Alloys
Section: 9. Metals | Syllabus: Cambridge IGCSE Chemistry (0620)
What Is an Alloy?
An alloy is a mixture of a metal with one or more other elements (usually other metals, sometimes carbon - a non-metal).
| Alloy | Composition |
|---|---|
| Brass | copper + zinc |
| Stainless steel | iron + other elements such as chromium, nickel and carbon |
Alloys are found everywhere - bronze statues, solder joints, coins - but brass and stainless steel are the two named compositions you need to recall.
Common Mistakes
- ❌ Mixing up brass and stainless steel's compositions
- ✓ brass = copper + zinc; stainless steel = iron + chromium/nickel/carbon
Why Alloys Are Harder and Stronger Than Pure Metals
Alloys are harder and stronger than the pure metals they're made from, which is exactly why they're more useful for demanding jobs.
In a pure metal, every atom is the same size, so the atoms pack into neat, regular layers. When a force is applied, these layers can slide smoothly over one another - this is what makes pure metals relatively soft.
In an alloy, atoms of a different size are mixed in among the original metal's atoms. This disrupts the regular layered arrangement, so the layers can no longer slide over each other easily - making the alloy harder and stronger.
Different-sized atoms in an alloy stop the layers sliding, making it harder than the pure metal
Worked Example: Identifying a Diagram of Structure
- Question: two particle diagrams are shown, one with atoms all the same size in regular rows, and one with a mixture of two different sized atoms. Which diagram represents an alloy, and why? [2]
- Answer: the diagram with two different sized atoms [1]; an alloy is a mixture of a metal with other elements, so it contains atoms of more than one size, unlike a pure metal [1]
Stainless Steel in Cutlery
Stainless steel (iron + chromium, and often nickel) is used for cutlery because it is:
- hard - resists scratching and wear from repeated washing and use
- resistant to rusting - the chromium forms a thin protective oxide layer, unlike pure iron
Worked Example
- Question: stainless steel is used to make cutlery. Which statement explains why stainless steel is more suitable than pure iron? (A) it is softer than pure iron (B) it is more resistant to rusting than the pure metal (C) it is a mixture of copper and zinc (D) it is more ductile than pure iron [1]
- Answer: B. Stainless steel resists rusting because of its chromium content, unlike pure iron. It is harder (not softer) than pure iron, and it is iron + chromium/nickel (not copper + zinc, which is brass).
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