Physical properties of iron
Why pure iron is not used, and its alloys are
1. Properties of iron
2. Iron everywhere, pure iron nowhere
Bridges, cars and tools are full of iron. Yet the book says pure iron has no industrial importance.
Pick the right answer
Predict: why is pure iron not used to build things?
- A. It is relatively soft.
- B. It is not attracted by a magnet.
- C. It melts at a low temperature.
- D. It is too hard to shape.
Explanation
Pure iron is relatively soft, with low hardness, so it is not used to build things. It is magnetic, melts at 1538 °C, and is malleable and ductile, so those are not the reason.
3. Soft, malleable, ductile, magnetic
The book describes pure iron like this:
| Property | Pure iron |
|---|---|
| Hardness | relatively soft, with low hardness |
| Malleable | yes: it can be hammered into sheets |
| Ductile | yes: it can be drawn into wires |
| Magnetic | yes: it is attracted by a magnet |
Easy to shape is good for making things. Easy to dent is bad for a bridge.
4. Melting point and density
Melting point 1538 °C
Density 7.87 g/cm³
Density = mass ÷ volume. So every 1 cm³ of pure iron has a mass of 7.87 g.
Volume of pure iron 1 cm³
Mass 7.87 g
5. Purity decides the properties
Its physical properties depend upon its purity and the nature of its impurities.
Impurities
Other elements mixed with the iron, in small amounts.
6. Many irons for many jobs
A large number of steels and other iron alloys are made with specific properties suitable for many uses.
Match the pairs
Match each everyday use to the property of iron it relies on.
- Hammered into thin sheets
- Drawn into fine wires
- Lifted by a magnet in a scrapyard
- Stays solid in a wood fire
7. Your turn
Fill in the blanks
A block of pure iron has a mass of 78.7 g. Its density is 7.87 g/cm³. Its volume is _____ cm³.
Explanation
78.7 ÷ 7.87 = 10 cm³.
Pick the right answer
A factory sample of iron is much harder than pure iron. Which is the best explanation?
- A. It contains other elements that block the slipping of its layers.
- B. It is purer than normal iron.
- C. It has a lower melting point.
- D. It has lost its magnetic properties.
Explanation
The factory iron is harder because it contains other elements: the physical properties of iron depend on its purity and the nature of its impurities, and these elements block the slipping of its layers. Pure iron itself is relatively soft.
True or false?
Pure iron is the most useful form of iron in industry.
Explanation
The statement is false: pure iron has no industrial importance because it is relatively soft. Industry uses steels and other iron alloys, whose properties suit many uses.
8. Summary
- Pure iron is relatively soft (low hardness), malleable, ductile and magnetic.
- It melts at 1538 °C. Its density is 7.87 g/cm³.
- Its physical properties depend on its purity and the nature of its impurities.
- So steels and iron alloys, not pure iron, are used in industry.
Back to the hook: can you explain why bridges are built from steel, not pure iron?
Reveal
Pure iron is relatively soft: its layers of atoms slip easily, so it dents easily. In steel, small carbon atoms block the slipping, so steel is harder.