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Why iron rusts and how rust is prevented, what makes metals useful, why alloys are harder than pure metals, and the properties of ceramics, polymers and composites.
Specification references this topic covers: AQA 5.2.2; Edexcel C1.42, C4.9.
Metals are strong and useful, but many of them react with the air and water around them. Iron turns to rust. The damage is called corrosion, and it costs a great deal of money every year in bridges, cars, ships and pipes.
This matters because choosing the right metal, or the right way to protect it, makes things last longer. Mixing metals to make alloys gives materials with better properties than the pure metals.
By the end you will be able to explain rusting and how to stop it, say why alloys are harder, and compare some common materials.
Corrosion is the destruction of a metal by reaction with substances in the environment. The corrosion of iron and steel is called rusting. Rust is hydrated iron(III) oxide, which is a flaky orange-brown solid.
Iron only rusts when it is in contact with both oxygen and water. The word equation is: iron + oxygen + water → hydrated iron(III) oxide. If either oxygen or water is missing, iron does not rust. Salt speeds rusting up, which is why cars rust faster near the sea or on salted roads.
Rusting is an oxidation reaction because the iron gains oxygen. Rust is weak and flakes off, so it exposes new iron underneath and the metal is slowly destroyed.
Barrier methods keep both oxygen and water away from the metal. Paint, oil or grease, and plastic coatings cover the surface. Electroplating covers it with a thin layer of another metal, such as chromium or tin. Each of these works only while the barrier stays unbroken.
The choice depends on the job. Paint is cheap for large objects such as bridges. Oil and grease suit moving parts such as bicycle chains, because paint would rub off. Plastic coats are used on things such as dish racks and garden furniture.
If a barrier is scratched, oxygen and water reach the iron and it rusts, unless a second type of protection is also used.
In sacrificial protection a more reactive metal is attached to the iron. The more reactive metal loses electrons and corrodes instead of the iron, so the iron is protected even if the surface is damaged. Blocks of zinc or magnesium are bolted to ships' hulls and to underground pipes.
Galvanising means coating iron or steel with zinc. The zinc is a barrier. If the coating is scratched, the zinc still protects the iron because it is more reactive and corrodes first. The zinc protects the iron by sacrificial protection as well.
This links to the reactivity series. A metal that is higher in the series reacts more readily with oxygen. A metal that is much lower, such as gold, is very resistant to oxidation.
Most metals are shiny solids with high melting points and high densities. They are good conductors of heat and electricity and are malleable, which means they can be hammered into shape. Most non-metals have low boiling points and are poor conductors of electricity.
Metals conduct because they have a giant structure of atoms held by metallic bonding, with delocalised electrons that are free to move and carry charge and energy. The layers of atoms can slide over one another, so pure metals can be bent and shaped.
Some metals resist corrosion for different reasons. Aluminium is quite reactive, but a thin, tough layer of aluminium oxide forms on its surface and stops further reaction. Gold and platinum are so unreactive that they hardly react with oxygen at all.
An alloy is a mixture of a metal with at least one other element. Pure metals are often too soft for many jobs, so they are mixed to make alloys that are harder. Steel is iron mixed with a small amount of carbon. Brass is copper and zinc. Bronze is copper and tin. Stainless steel contains chromium and nickel and does not rust.
An alloy is harder because the atoms of different elements are different sizes. The different-sized atoms disturb the regular layers, so the layers cannot slide over each other easily. This makes the alloy harder than the pure metal.
Alloys can be chosen for the job. Low-carbon steel is soft and easily shaped, high-carbon steel is hard but brittle, and aluminium alloys are strong but have a low density, which is why they are used in aircraft. The purity of gold is given in carats: 24 carat is pure gold, so 18 carat gold is 18 ÷ 24 × 100 = 75% gold.
Ceramics such as glass and pottery are hard and brittle, and they do not conduct electricity. Clay ceramics are made by shaping wet clay and heating it in a kiln. Glass is made by heating sand with other substances such as sodium carbonate and limestone.
Polymers are made of very long molecules. Their properties depend on the monomers and the conditions they are made in. Low-density poly(ethene) is soft and flexible. High-density poly(ethene) is stronger and has a higher melting point. Thermosoftening polymers soften when heated. Thermosetting polymers do not.
A composite has two parts: a reinforcement, such as fibres or fragments, embedded in a matrix or binder. Fibreglass has glass fibres in plastic. Carbon fibre has carbon fibres in plastic. Concrete has gravel in cement. The composite has properties that neither part has alone.
Iron rusts when it touches oxygen and water. Rust is hydrated iron(III) oxide.
Barriers such as paint, oil and plating keep oxygen and water out. Sacrificial protection uses a more reactive metal such as zinc. Galvanising coats steel with zinc.
Alloys are harder than pure metals because the different-sized atoms stop the layers sliding. Ceramics, polymers and composites have different properties suited to different jobs.
Name the two substances that must be present for iron to rust.
Answer: Oxygen and water
A galvanised steel bucket has a scratch through the zinc. Explain why the steel under the scratch does not rust quickly.
Answer: Zinc is more reactive than iron so it corrodes first and protects the steel
Explain why steel is harder than pure iron. A ring is marked 18 carat gold. Calculate the mass of gold in a ring of mass 20 g.
Answer: The different-sized carbon atoms distort the layers so they cannot slide. The ring has 15 g of gold
Corrosion, alloys and materials practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Iron rusts when it touches water and which gas from the air?
Answer: oxygen
What is the name for the corrosion of iron?
Answer: rusting
Is rusting an oxidation or a reduction reaction?
Answer: oxidation
Name one barrier method of preventing rust.
Answer: painting
What is an alloy?
Answer: a mixture of a metal with other elements
Name a metal used for sacrificial protection of iron.
Answer: zinc
What is the name for coating steel with zinc?
Answer: galvanising
Which two elements are in brass?
Answer: copper and zinc
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) State the two substances needed for iron to rust. [2 marks] (b) Describe two ways of preventing iron from rusting. [2 marks]
Answer: (a) oxygen and water (b) painting it, and coating it with zinc (galvanising)
(a) Explain why an alloy is harder than a pure metal. [2 marks] (b) 18 carat gold is an alloy. Calculate the percentage of gold in it. [2 marks]
Answer: (a) atoms of other elements are a different size so they distort the layers and the layers cannot slide easily (b) 75%
Saying iron rusts in water only
Iron needs both oxygen and water to rust.
Saying sacrificial protection uses a less reactive metal
The sacrificial metal is more reactive than iron, so it corrodes first.
Saying paint protects iron even when scratched
Paint is only a barrier. When it is scratched, the iron can rust.
Saying an alloy is a compound
An alloy is a mixture of a metal with other elements.
Saying atoms in alloys are all the same size
The different sizes of atoms stop the layers sliding, which makes the alloy harder.
Corrosion, alloys and materials is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 5.
Saying iron rusts in water only. Iron needs both oxygen and water to rust.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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