Properties of Metals and Alloys

This lesson covers the properties of metals and alloys as required by the AQA GCSE Combined Science Trilogy specification (8464), section 4.2.3. You need to understand why pure metals are soft and how alloying changes their properties.

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Properties of Pure Metals

Pure metals have a regular arrangement of identical atoms (or ions) in layers. This regular structure explains many of their properties.

Why Pure Metals Are Relatively Soft

In a pure metal, all the atoms are the same size. This means the layers are arranged in a very regular, orderly pattern. Because of this regularity, the layers can slide over each other relatively easily when a force is applied. This makes pure metals:

• Soft — they can be scratched or dented easily.
• Malleable — they can be hammered into different shapes.
• Ductile — they can be drawn into wires.

Exam Tip (AQA 8464): When explaining why pure metals are soft, always link it to the regular arrangement of identical atoms allowing layers to slide easily.

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What Is an Alloy?

An alloy is a mixture of a metal with one or more other elements (usually other metals, but sometimes non-metals such as carbon). Alloys are not compounds — they are mixtures, and the different atoms are not chemically bonded in a fixed ratio.

Common Alloys

Alloy	Main Metal	Other Element(s)	Key Use
Steel	Iron	Carbon (+ other metals)	Construction, vehicles, tools
Stainless steel	Iron	Chromium and nickel	Cutlery, surgical instruments
Brass	Copper	Zinc	Musical instruments, fittings
Bronze	Copper	Tin	Sculptures, bearings
Gold alloys (e.g. 9-carat)	Gold	Copper, silver	Jewellery

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Why Are Alloys Harder Than Pure Metals?

In an alloy, atoms of different sizes are mixed into the metal lattice. Because the atoms are not all the same size:

• The regular arrangement of layers is disrupted.
• The differently-sized atoms make it harder for the layers to slide over each other.
• This makes alloys harder and stronger than pure metals.

graph LR
    A["Pure Metal<br/>All atoms same size<br/>Regular layers"] -->|"Easy to slide"| B["Soft and<br/>malleable"]
    C["Alloy<br/>Different sized atoms<br/>Irregular layers"] -->|"Harder to slide"| D["Harder and<br/>stronger"]

    style A fill:#3498db,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#e67e22,color:#fff

Exam Tip: This is a very common 3-mark question. The answer follows a logical chain: (1) the different-sized atoms disrupt the regular arrangement, (2) this makes it harder for layers to slide, (3) so the alloy is harder than the pure metal.

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Types of Steel

Steel is an alloy of iron and carbon. The amount of carbon and other elements added determines the properties:

Type	Composition	Properties	Uses
Low-carbon steel (mild steel)	Iron + ~0.1–0.3% carbon	Soft, easy to shape, not very strong	Car bodies, nails, wire
High-carbon steel	Iron + ~0.6–1.5% carbon	Hard, strong, but brittle	Tools, blades, springs
Stainless steel	Iron + chromium + nickel	Hard, resistant to corrosion	Cutlery, surgical instruments, kitchen sinks

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Gold Alloys and Purity

Pure gold is very soft and easily scratched, making it impractical for jewellery. Gold is therefore alloyed with other metals such as copper and silver to make it harder:

Purity	Gold Content	Hardness
24-carat	100% gold	Very soft
18-carat	75% gold	Harder
9-carat	37.5% gold	Hardest (of common jewellery gold)

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Why Are Metals Useful? — Structure-Property-Use Relationships

graph TD
    A["Metallic Properties"] --> B["Good electrical<br/>conductor"]
    A --> C["Good thermal<br/>conductor"]
    A --> D["Malleable and<br/>ductile"]
    A --> E["High melting<br/>point"]
    A --> F["Strong"]
    B --> B1["Electrical wiring<br/>Circuits"]
    C --> C1["Saucepans<br/>Radiators"]
    D --> D1["Car body panels<br/>Wires"]
    E --> E1["Engines<br/>Furnace linings"]
    F --> F1["Bridge construction<br/>Building frames"]

    style A fill:#2c3e50,color:#fff
    style B fill:#3498db,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#8e44ad,color:#fff

Specific Metal Uses

Metal	Key Property Used	Application
Copper	Excellent electrical conductor, ductile	Electrical wiring
Copper	Good thermal conductor	Saucepans, heat exchangers
Aluminium	Low density, resistant to corrosion	Aircraft, drinks cans, power cables
Iron/Steel	Strong, high melting point	Construction, vehicles, bridges
Gold	Does not corrode, attractive	Jewellery, electrical contacts
Titanium	Strong, low density, corrosion resistant	Medical implants, aircraft

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Choosing the Right Metal or Alloy

When choosing a metal for a particular use, engineers consider:

1. Strength — how much force can it withstand?
2. Hardness — how resistant is it to scratching?
3. Density — is it lightweight?
4. Corrosion resistance — will it rust or corrode?
5. Cost — is it affordable?
6. Melting point — will it withstand high temperatures?
7. Conductivity — does it need to conduct electricity or heat?

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Common Mistakes

Mistake	Correction
"Alloys are compounds"	Alloys are mixtures, not compounds — the atoms are not chemically bonded in a fixed ratio
"Alloys are harder because the bonds are stronger"	Alloys are harder because the differently-sized atoms disrupt the regular layers, making it harder for them to slide
"Pure metals are hard"	Pure metals are generally soft because the regular layers can slide easily
"All steel is the same"	Different types of steel have different compositions and properties (e.g. low-carbon steel is soft, high-carbon steel is hard and brittle)
Confusing malleability with hardness	Malleability = can be shaped by hammering; Hardness = resistance to scratching or denting

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Summary