An alloy is a mixture of a metal with other elements, and it is harder than the pure metal because the different atom sizes block the layers from sliding. Your whole explanation rests on that one idea.
This lesson belongs to SPM Chemistry manufactured materials. The next lesson, comparing glass and ceramic applications, moves from metals to non-metallic materials.
What happens to the atoms in a pure metal?
In a pure metal, every atom is the same size. The atoms pack into regular layers held together by the metallic bond.
When you hit or bend the metal, one layer can slide over the next. The bond is not broken, because the atoms simply shift to a new neighbouring position. This is why pure copper is soft and can be drawn into wire.
What changes when a second element is added?
The added atoms have a different size from the host atoms. They sit in the rows and disturb the neat pattern.
Now a layer that tries to slide meets a larger or smaller atom in its way. The layers are blocked, so more force is needed to move them. The alloy is harder and stronger.
| Material | Atoms present | Sizes | Layers slide? |
|---|---|---|---|
| Copper | Cu only | All equal | Easily |
| Bronze | Cu and Sn | Two sizes | Blocked |
| Brass | Cu and Zn | Two sizes | Blocked |
| Steel | Fe and C | Two sizes | Blocked |
Worked example: bronze compared with copper
Question. Bronze is made of copper and tin. Explain why bronze is harder than pure copper. [3 marks]
Full-mark answer.
- Pure copper has atoms of the same size arranged in orderly layers.
- The layers can slide over each other easily when a force is applied, so copper is soft.
- In bronze, tin atoms are larger than copper atoms and disrupt the orderly arrangement, so the layers cannot slide easily and bronze is harder.
Common half-mark answer. “Bronze is harder because it is an alloy and alloys are strong.” This repeats the question. It names no atoms, no layers and no sliding, so it earns almost nothing.
The explanation frame to memorise
Use three sentences in this order.
- The pure metal: same-size atoms in orderly layers that slide easily.
- The alloy: atoms of different sizes disrupt the arrangement.
- The result: the layers are harder to slide, so the alloy is harder and stronger.
The frame works for bronze, brass, steel, pewter and duralumin. Only the names of the atoms change. Record any marks you drop on these questions in the mistake log and paper-error review.
Check yourself
Steel is made from iron with a small amount of carbon. Explain why steel is harder than pure iron, and state one reason a builder may prefer steel for a bridge.
Answer
Pure iron has atoms of the same size in orderly layers, and the layers slide easily. In steel, carbon atoms are smaller than iron atoms, and they disrupt the orderly arrangement of iron atoms. The layers cannot slide easily, so steel is harder and stronger.
A bridge must carry heavy loads without bending, so the strength of steel suits it better than soft pure iron.
What to study next
Move on to comparing glass and ceramic applications, then test yourself on the manufactured materials practice set.
If you want a teacher to check your written explanations with you, see online one-to-one Chemistry tuition.