Ch 2 · Uses of Metals · O Level Chemistry 5070
Topic 2 of 6 in Metals — O Level Chemistry 5070 — 38 lessons.
Introduction
Choosing the right metal for a job is an engineering decision, and Cambridge tests it in two ways: recall of a few set uses with their reasons, and data-response questions in which you must pick a metal from a table and justify the choice. The reasoning pattern is always the same: name the property, then connect it to the demand of the use. This lesson gives you the set examples and a method for the data questions.
Key ideas — Aluminium: three uses, three reasons
Aluminium is one of the most heavily used metals and the syllabus specifies three uses. Each is justified by a different property, so keep them distinct.
🧑🏫 Teacher's explanation
Think of choosing materials like choosing clothes for an activity. You wouldn't wear heavy denim boots to go swimming, nor would you wear thin silk to go mountain climbing. In chemistry, we select metals based on their 'personality'—their physical and chemical properties.
📖 Definitions
🔍 Worked example — Easy
An aircraft component is made of an aluminium alloy. If the volume of the component is 0.5 m^3 and the density of the alloy is 2700 kg/m^3, calculate the mass of the component.
🔍 Worked example — Standard
A student is asked to justify why aluminium is used for overhead power cables instead of steel. State two properties of aluminium that make it suitable for this use.
🔍 Worked example — Exam-style
Explain why aluminium is used for food cans despite being a reactive metal.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Copper in electrical wiring
Copper is used for household wiring, motor windings and printed circuits because of two properties:
🧑🏫 Teacher's explanation
Imagine you are trying to send a message through a long, thin garden hose. If the hose is too narrow or the material is too rough, the water (the electricity) won't flow smoothly. Copper is like a perfectly smooth, flexible hose. It allows electricity to flow with very little resistance, meaning we don't waste energy as heat.
📖 Definitions
🔍 Worked example — Easy
A copper wire has a mass of 50.0 g and a density of 8.96 g/cm^3. Calculate the volume of the wire.
🔍 Worked example — Standard
Explain why copper is preferred over silver for domestic electrical wiring, considering both conductivity and cost.
🔍 Worked example — Exam-style
A technician needs to choose between copper and aluminium for overhead power lines. Given that aluminium has a lower density than copper, justify why aluminium is used for long-distance overhead cables.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one physical property of copper that makes it suitable for making wires. [1] (b) Explain why silver is not used for this purpose despite having higher conductivity. [1] [2 marks]
Key ideas — Justifying a choice from data
Data questions give a table of properties for several metals, usually density, melting point, electrical conductivity, relative strength and cost, and ask which metal is best for a stated use. Use this routine:
🧑🏫 Teacher's explanation
Imagine you are buying a new smartphone. You don't just look at one feature; you look at a table of data: battery life, weight, camera quality, and price. If you want a phone for hiking, you'll prioritize low weight. If you want a phone for professional photography, you'll prioritize the camera. You might reject a phone that has a great camera but costs \2000 if your budget is \500.
📖 Definitions
🔍 Worked example — Easy
A table provides the following data for three metals: Metal A: Density 2.7 g/cm^3, Melting Point 660 °C, Cost \10/ extkg. Metal B: Density 7.8 g/cm^3, Melting Point 1538 °C, Cost \5/ extkg. Metal C: Density 1.7 g/cm^3, Melting Point 660 °C, Cost \50/ extkg. Which metal is best for making a lightweight, cheap cooking pot? Justify your answer.
🔍 Worked example — Standard
A manufacturer needs a metal for overhead power cables. The requirements are high electrical conductivity and low density. Metal X: Conductivity 5.9 × 10^7 S/m, Density 8.9 g/cm^3. Metal Y: Conductivity 5.7 × 10^7 S/m, Density 2.7 g/cm^3. Metal Z: Conductivity 4.0 × 10^7 S/m, Density 7.8 g/cm^3. Which metal is best?
🔍 Worked example — Exam-style
A new alloy is being tested for aircraft components. The component must be extremely light and strong. Metal 1: Density 2.7 g/cm^3, Strength 300 MPa. Metal 2: Density 7.8 g/cm^3, Strength 500 MPa. Metal 3: Density 4.5 g/cm^3, Strength 450 MPa. Which metal is best for the aircraft component?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
| :--- | :--- | :--- | :--- | | Aluminum | 2.7 | High | 5 | | Iron | 7.8 | Low | 2 | | Copper | 8.9 | Medium | 8 |
Worked example
Question. The table gives data for four metals.
🔍 Worked example — Easy
A metal used for food containers must be resistant to corrosion. Why is aluminium often chosen for this purpose instead of iron?
🔍 Worked example — Standard
Calculate the mass of a copper wire with a volume of 50 cm^3 and a density of 8.9 g cm^-3.
🔍 Worked example — Exam-style
A company needs to choose a metal for electrical wiring in a house. They have two options: Copper (Conductivity: 100, Cost: Medium) and Silver (Conductivity: 106, Very High). Justify why Copper is the standard choice.
⚠️ Common errors
Common mistakes
Practice
Answers Low density [B1]; good electrical conductivity [B1]. Aluminium reacts with oxygen in the air to form a layer of aluminium oxide [B1]; this layer is unreactive and coats the surface so the acid cannot reach the metal [B1]. Copper is a good electrical conductor so little energy is wasted [B1]; it is ductile so it can be drawn into wires [B1]. Titanium [B1]; it has the highest strength (9) of the metals given, so the frame can be made thin and still strong [B1]; its density (4.5 g cm⁻³) is much lower than steel (7.9 g cm⁻³), so the frame is lighter than a steel frame of similar strength, which matters more than cost for a racing bicycle [B1]. Accept aluminium with a justification based on low density and low cost, if the lower strength is acknowledged.