2.4.3Additional lessons
Properties of Alloys — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 35 of 98
Section 1 of 6What Is an Alloy
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.
Section 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.
Section 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.
| Alloy | Main components | Typical use |
|---|---|---|
| Brass | Cu, Zn | taps, instruments |
| Bronze | Cu, Sn | statues, bells |
| Steel | Fe, C | construction |
| Stainless steel | Fe, Cr, Ni | cutlery |
Section 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.
Section 5 of 6Worked Example
Question Explain why steel is harder than pure iron.
- Pure iron has a regular lattice of identical atoms in layers that can slide when a force is applied.
- In steel, carbon atoms of a different size are present between and among the iron atoms.
- 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.
Section 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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