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2.4.3Additional lessons

Properties of Alloys — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 35 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 6What Is an Alloy

2.4.3 Properties of Alloys

Pure metals are often too soft or too reactive for use, so they are mixed.

Definition
  • An alloy is a mixture of a metal with one or more other elements, usually other metals, in a solid solution.
Metallic bonding
  • Alloys retain metallic bonding, so they still conduct electricity and heat.
Designed properties
  • The mixture is chosen to give properties that suit a particular use better than the pure metal.
Not compounds
  • The composition of an alloy can vary, whereas a compound has a fixed formula.

Part 2 of 6Why Alloys Are Harder

1
In a pure metal all the atoms have the same size, so the layers are regular and can slide over each other easily.
2
In an alloy, atoms of a different size are present in the lattice.
3
The differently sized atoms disrupt the regular layers.
4
It is therefore harder for layers to slide when a force is applied.
5
As a result the alloy is generally harder and stronger, but less malleable and ductile than the pure metal.

Part 3 of 6Common Alloys

Common Alloys
Brass
  • Copper mixed with zinc is harder than pure copper and is used for instruments, taps and fittings.
Bronze
  • Copper mixed with tin is hard and corrosion resistant, used for statues, bells and bearings.
Steel
  • Iron containing a small amount of carbon is much harder and stronger than pure iron, so it is used in buildings and vehicles.
Stainless steel
  • Steel with chromium and nickel resists rusting, so it is used for cutlery and surgical tools.
AlloyMain componentsTypical use
BrassCu, Zntaps, instruments
BronzeCu, Snstatues, bells
SteelFe, Cconstruction
Stainless steelFe, Cr, Nicutlery

Part 4 of 6Effects on Other Properties

Conductivity
  • Foreign atoms scatter delocalised electrons, so alloys conduct electricity less well than pure metals.
Melting point
  • Alloys melt over a range of temperatures, and the melting point is often lower than that of the main metal.
Corrosion
  • Some alloys are resistant to corrosion because of a protective oxide layer, as with stainless steel.
Choice
  • Engineers choose the alloy composition to balance strength, flexibility, weight and cost.

Part 5 of 6Worked Example

Question Explain why steel is harder than pure iron.

  1. Pure iron has a regular lattice of identical atoms in layers that can slide when a force is applied.
  2. In steel, carbon atoms of a different size are present between and among the iron atoms.
  3. These atoms disrupt the layers and stop them sliding easily, so a greater force is needed to deform the metal.

Answer Atoms of a different size distort the lattice and prevent layers from sliding, so steel is harder.

Part 6 of 6Exam Checklist & Tips

2.4.3 – Key Points for the Exam

  • Alloy A mixture of a metal with other elements, giving a material with modified properties.
  • Solid solution A uniform mixture of atoms in a solid lattice.
  • Corrosion Gradual deterioration of a metal by reaction with its surroundings.
  • Hardness Resistance to scratching and deformation.

Common Mistakes

  • Saying an alloy is a compound.
  • Stating that alloys are more malleable than pure metals; they are usually less so.
  • Forgetting that alloys still have metallic bonding.
  • Claiming that alloys always conduct better than pure metals.

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