long periods to form, so their reserves are finite on a human timescale.

It cannot release energy when burned.

Fossil fuels are useful precisely because combustion releases energy.

It must be a solid at room temperature.

Physical state does not determine whether a resource is renewable. Natural gas is non-renewable even though it is gaseous.

A resource is non-renewable when it is consumed much faster than natural processes can replace it. Crude oil and natural gas are examples.

Natural gas consists mainly of which substance?

Methane, CH₄

Correct. Methane is the main component of natural gas.

Carbon dioxide, CO₂

Carbon dioxide may be produced when methane burns, but it is not the main component of natural gas.

Oxygen, O₂

Oxygen is required for combustion but is not the main component of natural gas.

Hydrogen, H₂

Hydrogen is a fuel, but natural gas consists mainly of methane rather than hydrogen.

Natural gas is a mixture, but its main chemical component is methane, CH₄.

Which substance is a hydrocarbon?

C₂H₅OH

Ethanol contains oxygen as well as carbon and hydrogen, so it is not a hydrocarbon.

CO₂

Carbon dioxide contains carbon and oxygen but no hydrogen.

C₄H₁₀

Correct. C₄H₁₀ contains carbon and hydrogen only.

CH₃COOH

Ethanoic acid contains oxygen as well as carbon and hydrogen.

A hydrocarbon is a compound containing only carbon and hydrogen.

Which physical property is mainly used to separate the hydrocarbons in crude oil?

Colour

The separation does not depend on colour.

Solubility in water

Fractional distillation does not separate the hydrocarbons according to water solubility.

Density

Differences in density are not the basis of fractional distillation.

Boiling point

Correct. Different hydrocarbons vaporise and condense at different temperatures because they have different boiling points.

Fractional distillation works because the components of crude oil have different boiling points.

Which temperature pattern is found in a crude-oil fractionating column?

Cool at the bottom and hot at the top

This is the opposite of the required temperature gradient.

Hot at the bottom and cooler towards the top

Correct. Hydrocarbon vapours cool as they rise through the column.

The same temperature throughout

A temperature gradient is needed so that different hydrocarbons condense at different heights.

Cold throughout the entire column

Crude oil must first be strongly heated so that its components can vaporise.

The fractionating column is hottest at the bottom and becomes cooler towards the top.

Natural Gas and Crude Oil Are Non-Renewable Energy Resources

Natural gas and crude oil are fossil fuels. They were formed from biological material over extremely long periods of time. Humans are extracting and using them much faster than natural processes can form new supplies.

For this reason, they are described as non-renewable.

Core Principle

Non-renewable does not mean “already used up”

A non-renewable resource may still have large reserves remaining. The important point is that once we use those reserves, they cannot be replaced within a useful human timescale.

Natural gas consists mainly of methane

Natural gas is a mixture of gases whose main component is methane, CH₄. Methane is useful as a fuel because combustion releases energy.

Complete combustion can be represented by:

CH₄ + 2O₂ → CO₂ + 2H₂O

Notice that natural gas should be described as mainly methane, rather than as pure methane. That distinction matters because natural gas is a mixture.

Crude Oil Is a Mixture of Hydrocarbons

Crude oil is not one compound. It contains many different substances mixed together, and most of these substances are hydrocarbons.

Definition

A hydrocarbon contains carbon and hydrogen only

The word tells us exactly what is present: hydro- refers to hydrogen and carbon refers to carbon.

CH₄, C₄H₁₀ and C₈H₁₈ are hydrocarbons because they contain only carbon and hydrogen. C₂H₅OH is not a hydrocarbon because it also contains oxygen.

Different hydrocarbons contain molecules of different sizes and therefore have different physical properties, including different boiling points. This difference allows crude oil to be separated.

Important Distinction

Crude oil and a crude-oil fraction are both mixtures

Crude oil contains a very large number of hydrocarbons. After fractional distillation, each fraction still contains several hydrocarbons, but the hydrocarbons within a fraction have similar boiling points.

Fractional Distillation Separates Crude Oil

Crude oil would be far less useful if all its hydrocarbons remained mixed together. Different groups of hydrocarbons are suited to different purposes, so the mixture is separated into fractions.

The process used is fractional distillation.

$Fractional distillation of crude oil showing eight fractions from liquid petroleum gas at the top to bitumen at the bottom, with lower-boiling hydrocarbons condensing higher in the column and higher-boiling hydrocarbons condensing lower down.$

Step 1: Crude oil is strongly heated

The crude oil is heated so that most of its hydrocarbons vaporise. The resulting mixture of hot vapours enters the fractionating column near the bottom.

Step 2: The vapours rise through a temperature gradient

The fractionating column is hot at the bottom and becomes progressively cooler towards the top.

As the vapours rise, they therefore encounter lower and lower temperatures.

Step 3: Different hydrocarbons condense at different heights

Each hydrocarbon has a particular boiling point. A vapour remains gaseous while the surrounding temperature is sufficiently high. When it reaches a region that is cool enough, it condenses to form a liquid.

Because the hydrocarbons have different boiling points, they condense at different levels in the column and can be collected separately as fractions.

Follow the Logic

Boiling point determines where a hydrocarbon is collected

Hydrocarbons with high boiling points condense while they are still relatively low in the hot column.

Hydrocarbons with lower boiling points remain as vapours for longer, rise further and condense higher up where the column is cooler.

Why Smaller Hydrocarbons Rise Further Up the Column

In general, smaller hydrocarbon molecules have lower boiling points than larger hydrocarbon molecules.

This gives us a useful reasoning chain:

smaller molecules → lower boiling points → remain gaseous to lower temperatures → rise further up the column

Conversely:

larger molecules → higher boiling points → condense at higher temperatures → collected lower in the column

Worked Reasoning

Where would a low-boiling fraction be collected?

Suppose one fraction contains hydrocarbons with relatively low boiling points. These hydrocarbons remain gaseous even after the vapours have travelled into cooler regions of the fractionating column.

They therefore travel further upwards before condensing, so the fraction is collected towards the upper part of the column.

Fractional distillation is a physical separation. Vaporising and condensing a hydrocarbon does not turn it into a different hydrocarbon. The molecules present before the separation are still the same molecules afterwards.

Crude-Oil Fractions Have Competing Uses

Once crude oil has been separated, its fractions are valuable for two broad reasons. They can be used as fuels, or they can be used as chemical feedstock.

Use	What happens?	Purpose
Fuel	The hydrocarbons are burned.	Chemical energy is released for transport, heating or other energy needs.
Chemical feedstock	The hydrocarbons are used as starting materials.	They are converted into other useful chemicals and materials.

What does chemical feedstock mean?

A feedstock is simply a starting material for an industrial chemical process. Hydrocarbons from crude oil can be used to manufacture substances such as plastics, detergents and other useful organic chemicals.

Why are these described as competing uses?

Crude oil is finite. A hydrocarbon molecule that is burned as a fuel is used up in the combustion reaction. It can no longer be used as a starting material for manufacturing something else.

There is therefore competition between the immediate value of obtaining energy and the chemical value of keeping hydrocarbons as raw materials.

Bioethanol Is a Renewable Fuel

The finite supply of fossil fuels encourages us to consider alternative energy sources. One example is bioethanol, which is ethanol produced from plant material.

Sugarcane can be grown to provide sugars. These sugars can then undergo fermentation to produce ethanol.

The fermentation of glucose can be represented by:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Sugarcane can be planted and grown again after harvesting. Bioethanol is therefore described as a renewable fuel, unlike crude oil and natural gas.

Do Not Confuse These Ideas

Renewable does not mean that combustion produces no carbon dioxide

Ethanol contains carbon, so complete combustion still produces CO₂:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

The environmental advantage comes from considering where that carbon came from, not from claiming that no CO₂ is produced.

Biofuels and the Carbon Dioxide Balance

When sugarcane grows, it removes carbon dioxide from the atmosphere during photosynthesis. The carbon becomes incorporated into substances in the plant and can eventually become part of the ethanol molecule.

When the bioethanol is burned, carbon dioxide is released back into the atmosphere. Therefore, some of the CO₂ released during combustion has previously been removed from the atmosphere by the growing crop.

Fossil fuels are different. Their carbon has been stored underground for extremely long periods. When fossil fuels are burned, this stored carbon is transferred into the modern atmosphere as CO₂.

Comparison of fossil fuels and bioethanol from sugarcane showing that fossil fuels transfer long-term underground carbon into the atmosphere while sugarcane absorbs carbon dioxide during growth, reducing the net addition of carbon dioxide from bioethanol.

Fossil fuels	Bioethanol from sugarcane
Non-renewable	Renewable when the crop is replanted
Carbon comes from long-term underground stores	Carbon was recently taken from atmospheric CO₂ by the plant
Burning produces a net addition of fossil carbon to the atmosphere	Plant growth can offset some of the CO₂ released when the fuel is burned

Exam-Safe Explanation

Why can bioethanol be more environmentally sustainable?

Sugarcane absorbs CO₂ from the atmosphere while it grows. This offsets some of the CO₂ released when the bioethanol is burned. Burning fossil fuels, in contrast, releases carbon that was previously stored underground, producing a greater net addition of CO₂ to the atmosphere.

Why we should be careful with the phrase “carbon neutral”

Growing, harvesting, transporting and processing sugarcane all require energy. Producing and purifying ethanol also requires energy. If fossil fuels are used during these stages, additional CO₂ is released.

It is therefore safer to explain that bioethanol can result in a smaller net addition of CO₂ rather than simply claiming that it produces no net carbon dioxide under all circumstances.

Renewable and sustainable are related, but they are not identical

Renewable describes whether a resource can be replenished. Sugarcane can be replanted, so the source of bioethanol is renewable.

Sustainability is a wider idea. Large-scale biofuel production may require land, water, fertilisers and energy. Land used to grow fuel crops may also compete with land used for food production.

Biofuels can therefore offer important advantages without being completely free of environmental costs.

Common Mistakes

Common mistake	Why it is wrong	Better Chemistry
“Crude oil is a hydrocarbon.”	Crude oil is not one compound.	Crude oil is a mixture of hydrocarbons.
“A crude-oil fraction is a pure substance.”	A fraction normally contains several hydrocarbons.	A fraction contains hydrocarbons with similar boiling points.
“Natural gas is methane.”	This suggests that natural gas is pure methane.	Natural gas consists mainly of methane.
“The fractionating column is hottest at the top.”	Vapours must cool as they rise.	The column is hot at the bottom and cooler at the top.
“Large hydrocarbons collect at the top.”	Larger molecules generally have higher boiling points.	They condense at higher temperatures and are therefore collected lower down.
“Fractional distillation creates new hydrocarbons.”	No chemical reaction is needed for the separation.	It is a physical separation based on boiling points.
“Bioethanol does not produce CO₂.”	Ethanol contains carbon and forms CO₂ during complete combustion.	Growing sugarcane can offset some of the CO₂ released.
“Renewable means environmentally harmless.”	Growing and processing crops can still have environmental effects.	Renewability and overall sustainability should be considered separately.

Fuels and Crude Oil Checklist

Energy Resources

I can explain why natural gas and crude oil are non-renewable and state that natural gas consists mainly of methane.

Hydrocarbons

I can define a hydrocarbon as a compound containing carbon and hydrogen only and explain why crude oil is a mixture.

Fractional Distillation

I can explain how heating, vaporisation, cooling and condensation separate crude oil into fractions.

Fractionating Column

I know that the column is hot at the bottom and cooler at the top, and I can predict where hydrocarbons with different boiling points are collected.

Competing Uses

I can distinguish between burning hydrocarbons as fuels and using them as chemical feedstock.

Biofuels

I can explain why bioethanol is renewable and why its use can result in a smaller net addition of CO₂ than fossil fuels.

Mastery Quiz

Fuels and Crude Oil Mastery

Now apply what you have learned. These questions focus on reasoning rather than simply recalling definitions.

10 exam-style questions
Application and misconception testing
Detailed feedback for every option

Question 1 of 10

Score: 0

Hydrocarbon P has a higher boiling point than hydrocarbon Q. Where would P most likely condense compared with Q in a fractionating column?

Lower in the column

Correct. P condenses at a higher temperature, so it condenses in a hotter region lower in the column.

Higher in the column

Higher parts are cooler and are associated with hydrocarbons having lower boiling points.

At exactly the same height

Their different boiling points mean that they generally condense at different temperatures and therefore different heights.

P cannot condense in the column

A high boiling point does not prevent condensation. It means condensation occurs at a relatively high temperature.

Higher boiling point → condensation at a higher temperature → lower, hotter part of the fractionating column.

Two fractions, X and Y, are collected from the same fractionating column. X is collected much higher than Y. Which conclusion is most reasonable?

X contains hydrocarbons with higher boiling points than Y.

Higher-boiling hydrocarbons condense lower down, not higher up.

X contains hydrocarbons with lower boiling points than Y.

Correct. X remains gaseous until it reaches a cooler region higher in the column.

X must be a pure hydrocarbon.

A fraction normally contains several hydrocarbons with similar boiling points.

Y must contain no hydrocarbons.

Crude-oil fractions consist mainly of hydrocarbons.

The higher a fraction is collected, the lower the boiling range of its hydrocarbons is expected to be.

A student says, “When crude oil is fractionally distilled, large hydrocarbon molecules are chemically changed into small molecules.” What is wrong with this statement?

Crude oil contains no large hydrocarbon molecules.

Crude oil contains hydrocarbons of many different molecular sizes.

Fractional distillation only works on pure substances.

Fractional distillation is specifically useful for separating mixtures of liquids with different boiling points.

Fractional distillation separates molecules without changing their chemical identity.

Correct. Vaporisation and condensation are physical changes.

All hydrocarbons have the same boiling point.

Fractional distillation works because different hydrocarbons have different boiling points.

Fractional distillation is a physical separation process. It does not convert one hydrocarbon into another.

Hydrocarbon P rises further up a fractionating column before it condenses than hydrocarbon Q. What can be concluded?

P has a lower boiling point than Q.

Correct. P must reach a cooler part of the column before it condenses, so its boiling point is lower than that of Q.

P has a higher boiling point than Q.

A higher-boiling hydrocarbon would condense sooner in the hotter, lower part of the column rather than rising further.

P and Q must have the same boiling point.

If they had the same boiling point, they would tend to condense at similar temperatures rather than at clearly different heights.

P cannot be a hydrocarbon.

Hydrocarbons can have different boiling points, so different hydrocarbons naturally condense at different heights in the column.

P rises further → reaches a cooler region → condenses at a lower temperature → P has a lower boiling point than Q.

Why is crude oil described as having competing uses?

Crude oil can only be used by one country at a time.

The competition refers to different uses of a finite chemical resource, not competition between countries.

A hydrocarbon cannot release energy when burned.

Hydrocarbon fuels are useful because their combustion releases energy.

Fractional distillation destroys most of the crude oil.

Fractional distillation physically separates the components; it does not destroy them.

Fractions may either be burned as fuels or retained as raw materials for making chemicals.

Correct. Using a finite hydrocarbon as a fuel means that molecule is no longer available as chemical feedstock.

Crude oil is valuable both as an energy source and as a source of starting materials for the chemical industry.

A company decides to use a crude-oil fraction as chemical feedstock instead of burning it. What does this mean?

The fraction will be thrown away without being used.

Feedstock is useful raw material rather than waste.

Its hydrocarbons will be used as starting materials for making other substances.

Correct. A chemical feedstock is a starting material for chemical manufacture.

Its hydrocarbons will be separated by filtration.

Feedstock describes how the material is used, not a separation technique.

The fraction must first be converted entirely into carbon dioxide.

Converting the fraction to CO₂ would destroy the hydrocarbon molecules instead of preserving them as raw materials.

Chemical feedstock means raw material used to manufacture other chemicals.

Sugarcane is harvested, processed and fermented to produce ethanol. Why can this ethanol be described as a renewable fuel?

Ethanol contains no carbon.

Ethanol, C₂H₅OH, contains carbon.

Ethanol produces no products when burned.

Complete combustion of ethanol produces CO₂ and H₂O.

More sugarcane can be grown to replace the crop that was harvested.

Correct. The biological source can be replenished on a relatively short timescale.

Ethanol is found naturally in unlimited underground reserves.

Bioethanol is produced from biological material rather than extracted from unlimited underground reserves.

Bioethanol is renewable because the plant material used to make it can be grown again.

A student argues, “Bioethanol cannot be better than petrol because both produce CO₂ when burned.” Which response gives the best Chemistry?

Bioethanol actually produces oxygen instead of carbon dioxide.

Complete combustion of ethanol produces CO₂ and H₂O, not oxygen.

Bioethanol does not contain carbon.

Its formula, C₂H₅OH, clearly contains carbon.

Sugarcane absorbs atmospheric CO₂ while growing, offsetting some of the CO₂ later released.

Correct. The comparison concerns the net addition of CO₂, not whether combustion produces CO₂ at all.

Fossil fuels do not contain carbon.

Fossil fuels contain carbon and their combustion is an important source of CO₂.

Both fuels release CO₂, but plants used for biofuels absorb CO₂ during growth. This can reduce the net increase in atmospheric CO₂.

Which statement gives the best reason why bioethanol should not automatically be described as completely carbon neutral?

Sugarcane cannot absorb carbon dioxide.

Plants do absorb CO₂ during photosynthesis.

Ethanol cannot be burned.

Ethanol is a combustible fuel.

Bioethanol is made from crude oil.

Bioethanol is produced from biological material such as sugarcane.

Farming, processing and transporting the fuel may themselves release CO₂.

Correct. The whole production process must be considered when assessing net carbon dioxide emissions.

Plant growth offsets some emissions, but additional energy used throughout production and transport may also generate CO₂.

Which statement correctly distinguishes renewable from sustainable?

Renewable and sustainable always mean exactly the same thing.

A renewable resource can still be produced in ways that create environmental or social problems.

Renewable concerns whether a resource can be replenished, while sustainability considers wider long-term impacts.

Correct. These ideas are related but not identical.

A sustainable fuel must produce no substances during combustion.

Sustainability does not require combustion to produce no products.

A renewable resource can never run short temporarily.

Renewable resources still need to be managed and replenished appropriately.

Renewable describes replacement of the resource. Sustainability considers whether its use can continue responsibly over the long term.

Frequently Asked Questions

Questions Students Often Ask About Fuels and Crude Oil

These questions connect the ideas in the chapter and clear up a few points that often cause confusion.

Why can crude oil not simply be used without separating it?

Crude oil contains many hydrocarbons with different boiling points and different useful properties. Separating it into fractions allows groups of hydrocarbons to be directed towards purposes for which they are better suited.

Is each fraction from crude oil one hydrocarbon?

No. A fraction is still a mixture. It contains several hydrocarbons whose boiling points are similar enough for them to be collected over the same region of the fractionating column.

Why do low-boiling hydrocarbons appear near the top of the column?

The upper part of the column is cooler. A low-boiling hydrocarbon can remain gaseous until it reaches these cooler regions, so it travels further up the column before condensing.

Does fractional distillation break covalent bonds inside hydrocarbon molecules?

No. Fractional distillation involves physical changes of state. Hydrocarbons vaporise and condense, but their molecules remain chemically unchanged.

Why is crude oil valuable even if we stop thinking of it as a fuel?

Crude oil provides carbon-containing molecules that can serve as starting materials for manufacturing many useful chemicals. Its value therefore extends beyond the energy released by combustion.

If bioethanol releases CO₂, why is it considered more sustainable?

The sugarcane used to make bioethanol absorbs CO₂ while growing. This can offset part of the CO₂ released when the fuel is burned. Fossil fuels instead introduce carbon from long-term underground stores into the atmosphere.

Is bioethanol completely carbon neutral?

Not necessarily. Farming, harvesting, processing, distillation and transport may require energy and cause additional CO₂ emissions. It is therefore more accurate to discuss a lower net CO₂ contribution rather than assume that the entire process has zero net emissions.

Fuels and crude oil connect several important Chemistry ideas. Crude oil is a finite mixture of hydrocarbons whose different boiling points allow it to be separated by fractional distillation. The resulting fractions are valuable both as fuels and as chemical feedstock, which is why their uses compete. Bioethanol then gives us a useful comparison: energy can also come from renewable biological material, although we must consider the entire carbon cycle rather than simply assuming that renewable means emission-free.