Alloys as useful materials (HT)
An alloy is a mixture of a metal with at least one other element (often another metal). Alloys are designed because pure metals are usually too soft for many uses — adding atoms of different sizes disrupts the regular layers, making it harder for them to slide.
Why alloys are harder than pure metals
In a pure metal, identical atoms form a regular lattice. Layers can slide over one another, making the metal soft and ductile.
In an alloy, the foreign atoms are a different size — they distort the lattice and prevent layers sliding past each other. The result is a harder, stronger material.
Common alloys you must know
Bronze (Cu + Sn — copper + tin)
- Hard, corrosion-resistant.
- Uses: ship propellers, statues, bells, coins.
Brass (Cu + Zn — copper + zinc)
- Workable, golden colour.
- Uses: door fittings, musical instruments (trumpet, sax).
Steels (Fe + C, sometimes other metals)
| Steel | Composition | Properties / use |
|---|---|---|
| Low-carbon steel | Fe + 0.05–0.25% C | Soft, easily shaped — car bodies, bridges |
| High-carbon steel | Fe + 0.6–1.5% C | Hard, brittle — blades, cutting tools |
| Stainless steel | Fe + Cr + Ni | Hard, corrosion-resistant — cutlery, sinks |
Gold alloys
Pure gold (24 carat) is too soft for jewellery. Gold is alloyed with silver, copper or zinc.
- 18 carat = 18/24 = 75% gold.
- 9 carat = 9/24 = 37.5% gold.
The carat number = parts gold out of 24.
Aluminium alloys
Pure Al is light but soft. Alloying with Cu, Mg, Si etc. makes it strong while staying light.
- Used in aircraft frames, lightweight bicycle frames, drinks cans.
Why use alloys?
- Hardness — much greater than pure metal.
- Strength — supports more load.
- Specific properties — corrosion resistance (stainless steel), low density (Al alloys).
- Cost control — diluting expensive metals.
✦Worked example
A pure copper coin would deform with use. What alloy would be more practical?
Bronze (Cu + Sn) — harder, more durable. Many coins are bronze or "cupronickel" (Cu + Ni).
⚠Common mistakes
- Saying alloys are compounds — they're mixtures (no chemical bond between Fe and C).
- Confusing brass with bronze — brass = Cu + Zn; bronze = Cu + Sn.
- Ignoring gold purity in jewellery — pure (24 carat) gold is rare in everyday jewellery.
- Saying stainless steel is just iron + chromium — it usually contains nickel and other metals too.
Links
Builds on C2.4 (metallic bonding). Connects to C10.6 (recycling), C10.7 (corrosion).
AI-generated · claude-opus-4-7 · v3-deep-chemistry