An alloy is harder than its pure metal because atoms of a different size disturb the regular layers, so the layers slide less easily. That one sentence, with the word “layers”, is the core of a full-mark answer.
This lesson is part of industrial chemistry. The next lesson, comparing polymers, glass and ceramics, widens the comparison beyond metals.
Why can a pure metal be bent so easily?
In a pure metal, all atoms are the same size. They pack in neat, regular layers, like oranges stacked in a crate.
When a force pushes on the metal, one layer can slide over the next because the layers line up. That sliding is why pure metals can be bent, hammered or stretched, and why they are too soft for some jobs.
What does an alloy change?
An alloy adds atoms of another element, and these atoms are a different size. The larger or smaller atoms sit between the regular ones and upset the neat layers.
Now a layer cannot slide freely, because the different atoms block its path. More force is needed to move it, so the alloy is harder and stronger than the pure metal.
Worked example: reading hardness data
An original experiment presses a steel ball into three samples with the same force. A smaller dent means a harder sample.
| Sample | Composition | Dent diameter (mm) |
|---|---|---|
| X | Pure copper | 4.0 |
| Y | Copper with some tin (bronze) | 2.5 |
| Z | Copper with some zinc (brass) | 3.0 |
Question (3 marks): Compare the hardness of X and Y and explain the difference.
Step 1: compare the evidence. X has a dent of 4.0 mm and Y has 2.5 mm. The dent in Y is 1.5 mm smaller, so Y is harder.
Step 2: explain with atoms. Tin atoms are a different size from copper atoms. They disturb the regular layers of copper atoms, so the layers slide less easily.
Step 3: state an assumption. The comparison holds only if the same ball, force and sample thickness were used for all three.
The mistake that costs marks
A weak answer says, “Alloys are harder because they are stronger mixtures.” That repeats the question and gives no reason.
| Wrong | Right |
|---|---|
| “The alloy is a stronger mixture” | “Atoms of different sizes disturb the layers” |
| “The atoms are fixed” | “The layers cannot slide easily” |
| “The alloy is a new compound” | “The alloy is a mixture of metals” |
The repair is always the same. Name the atoms, say what they disturb, and say what happens to the layers.
Check yourself
Pure gold is soft. A jeweller mixes it with a small amount of copper to make a ring. Explain why the ring is harder than pure gold, and name one assumption about the comparison.
Answer
Copper atoms are a different size from gold atoms, so they disturb the regular layers of gold atoms. The layers slide less easily, so the alloy is harder than pure gold.
Assumption: the two samples are compared using the same test, for example the same force pressing on the same thickness of metal.
What to study next
Hardness is one property among several. Continue with comparing polymers, glass and ceramics, then use the graph evidence and fair-comparison lab to practise reading material data.
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