9.2 Uses of Metals - Revision Notes
1. Overview
The choice of a metal for a specific job depends on its physical and chemical properties. Metals like aluminium and copper are widely used in industry and daily life because they possess unique combinations of properties such as low density, high conductivity, and resistance to chemical attack.
Key Definitions
- Density: The mass of a substance per unit volume (how "heavy" it is for its size).
- Electrical Conductivity: The ability of a material to allow an electric current to flow through it.
- Ductility: The ability of a material to be drawn out into thin wires without breaking.
- Malleability: The ability of a material to be hammered or pressed into thin sheets.
- Corrosion Resistance: The ability of a metal to resist damage caused by chemical reactions with its environment (such as rusting or oxidation).
Core Content
Aluminium
Aluminium is one of the most versatile metals in the world due to its unique physical and chemical characteristics.
- Manufacture of Aircraft:
- Property: Low density.
- Reason: It makes the aircraft lightweight, which is essential for flight and fuel efficiency. Note: It is often alloyed with other metals to increase its strength while keeping it light.
- Overhead Electrical Cables:
- Property: Low density and good electrical conductivity.
- Reason: Although copper is a better conductor, aluminium is much lighter and cheaper. Its low density prevents the long cables from sagging too much or breaking under their own weight.
- Food Containers (Cans and Foil):
- Property: Resistance to corrosion.
- Reason: Aluminium reacts very quickly with oxygen in the air to form a very thin, tough, and transparent layer of aluminium oxide ($Al_2O_3$). This layer sticks to the surface and prevents any further reaction with food, water, or air.
Copper
Copper is a reddish-brown metal known primarily for its excellent conductive properties.
- Electrical Wiring:
- Property: Good electrical conductivity and ductility.
- Reason: It allows electricity to pass through it with very little resistance (reducing energy loss). Because it is ductile, it can be easily pulled into the thin, flexible wires needed for domestic and industrial circuits.
Extended Content (Extended Only)
There is no additional supplement content for this specific sub-topic in the IGCSE Chemistry syllabus.
Key Equations
While the uses of metals focus on physical properties, the corrosion resistance of aluminium is due to a chemical reaction that forms a protective oxide layer:
Word Equation: Aluminium + Oxygen → Aluminium oxide
Balanced Symbol Equation: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$
Common Mistakes to Avoid
- ❌ Wrong: Aluminium is used for aircraft because it is a strong metal.
- ✅ Right: Aluminium is used because it has a low density. (Pure aluminium is actually quite soft; it is the low density that is the primary reason for its use in aviation).
- ❌ Wrong: Aluminium does not react with oxygen.
- ✅ Right: Aluminium is highly reactive, but it reacts instantly to form a protective oxide layer that prevents further corrosion.
- ❌ Wrong: Copper is used for overhead cables.
- ✅ Right: Copper is too dense (heavy) for overhead cables; aluminium is used for overhead cables, while copper is used for indoor wiring.
Exam Tips
- Command Words: If a question asks you to "Explain" a use, you must mention both the property (e.g., low density) and the reason it fits the use (e.g., makes the plane light enough to fly).
- The "Oxide Layer": In questions about aluminium's resistance to corrosion, always mention the "protective oxide layer." Simply saying "it doesn't rust" is usually not enough for full marks.
- Typical Contexts: Expect questions asking why copper is preferred for home wiring (conductivity/ductility) versus why aluminium is used for long-distance power lines (low density/conductivity).
- Comparison: Be prepared to compare metals. For example, why is aluminium better than steel for food cans? (Aluminium doesn't corrode as easily as steel/iron).