Alloy

Summary: An alloy is a mixture of a metal with one or more other elements (usually metals or carbon). Alloys are harder and stronger than pure metals because atoms of different sizes disrupt the regular lattice, preventing layers of atoms from sliding past each other. Key alloys: steel (Fe + C), stainless steel (Fe + Cr + Ni), brass (Cu + Zn), bronze (Cu + Sn), solder (Pb + Sn), duralumin (Al + Cu + Mg). Tags: igcse chemistry metals alloys structure Created: 2026-07-26 Last Updated: 2026-07-26

What Is an Alloy?

An alloy is a mixture of a metal with one or more other elements. The other elements are usually other metals, but can also be non-metals (e.g., carbon in steel). Alloys are designed to have improved properties compared to the pure metal — greater hardness, strength, corrosion resistance, or specific characteristics for particular applications.

Why Make Alloys?

Pure metals have limitations:

• Pure iron is relatively soft and rusts easily
• Pure aluminium is too weak for structural use
• Pure gold (24 carat) is too soft for jewellery
• Pure copper is too soft for tools

Alloys overcome these limitations by modifying the metal’s structure at the atomic level.

Structure of Pure Metals vs Alloys

Pure Metal Structure

In a pure metal, all atoms are the same size and are arranged in regular, repeating layers (a giant metallic lattice). The layers are held together by metallic bonding — the electrostatic attraction between positive metal ions and a “sea” of delocalised electrons.

When a force is applied, entire layers of atoms can slide over each other easily. The delocalised electrons continue to hold the structure together. This is why pure metals are malleable (can be hammered into shape) and ductile (can be drawn into wires).

Alloy Structure

In an alloy, atoms of different sizes are present. The larger (or smaller) foreign atoms disrupt/distort the regular arrangement of the metal lattice. When a force is applied, the layers of atoms can no longer slide easily past each other because the differently-sized atoms act as “blocks.”

This disruption means:

• Greater hardness — more resistant to scratching and indentation
• Greater strength — more resistant to deformation under load
• Reduced malleability and ductility — the trade-off for increased strength

The Standard IGCSE Diagram Question

Examiners expect you to draw or interpret:

• Pure metal: Regular rows of same-sized circles
• Alloy: Regular rows disrupted by circles of a different size

Common Alloys in the IGCSE Syllabus

Iron-Based Alloys (Steels)

Copper-Based Alloys

Aluminium-Based Alloys

Other Important Alloys

Why Alloys Are Harder — The Detailed Explanation

At the Atomic Level

1. Pure metal: The regular lattice allows slip planes to move. Dislocations (line defects in the crystal) can move through the lattice with little resistance → the metal deforms plastically at relatively low stress.

2. Alloy: Foreign atoms act as obstacles to dislocation movement. Dislocations cannot move freely — they get “pinned” by the foreign atoms. This is called solid solution strengthening (or alloy hardening). Much higher stress is needed to cause plastic deformation.

3. Precipitation hardening: In some alloys (e.g., duralumin), the added elements form very small particles (precipitates) within the metal lattice. These precipitates block dislocation movement even more effectively → even greater strength.

The IGCSE-Level Answer (2–3 marks)

“In a pure metal, all the atoms are the same size and are arranged in regular layers. The layers can slide over each other easily, so the metal is soft. In an alloy, atoms of different sizes are present. These disrupt the regular arrangement of layers, preventing them from sliding past each other. This makes the alloy harder and stronger than the pure metal.”

Key Concepts from Past Papers

Definitions You MUST Know

• Alloy: a mixture of a metal with one or more other elements (usually other metals, sometimes non-metals like carbon)
• Pure metal: a metal consisting of only one type of atom, with no other elements mixed in
• Malleable: can be hammered or pressed into shape without breaking or cracking
• Ductile: can be drawn out into a thin wire
• Lattice: a regular, repeating three-dimensional arrangement of atoms/ions

Recurring Mark Scheme Answers

• “In an alloy, atoms of different sizes disrupt the regular arrangement / lattice”
• “The layers of atoms cannot slide over each other as easily / the alloy is harder/stronger”
• “Alloys are harder than pure metals because the different-sized atoms prevent layers sliding”
• “Stainless steel is an alloy of iron with chromium and nickel”
• “Brass is an alloy of copper and zinc — it is harder than pure copper”
• “Steel is an alloy of iron with controlled amounts of carbon”

Common Mistakes

• Saying “alloys are compounds”: Alloys are mixtures, not compounds — the elements are not chemically combined in fixed ratios
• Confusing the composition of common alloys: Brass = Cu+Zn (not Cu+Sn — that’s bronze); Bronze = Cu+Sn; Solder = Pb+Sn
• Forgetting that carbon is a non-metal but is the key alloying element in steel
• Saying “alloys are made by chemically reacting metals”: They are made by melting metals together and allowing the mixture to cool — a physical process, not a chemical reaction
• Drawing alloy diagrams with different-sized circles randomly placed: Must show them within the same layers/rows, but disrupting the regularity
• Saying “alloys have stronger bonds”: The metallic bonds are the same type — it’s the physical obstruction of layers sliding that makes them harder, not stronger bonds

Common Question Types

Type 1: “Explain why an alloy is harder than a pure metal” (2–3 marks)

• Frequency: ~45% of Papers 3/4
• Must include: (1) Pure metal — regular layers, layers slide easily; (2) Alloy — different-sized atoms, layers cannot slide; (3) Therefore harder/stronger

Type 2: “Draw/interpret diagrams of pure metal vs alloy structure” (2–3 marks)

• Frequency: ~35% of papers
• Must show: Pure = regular rows of same-sized circles; Alloy = some circles of clearly different sizes disrupting the regularity

Type 3: “Name an alloy of [metal X] and state its use” (1–2 marks)

• Frequency: ~30% of papers
• Examples: Fe → steel (construction); Cu → brass (musical instruments); Al → duralumin (aircraft)

Type 4: “Suggest why [alloy X] is used rather than [pure metal Y] for [purpose Z]” (2–3 marks)

• Frequency: ~35% of papers
• Approach: State the pure metal’s limitation, state how the alloy overcomes it

Key Facts to Memorize

• Alloy = mixture of metal + other elements (NOT a compound)
• Alloys are harder/stronger than pure metals because different-sized atoms disrupt the lattice → layers cannot slide
• Steel = Fe + C (the most important alloy by volume — over 1.8 billion tonnes produced annually)
• Stainless steel = Fe + Cr (~18%) + Ni (~8%) — corrosion resistant
• Brass = Cu + Zn — harder than Cu, golden colour, musical instruments
• Bronze = Cu + Sn — hard, corrosion resistant, statues and ship propellers
• Solder = Pb + Sn — low melting point, electronics
• Duralumin = Al + Cu + Mg — low density + strong, aircraft
• Pure metals are softer → useful where malleability is needed (e.g., jewellery gold, electrical wiring)
• Alloys sacrifice some ductility for greatly improved strength and hardness

Related Notes

• Uses of Metals — Why specific metals and alloys are chosen for specific applications
• Metallic Bonding — The bonding in pure metals and how it changes in alloys
• Reactivity of Metals — Reactivity and alloy formation
• Rusting of Iron — Why stainless steel doesn’t rust (Cr₂O₃ protective layer)
• Metal Extraction — Extracting the pure metals that are then alloyed
• Transition Metals — Many key alloying elements are transition metals
• Brass — Detailed brass properties
• Stainless Steel — Detailed stainless steel composition and uses
• IGCSE-Chem-Index

Past Paper Sources

• 0620/42 M/J 2023 Q5(d): Draw diagrams to show the structure of a pure metal and an alloy (3 marks)
• 0971/32 O/N 2022 Q4(b): Explain why alloys are harder than pure metals (3 marks)
• 0620/43 M/J 2019 Q7(c): Name an alloy of iron and state why it is used instead of pure iron (2 marks)
• 0620/62 M/J 2020 Q3: State the composition of brass and bronze (2 marks)
• 0971/52 O/N 2023 Q4(c): Explain why duralumin is used for aircraft rather than pure aluminium (3 marks)
• 0620/32 O/N 2018 Q5(b): Why does adding carbon to iron make it harder? (2 marks)

IGCSE Chemistry (0620/0971) wiki. Alloys are one of the oldest technologies — bronze was made as early as 3500 BC — yet the atomic-level explanation of why they work is a key part of modern materials science.