Cambridge O Level Chemistry · Syllabus 5070 · Metals
Density
What is Density?
The mass of a material per unit volume. A low-density material gives a large object for a small mass, which is why aluminium rather than a denser metal is chosen for aircraft bodies and for overhead cables that must not sag under their own weight.
This definition is part of the Metals chapter in Cambridge O Level Chemistry.
Common mistakes with Density
- 9. “Overhead cables should be copper, because copper is the better conductor.” Defect Optimises the wrong property for the situation. Reason The cable also has to hold itself up over a long span. Aluminium’s low density is what makes that possible. Repair “Aluminium is used, because it has a low density and is a good conductor.” Transfer Why is copper right for house wiring? A short wire inside a wall does not have to support itself, so the better conductivity wins.
Examiner tips on Density
- Two-mark structure. “Aluminium is used for aircraft because it has a low density” is one mark. Adding “so the aircraft has a smaller mass and uses less fuel” is the second. The property alone is never the whole answer.
Questions students ask about Density
Why is aluminium used for overhead cables and aircraft when other metals are stronger?
Aluminium has a low density, so a cable or aircraft body made from it has a smaller mass for the same size. A lighter overhead cable sags less between pylons, and a lighter aircraft uses less fuel. Density, not strength, is the property that decides this use, which is why "aluminium is strong" does not earn the mark.
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