9.3 Alloys and their properties
- Syllabus
- 0620–2026–2027
- Topic
- 9.3
- Level
- —
Learning objectives
9.3.1An alloy as a mixture of a metal with• Describe an alloy as a mixture of a metal with other elements, (a) brass as a mixture of copper and zinc (b) stainless steel as a mixture of iron and other elements such as chromium, nickel and carbon9.3.2Alloys can be harder and stronger than• State: alloys can be harder and stronger than the pure metals and are more useful9.3.3Uses of alloys in terms of their• Describe uses of alloys in terms of their physical properties, including stainless steel in cutlery because of its hardness and resistance to rusting9.3.4Representations of alloys from• Identify representations of alloys from diagrams of structure9.3.5Alloys can be harder/stronger than• Explain why alloys can be harder/stronger than pure metals: different-sized atoms stop layers sliding easily
Define alloys and name their components
An alloy is a mixture of a metal with one or more other elements. The added elements may be metals or non-metals; an alloy is a mixture, not a compound.
| Alloy | Main metal | Other required elements |
|---|---|---|
| brass | copper | zinc |
| stainless steel | iron | elements such as chromium, nickel and carbon |
Describe composition with 'mixture of'. Brass contains copper and zinc; stainless steel contains iron with other elements and may include both metallic chromium/nickel and non-metallic carbon.
An alloy does not have to contain two metals: stainless steel contains carbon. Because composition can vary, an alloy is not a pure element or a fixed-ratio compound.
Compare alloys with pure metals
Alloys can be harder and stronger than the pure metals from which they are made, so they are often more useful.
| Material form | Typical response to force | Practical consequence |
|---|---|---|
| pure metal | often softer and easier to deform | useful where shaping is important |
| alloy | can resist scratching, bending or breaking more strongly | useful where hardness or strength is needed |
Choose the material by the required property. Alloying changes properties and can improve hardness or strength, but the exact benefit depends on the alloy's composition.
More useful does not mean better for every purpose. A harder alloy may be less malleable; property requirements, not the word 'alloy' alone, control the choice.
Link stainless-steel cutlery to its properties
Stainless steel is used for cutlery because it is hard and resistant to rusting.
| Cutlery requirement | Stainless-steel property | Why it fits |
|---|---|---|
| keeps its shape and resists scratching in use | hardness | withstands repeated contact and force |
| remains serviceable around water and food | resistance to rusting | avoids rapid corrosion and surface damage |
A complete explanation names the use and both properties: stainless steel suits cutlery because it is hard and resists rusting.
Strength alone is not the prescribed explanation. For cutlery, state hardness and resistance to rusting; conductivity is irrelevant to this use.
Identify an alloy structure representation
| Text-first particle representation | Identification | Diagnostic clue |
|---|---|---|
| ● ● ● ● / ● ● ● ● / ● ● ● ● | pure metal | one atom size/type in regular layers |
| ● ● ○ ● / ● ◉ ● ● / ○ ● ● ◉ | alloy | different atom sizes/types mixed through the metallic structure |
Ignore the symbols' names and compare sizes or types. A regular array containing only identical atoms represents a pure metal; a metallic array containing a second size or type represents an alloy.
The different symbols are interspersed rather than arranged in a fixed repeating ratio, consistent with an alloy being a mixture.
Do not label every diagram with two atom types as an alloy. It must represent a metal-based mixture; an ordered repeating pattern of unlike atoms may instead represent a compound.
Explain why alloys are harder and stronger
In a pure metal, equal-sized atoms form regular layers that can slide over one another when a force is applied.
| Structure | Effect on layers | Mechanical result |
|---|---|---|
| pure metal: similar-sized atoms | layers remain regular and slide relatively easily | softer, more malleable |
| alloy: different-sized atoms | regular layers are distorted and cannot slide easily | harder and stronger |
Use the full causal chain: different-sized atoms → distorted layers → layers cannot slide over each other easily → greater hardness and strength.
Do not explain alloy strength by saying the added element is simply 'hard' or makes stronger bonds. The required particle explanation is obstruction of layer sliding by different-sized atoms.