Key Stage 4 science · Chemistry – Chemical and allied industries

GCSE chemistry: crude oil, cracking and metal extraction

This GCSE chemistry topic looks at two raw materials that modern life depends on: crude oil and metal ores. Students learn how crude oil is separated into useful fractions, how cracking turns long molecules into more valuable short ones, and why oil is both a fuel and the starting material for plastics. They also learn how a metal’s place in the reactivity series decides how it is extracted from its ore.

Crude oil, cracking and metal extraction, explained

Crude oil is a mixture of many hydrocarbons, compounds made only of hydrogen and carbon. Most are alkanes, which have only single bonds between carbon atoms and the general formula CₙH₂ₙ₊₂, such as methane CH₄ and octane C₈H₁₈. Crude oil formed over millions of years from the remains of ancient plankton buried in mud, and it is finite. Hydrocarbons are used as fuels, which are burned for energy, and as feedstock, the starting materials for making plastics, solvents and many other chemicals. These two uses compete for a limited resource, which is why students discuss whether burning so much of it is wise.

Fractional distillation separates crude oil into fractions, each a mixture of hydrocarbons with similar boiling points. The oil is heated until it vaporises and enters a tall column that is hot at the bottom and cooler towards the top. Each vapour rises until it reaches a level cool enough for it to condense, and is drawn off there. Small molecules, such as those in refinery gases and petrol, have low boiling points and leave near the top. Long molecules, such as those in bitumen, have high boiling points and leave at the bottom. As chain length increases, the fraction is also more viscous and harder to ignite. This is a physical separation. No bonds are broken.

Demand for the short fractions is far greater than the supply, while long fractions are less useful. Cracking is a chemical reaction that breaks long hydrocarbons into shorter, more useful ones. In catalytic cracking the vapour passes over a hot catalyst, and in steam cracking it is mixed with steam at high temperature. Cracking an alkane gives a shorter alkane and an alkene, a hydrocarbon with a carbon-carbon double bond. For example, C₁₀H₂₂ → C₈H₁₈ + C₂H₄. Alkenes are more reactive than alkanes and are used to make polymers. They also turn orange bromine water colourless, which is the standard test.

Metals are found in the ground as ores, often as the metal oxide. How the metal is extracted depends on its position in the reactivity series. A metal less reactive than carbon, such as iron, can be extracted by heating its oxide with carbon, which takes away the oxygen. A metal more reactive than carbon, such as aluminium, holds on to its oxygen too firmly, so a molten compound of aluminium is electrolysed, which uses a lot of electrical energy. The least reactive metals, such as gold, are found uncombined. Extracted metals are sometimes impure, and copper, for example, is purified by electrolysis.

Words your child will hear

Hydrocarbon: A compound made of hydrogen and carbon atoms only.
Alkane: A hydrocarbon with only single bonds between carbon atoms, such as methane or octane.
Alkene: A hydrocarbon with a carbon-carbon double bond, such as ethene, C₂H₄.
Fraction: A group of hydrocarbons with similar boiling points, collected together from crude oil.
Feedstock: A raw material that is used to make other chemicals, such as plastics.
Cracking: Breaking long hydrocarbon molecules into shorter, more useful ones.
Ore: Rock that contains enough of a metal compound to make extracting the metal worthwhile.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about carbon compounds, both as fuels and feedstock, and the competing demands for limited resources
Students should be taught about fractional distillation of crude oil and cracking to make more useful materials
Students should be taught about extraction and purification of metals related to the position of carbon in a reactivity series

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Use the reactivity series (potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold) to decide how iron, aluminium and gold would be obtained, and say what is oxidised and reduced in the case of iron.

Find carbon in the series. It sits between aluminium and zinc. Metals above carbon are more reactive than it, and metals below it are less reactive.
Iron is below carbon, so carbon can take the oxygen away from iron oxide. Heating the ore with carbon is cheap and effective.

Write the simplified equation and check that it balances.

2Fe₂O₃ + 3C → 4Fe + 3CO₂
Fe: 4 = 4  O: 6 = 6  C: 3 = 3

Iron oxide loses oxygen, so it is reduced. Carbon gains oxygen, so it is oxidised.
Aluminium is above carbon, so carbon cannot remove its oxygen. Aluminium oxide must be electrolysed instead. It is dissolved in molten cryolite to lower the temperature needed, but the process still uses a lot of electricity.
Gold is very unreactive, so it is found as the metal itself and needs no chemical extraction, only separating from rock.

Iron is extracted by heating its ore with carbon, with iron oxide reduced and carbon oxidised. Aluminium is extracted by electrolysis of the molten compound. Gold occurs uncombined.

Common mistakes and how to help

Confusing fractional distillation with cracking

Both happen at oil refineries and both involve heating, so they blur together.

What helps: Distillation separates molecules that are already there by boiling point, so no new substances form. Cracking breaks bonds and makes new molecules. Ask “are the molecules the same size afterwards?”

Thinking long-chain hydrocarbons are runny and easy to ignite

Students link big with more, so assume a longer molecule burns more readily.

What helps: As molecules get longer, they have stronger forces between them, so the fraction becomes thicker, has a higher boiling point and is harder to ignite. Bitumen is a thick tar, while the gas in a camping stove is a very small molecule.

Believing carbon can extract any metal

Carbon is cheap and the method is the first one students meet.

What helps: Carbon can only remove oxygen from metals below it in the reactivity series. For the metals above, the molten compound must be electrolysed. That is why extracting aluminium uses so much electricity and is costly.

Mixing up oxidation and reduction in extraction

Reduction sounds like making something smaller, and students expect the metal to be “used up”.

What helps: Reduction is loss of oxygen. The ore loses oxygen and its mass drops, while the metal forms. Ask “what gained the oxygen?” to find the oxidised substance.

How to help at home

Hunt for crude oil products. Walk round your home and list everything made from oil: plastic containers, polyester clothing, car tyres, bin liners, candle wax, the cases on electronics. It shows how much oil is used as a feedstock and not just as fuel.
Model cracking with spaghetti. Snap a long strand of dried spaghetti into shorter pieces and talk about how a long molecule becomes shorter ones. Real cracking also forms a double bond, so it is a simplified model, but it makes the idea of breaking a chain visible.
Compare how liquids flow. Tilt a clean baking tray and compare how fast a spoonful of water, cooking oil and runny honey travel down it. Thicker liquids have stronger attractions between molecules, which is a helpful picture for why long-chain fractions are viscous. Wipe up carefully, as it will be slippery.
Look at metals around you. Find aluminium foil, a copper pipe, an iron nail and a steel pan. Ask your child to say where each sits in the reactivity series and how it was probably extracted. Linking the order of the series to how hard a metal is to obtain makes the series feel useful.

Builds on

Key Stage 3: Chemical reactions and equations
Key Stage 3: Acids, alkalis and neutralisation