Topic 9.3: Alloys and Their Properties Revision Notes

1. Overview

Alloys are a critical category of materials in chemistry and engineering. While pure metals are often too soft or chemically reactive for many practical purposes, mixing them with other elements creates alloys with enhanced physical and chemical properties. This topic explores why alloys are used instead of pure metals and how their internal structure determines their strength.

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Key Definitions

• Alloy: A mixture of a metal with one or more other elements (usually other metals or carbon).
• Brass: An alloy composed of copper ($Cu$) and zinc ($Zn$).
• Stainless Steel: An alloy composed of iron ($Fe$) mixed with chromium ($Cr$), nickel ($Ni$), and carbon ($C$).
• Malleability: The ability of a material to be hammered or pressed into shape without cracking or breaking.

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Core Content

Common Alloys and Their Compositions

You must know the specific composition of these two alloys:

• Brass: Copper + Zinc
• Stainless Steel: Iron + Chromium + Nickel + Carbon

Properties and Uses

Alloys are generally harder and stronger than the pure metals from which they are made. This makes them significantly more useful in construction and manufacturing.

Example: Stainless Steel

• Properties: Extremely hard and highly resistant to rusting (corrosion).
• Uses: Used extensively in cutlery and kitchen sinks because it does not react with food or water and can withstand the physical wear of daily use.
• Reaction context: Unlike pure iron, which reacts with oxygen and water to form rust, the chromium in stainless steel forms a thin, invisible layer of chromium oxide that protects the metal.

Identifying Alloys from Diagrams

In exams, you may be asked to identify an alloy from a particle diagram.

• Pure Metal: Shows regular rows of identical-sized atoms arranged in a lattice.
• Alloy: Shows a lattice where some atoms are replaced by atoms of a different size, disrupting the regular pattern.

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Extended Content (Extended Curriculum Only)

Why Alloys are Harder than Pure Metals

To understand why alloys are stronger, we must look at the arrangement of atoms (the structure):

1. Pure Metals: In a pure metal, all atoms are the same size. They are arranged in regular, repeating layers. When a force is applied, these layers can easily slide over each other. This is why pure metals like gold or lead are soft and easily shaped.
2. Alloys: In an alloy, the added elements have different sized atoms. These different sized atoms distort the regular arrangement of the metal lattice.
3. The Result: Because the layers are no longer uniform, they can no longer slide over each other easily. This "locks" the structure in place, making the material much harder and less malleable.

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Key Equations

While alloys are mixtures and not compounds (so they don't have a single chemical formula), the process of rusting is often discussed in contrast to the resistance of stainless steel.

The Rusting of Iron (which stainless steel prevents):

• Word Equation: Iron + Oxygen + Water → Hydrated Iron(III) Oxide
• Symbol Equation: $4Fe(s) + 3O_2(g) + 2nH_2O(l) \rightarrow 2Fe_2O_3 \cdot nH_2O(s)$

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Common Mistakes to Avoid

• ❌ Wrong: Describing an alloy as a "compound."
• ✓ Right: An alloy is a mixture. The elements are not chemically bonded in fixed proportions.
• ❌ Wrong: Thinking that adding any element makes a metal softer.
• ✓ Right: Adding different sized atoms almost always makes the metal harder because it prevents layers from sliding.
• ❌ Wrong: Forgetting that carbon is often an ingredient in steel alloys.
• ✓ Right: Steel is a mixture of iron and carbon; stainless steel adds chromium and nickel to that mix.

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Exam Tips

• Command Words: If a question asks you to "Describe" the structure of an alloy, mention the different sized atoms. If it asks you to "Explain" why it is harder, you must mention that the different sized atoms prevent the layers from sliding.
• Contextual Questions: Be prepared to explain why stainless steel is used for medical instruments or cutlery—the answer is always its hardness and resistance to corrosion/rusting.
• Identification: If shown a diagram with two different types of atoms, it is an alloy. If all atoms are the same, it is a pure metal.
• Composition: Memorize that Brass = $Cu + Zn$. A common trick is to swap it with Bronze ($Cu + Sn$), but you only need to know Brass for the IGCSE syllabus.