This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4. It may also appear in Paper 5 and Paper 6 within the context of experimental skills.
An
alloy is a
mixture of a metal with other elements. These other elements can be metals or non-metals.
It is crucial to understand that an alloy is a mixture, not a compound. The atoms are physically mixed together but not chemically bonded in fixed ratios like in a chemical compound. This means the properties of the alloy are different from the pure metal it is based on.
Building on previous concepts: In pure metals, atoms are arranged in regular layers that can slide over each other, making them soft and malleable. Alloys disrupt this structure to create stronger materials.
An alloy is defined as a mixture of a metal with other elements (which may include other metals or non-metals).
Here are the specific alloys you must know, including their composition and why they are used:
| Alloy Name | Composition | Properties & Uses |
|---|
| Brass | Copper + Zinc | Harder and stronger than pure copper. Used for musical instruments, door handles, and locks. |
| Stainless Steel | Iron + Chromium + Nickel + Carbon | Hard, strong, and resistant to rusting (corrosion). Used for cutlery, surgical instruments, and kitchen sinks. |
| Steel (Carbon Steel) | Iron + Carbon | Stronger than pure iron. Used for bridges, cars, and construction. |
Note on Stainless Steel: It is not just iron and carbon. The addition of chromium and nickel is what gives it the specific property of resisting rusting, which pure steel does not do well.
The Error: Students often describe alloys as 'compounds' or say that the metals are 'chemically combined.'
The Correct Understanding: Alloys are mixtures. The atoms are mixed together physically, not chemically bonded. You cannot write a chemical formula for an alloy (like H_2O) because the ratio of elements can vary.
When to use this: When asked to explain why alloys are harder or stronger than pure metals.
The Reasoning: Examiners look for a two-part explanation: the cause (different sized atoms) and the effect (layers cannot slide).
Correct Phrasing: 'Alloys contain atoms of different sizes. These different-sized atoms disturb/disrupt the regular layers of atoms. This prevents the layers from sliding over each other easily.'
Why this works: It directly links the structural change (size difference) to the mechanical property (hardness/strength).
Q:Define the term 'alloy'.
A:A mixture of a metal with other elements.
Q:State two metals that are mixed to make brass.
Q:Explain why stainless steel is used for cutlery instead of pure iron.
A:Stainless steel is harder/stronger than pure iron and is resistant to rusting/corrosion. Pure iron would rust easily and is softer.
Q:Explain in terms of structure why alloys are harder than pure metals.
A:Alloys contain atoms of different sizes. These atoms disturb the regular arrangement/layers of atoms. This prevents the layers from sliding over each other.
Q:Which elements are mixed with iron to make stainless steel?
A:Iron is mixed with chromium, nickel, and carbon.