Corrosion & Rusting T
Sections 7–11 (materials, the Haber process and fertilisers) are Chemistry only — not on the Combined Science papers.
Corrosion is the destruction of a metal by chemical reactions with substances in its environment. Rusting is the corrosion of iron specifically — and it needs both air (oxygen) and water.
Only the nail with both air and water rusts. Boiling the water and sealing it with oil removes the air; calcium chloride removes the water — and neither nail rusts.
Watch it rust over a week
Slide the day forward (or press play). Only the nail with both air and water rusts — the controls stay shiny, proving both are needed.
Day 0 — every nail is shiny steel. Move the days forward and watch.
Build your own tube
- Barrier methods — keep out air and water with a coating: painting, oiling, greasing, coating in plastic, or electroplating. (If the barrier is scratched, the iron underneath rusts again.)
- Aluminium protects itself — it forms a tough oxide layer that stops further corrosion.
- Galvanising — coating iron with zinc. The zinc is a barrier and gives sacrificial protection.
- Sacrificial protection — attach a more reactive metal (zinc or magnesium — check the C4 reactivity series). Being more reactive, it loses electrons and corrodes in preference to the iron, so the iron is protected even if the surface is scratched. (Magnesium blocks are bolted to ships’ hulls.)
- Corrosion is the general term; rusting is corrosion of iron specifically. Don’t use them interchangeably.
- Sacrificial metal must be more reactive than iron (zinc, magnesium) — it corrodes instead of the iron.
- Barrier vs sacrificial. A scratched barrier stops working; sacrificial protection keeps working even when scratched, because the reactive metal still corrodes first.
🧪 Exam-style questions
Which two substances are needed for iron to rust? Tick (✓) two boxes, then press Check.
In the rusting experiment, one tube has boiled water with a layer of oil on top. Why does the nail in this tube not rust? Tick (✓) one box.
Blocks of zinc are attached to an iron ship’s hull. Explain how this protects the iron. Tick (✓) one box.
Which of these is a barrier method of rust prevention? Tick (✓) one box.
Alloys T
Most metals in everyday use are alloys — a metal mixed with one or more other elements. Alloys are usually harder and stronger than the pure metal, and the reason is all about how the atoms are arranged.
Two metal structures side by side — a pure metal and an alloy. Press the button to apply a force and watch whether the layers can slide over each other.
Why alloys are harder. Pure metal (left): identical ions in regular layers slide over each other easily. Alloy (right): atoms of a different size distort the layers, so they bump into each other and cannot slide — making the alloy harder and stronger.
- Bronze = copper + tin — harder than copper; used for statues, ornaments and medals.
- Brass = copper + zinc — corrosion-resistant; used for fittings and musical instruments.
- Gold alloys (with silver, copper, zinc) — for jewellery; purity in carats (24 carat = 100% gold, 18 carat = 75%).
- Steels = iron + carbon (+ other metals): high-carbon steel is strong but brittle; low-carbon steel is soft and easily shaped; stainless steel (with chromium and nickel) is hard and corrosion-resistant.
- Aluminium alloys — low density, used for aircraft.
- An alloy is a mixture, not a compound — the elements are not chemically combined.
- Carats are a fraction of 24. 24 carat = pure (100%); 18 carat = 18/24 = 75%; 12 carat = 50%.
- Don’t confuse an alloy with a composite. An alloy is a uniform mix of metals; a composite has two distinguishable materials (section 9).
🧪 Exam-style questions
Why is an alloy usually harder than the pure metal? Tick (✓) one box.
Bronze is an alloy of copper and one other metal. Name the other metal.
Show answer
Tin. 1 mark (Bronze = copper + tin. Don’t confuse it with brass = copper + zinc.)
An 18-carat gold ring has a mass of 8.0 g. Pure gold is 24 carat. Calculate the mass of gold in the ring.
Show answer
- Fraction of gold = 18 ÷ 24 = 0.75 (75%). 1 mark
- Mass of gold = 0.75 × 8.0 = 6.0 g. 1 mark
Stainless steel resists corrosion. Which elements are added to the steel to make it stainless? Tick (✓) one box.
Ceramics, Polymers & Composites T
The materials we build with fall into a few families — ceramics, polymers and composites — and their properties come from their structure. Match the material to the job.
Ceramics
- Soda-lime glass — the common glass, made by heating sand, sodium carbonate and limestone until it melts.
- Borosilicate glass — made from sand and boron trioxide; it melts at a higher temperature than soda-lime glass (so it tolerates heat better).
- Clay ceramics (pottery, bricks) — made by shaping wet clay then firing it (heating in a furnace) to make it hard.
Polymers
A polymer’s properties depend on the monomer it’s made from and the conditions of making it. The same monomer can give different materials: low density (LD) and high density (HD) poly(ethene) are both made from ethene, but under different conditions — LD poly(ethene) (made at high pressure) is flexible for bags; HD poly(ethene) (made with a catalyst at lower temperature/pressure) is more rigid for pipes and tanks. (Ethene is an alkene, and the small monomers join by addition polymerisation, from C7.)
Think loose spaghetti versus a knotted net — and it’s why thermosoftening polymers can be recycled by melting and remoulding, but thermosetting polymers can’t.
Composites
A composite is made of two materials: a matrix (binder) surrounding reinforcement (fibres or fragments). The combination gives properties neither has alone. Examples: fibreglass, carbon fibre, steel-reinforced concrete, and wood (a natural composite of fibres in a polymer matrix).
- Thermosetting does NOT melt — its strong cross-links hold the structure together. Thermosoftening does melt because only weak intermolecular forces hold its separate chains together.
- LD and HD poly(ethene) are both made from ethene — the difference is the conditions of manufacture, not the monomer.
- Composite vs alloy. A composite has two distinguishable materials (matrix + reinforcement); an alloy is a uniform mixture of metals.
🧪 Exam-style questions
Why does a thermosetting polymer not melt when heated? Tick (✓) one box.
Name the three substances that are heated together to make soda-lime glass.
Show answer
Sand, sodium carbonate and limestone, heated until they melt. 1 mark (All three needed. Sand + boron trioxide makes borosilicate glass instead.)
Which of these is a composite material? Tick (✓) one box.
A thermosoftening polymer can be melted and remoulded. Explain why, in terms of its structure. Tick (✓) one box.
A bridge is to be built from concrete. An engineer has the data in the table below.
| Material | Compressive strength (MPa) | Tensile strength (MPa) |
|---|---|---|
| Concrete | 40 | 3 |
| Steel-reinforced concrete (a composite) | 40 | 40 |
Compare the two materials using the data, and explain why the engineer chooses steel-reinforced concrete for the bridge.
Show answer
- Both materials have the same compressive strength (40 MPa), so they resist being squashed equally well. 1 mark
- But the composite has a much higher tensile strength (40 MPa vs 3 MPa) — about 13 times higher — so it is far better at resisting being stretched/pulled apart. 1 mark
- A bridge is loaded so that parts are stretched, and plain concrete would crack at low tension; the steel reinforcement gives the tensile strength the concrete lacks, so the composite combines the best of both. 1 mark
Allow: any correct use of the figures (e.g. quoting both tensile values, or the difference 40 − 3 = 37 MPa, or that the composite is ~13× stronger in tension). Do not accept: “the composite is stronger” with no reference to the data.