Alloys and Their Uses
Alloys for IGCSE Chemistry 0620: why different-sized atoms make alloys harder than pure metals, plus brass, stainless steel and mild steel uses that score.
The IGCSE Chemistry Specialist Team · founded by Rig
Written to the Cambridge IGCSE Chemistry (0620) syllabus and mark-scheme conventions. Last updated 2026-06-11.
The alloy question is one of the few places in 0620 where a diagram can earn the full explanation marks by itself: neat rows of identical atoms for the pure metal, disrupted rows with odd-sized atoms for the alloy. Students lose the marks by answering in everyday language (“mixing metals makes them stronger”) instead of particle language. Within the Metals topic, this subtopic is short, visual, and worth banking early.
What an alloy is
An alloy is a mixture of a metal with one or more other elements, usually other metals, but carbon counts too (steel is iron with a small amount of carbon). Because alloys are mixtures, not compounds, there is no fixed formula; the proportions are chosen to tune the properties. Brass can be roughly two-thirds copper, one-third zinc, adjusted for the job.
The exam definition mark wants “mixture”, “metal”, and “other element(s)”. “Two metals joined together” is too loose, and “a compound of metals” is wrong outright.
Why alloys are harder: the particle explanation
A pure metal is a giant lattice of identical atoms arranged in regular layers. Under force, the layers slide over each other, which is why pure metals are malleable and comparatively soft (the structural detail behind properties of metals).
In an alloy, atoms of a different element, and crucially a different size, sit inside the lattice. They disrupt the regular arrangement, so the layers can no longer slide over each other easily. The alloy is harder and stronger than the pure metal.
Three ideas, in order, earn the marks:
- The added atoms are a different size.
- They disrupt the regular layered arrangement.
- The layers can no longer slide over each other.
Draw it whenever the question allows: regular rows for the pure metal, rows broken by larger (or smaller) circles for the alloy. Label “layers slide” and “layers cannot slide” and the diagram does the talking.
The named alloys and their uses
| Alloy | Composition | Use | Why it suits the use |
|---|---|---|---|
| Brass | Copper + zinc | Musical instruments, door fittings, plugs | Harder than copper, resists corrosion, attractive finish |
| Stainless steel | Iron + chromium (+ nickel) | Cutlery, surgical instruments, chemical plant | Hard and resists rusting |
| Mild steel | Iron + ~0.25% carbon | Car bodies, machinery, girders | Strong, malleable, cheap |
Stainless steel earns a cross-topic line: the chromium resists corrosion, making stainless steel the alloy answer to the problem solved by coatings in rusting and prevention. A question can fairly ask why cutlery is stainless steel rather than painted mild steel. Paint would wear off into food; the corrosion resistance of stainless steel is built into the metal itself.
Use-questions are marked on the property-to-job link. “Brass is used for instruments because it is brass” is circular; “brass is harder than pure copper and resists corrosion” scores.
Why use an alloy instead of the pure metal?
This is the evaluation angle: pure iron is too soft for construction; adding carbon makes mild steel, which is strong enough. Pure copper is too soft for fittings; brass is harder. Pure aluminium is too weak for aircraft; aluminium alloys keep the low density and add strength. The pattern is always “pure metal too soft for the job → alloying hardens it because the layers can no longer slide.”
Worked exam question
Brass is an alloy of copper and zinc. (a) Explain, with the help of a labelled diagram, why brass is harder than pure copper. [3] (b) State one use of brass. [1] (c) Stainless steel is used for cutlery rather than mild steel. Explain why. [1]
Model answer: (a) Diagram: regular layers of identical atoms for copper; layers disrupted by different-sized zinc atoms for brass. Zinc atoms are a different size from copper atoms (1); they disrupt the regular layered arrangement (1); so the layers can no longer slide over each other (1). (b) Musical instruments / door fittings / electrical plug pins (1). (c) Stainless steel contains chromium and resists rusting/corrosion, so it is suitable for repeated contact with water and food (1).
Mark-by-mark: in (a), all three particle ideas are needed; a correct, fully labelled diagram can carry them. “Brass is a mixture so it is stronger” earns nothing. (b) any standard use scores. (c) the operative idea is corrosion resistance, not “it looks nicer.”
The mistakes that cost marks
- No particle language. “Alloys are stronger because the metals combine” scores zero. The mark scheme wants atoms, sizes, layers, sliding.
- Calling an alloy a compound. Alloys are mixtures. The word compound implies fixed proportions and chemical bonding, and examiners penalise it in the definition.
- Same-sized atoms in the diagram. If the drawn alloy atoms are the same size as the metal atoms, the disruption argument collapses and the diagram earns nothing.
- Uses without properties. Every “state a use and explain” question needs the property doing the work: hardness, corrosion resistance, strength, cost.
How examiners want it phrased
| Student wording | Mark-scheme wording |
|---|---|
| ”Mixing metals makes them stronger" | "Atoms of different sizes disrupt the layers, so the layers cannot slide over each other" |
| "Brass is a metal made of two metals" | "Brass is an alloy, a mixture of copper and zinc" |
| "Stainless steel doesn’t get dirty" | "Stainless steel contains chromium, so it resists rusting" |
| "Pure iron isn’t good enough" | "Pure iron is too soft; adding carbon disrupts the layers and makes mild steel harder” |
One diagram, three phrases, three named alloys: this is a subtopic you can finish in a single revision session. If you want a tutor to pressure-test the rest of Metals the same way, the free trial lesson exists for exactly that.
Test yourself
Answer in particle language (atoms, sizes, layers), then click to check.
Q1 (3 marks). Mild steel contains iron with a small amount of carbon. Explain why mild steel is harder than pure iron.
Show answer
• the carbon atoms are a different size from the iron atoms [1] • they disrupt the regular layered arrangement of the lattice [1] • so the layers can no longer slide over each other easily [1]
Q2 (2 marks). State the elements in stainless steel and give one use, linked to a property.
Show answer
• iron with chromium (and nickel) [1] • cutlery / surgical instruments / chemical plant, because it is hard and resists rusting [1]
Q3 (2 marks). A student describes brass as “a compound of copper and zinc”. Give two reasons why this description is wrong.
Show answer
• brass is a mixture, not a compound: the copper and zinc are not chemically combined in a fixed ratio [1] • its proportions can vary / it has no fixed formula, unlike a compound [1]
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Frequently asked questions
Why are alloys harder than pure metals?
Atoms of different sizes disrupt the regular layers of the metal lattice, so the layers can no longer slide over each other. Three scoring ideas: different-sized atoms, disrupted layers, layers cannot slide.
Which named alloys do I need for 0620?
Brass (copper and zinc) for musical instruments and fittings; stainless steel (iron with chromium and nickel) for cutlery and chemical plant because it resists rusting; mild steel for car bodies and machinery.
Is an alloy a compound or a mixture?
A mixture of a metal with one or more other elements, which can be metals or carbon (as in steel). There are no fixed proportions and no chemical bonds defining a formula.