Uses of metals

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).

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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.

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Extended Content (Extended Only)

There is no additional supplement content for this specific sub-topic in the IGCSE Chemistry syllabus.

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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: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$

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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.

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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).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) Aluminium is used in the manufacture of overhead electrical cables. State two physical properties of aluminium that make it suitable for this use. [2]

(b) Explain why aluminium is often used in food containers. [3]

Worked Solution:

(a)

1. Low density. Aluminium is lightweight, so the cables do not sag excessively between supports.

2. Good electrical conductivity. Aluminium allows electricity to flow easily through the cable.

How to earn full marks:

• One mark for each correct physical property.
• Accept "light weight" for "low density."
• Do not accept "strong" as a physical property related to this use.

(b)

1. Aluminium is resistant to corrosion. Aluminium forms a thin layer of aluminium oxide which prevents further reaction with food.

2. This layer is inert and non-toxic. The oxide layer does not react with the food.

3. Aluminium is malleable. It can be easily shaped into containers.

How to earn full marks:

• One mark for stating aluminium is resistant to corrosion.
• One mark for explaining the role of the aluminium oxide layer.
• One mark for stating the oxide layer is inert/non-toxic.

Common Pitfall: Many students only mention conductivity when discussing aluminium in overhead cables, forgetting the crucial role of low density in preventing excessive sagging. Also, remember that "strong" is too vague; specify which type of strength (tensile, compressive) if relevant.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

(a) State two reasons why copper is used in electrical wiring. [2]

(b) Describe the property of copper that allows it to be drawn into wires. [2]

(c) Explain why copper is not used to make aircraft bodies. [2]

Worked Solution:

(a)

1. Good electrical conductivity. Copper allows electricity to flow easily through the wire.
2. Ductility. Copper can be drawn into wires.

How to earn full marks:

• One mark for each correct property.
• Accept "high conductivity" or "conducts electricity well."
• Do not accept "strong" or "cheap."

(b)

1. Ductility is the ability of a metal to be drawn into wires. A ductile metal can be stretched into a thin wire without breaking.

2. This is because the layers of atoms can slide over each other without disrupting the metallic bonding. The metallic bonds are strong enough to hold the atoms together but allow them to move.

How to earn full marks:

• One mark for defining ductility as the ability to be drawn into wires.
• One mark for mentioning the sliding layers of atoms.

(c)

1. Copper is too dense/heavy. Aircraft need to be lightweight to fly efficiently.

2. This would increase the weight of the aircraft significantly. Making it more expensive to operate.

How to earn full marks:

• One mark for stating copper is too dense/heavy.
• One mark for explaining the consequence of increased weight (reduced efficiency or increased cost).

Common Pitfall: Students sometimes confuse malleability and ductility. Remember that ductility is specifically about being drawn into wires, while malleability is about being hammered into sheets. Also, don't just say "heavy"; use the term "dense" or "high density" for scientific accuracy.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student investigates the suitability of aluminium and steel for making saucepans. They test the rate at which each metal heats up when placed on a hotplate.

(a) State two other properties of aluminium that make it suitable for making saucepans. [2]

(b) Describe an experiment the student could carry out to compare the rate of heating of aluminium and steel saucepans. Include details of the apparatus, procedure, and measurements taken. [4]

(c) Suggest one safety precaution the student should take during the experiment and explain why it is necessary. [2]

Worked Solution:

(a)

1. Good conductor of heat. Allows heat to be transferred evenly throughout the saucepan to cook the food.

2. Resistance to corrosion. Prevents the saucepan from rusting or corroding when exposed to water and food.

How to earn full marks:

• One mark for each correct property.
• Accept "high thermal conductivity" for "good conductor of heat."
• Do not accept "strong" or "cheap."

(b)

1. Use two saucepans, one made of aluminium and one made of steel, of the same size and shape. Ensures a fair comparison between the two metals.

2. Place the same volume of water (e.g., 200 cm$^3$) in each saucepan. Keeps the initial conditions the same.

3. Place each saucepan on an identical hotplate set to the same power setting. Ensures the same amount of heat is supplied to each saucepan.

4. Measure the temperature of the water in each saucepan using a thermometer at regular intervals (e.g., every minute) for a set period of time (e.g., 10 minutes). Allows comparison of the temperature increase over time.

How to earn full marks:

• One mark for using two saucepans of the same size/shape, one aluminium and one steel.
• One mark for using the same volume of water in each saucepan.
• One mark for using identical hotplates at the same power setting.
• One mark for measuring the temperature of the water at regular intervals.

(c)

1. Wear heatproof gloves. To prevent burns from handling the hot saucepans or hotplate.

2. Alternatively, use tongs to handle the hot saucepans. To avoid direct contact with the hot surface.

How to earn full marks:

• One mark for stating a valid safety precaution (e.g., heatproof gloves, tongs).
• One mark for explaining why the precaution is necessary (to prevent burns).
• Accept other reasonable safety precautions related to heat or electricity.

Common Pitfall: When describing experiments, be specific about how you'll control variables to ensure a fair test. For example, stating "same amount of water" isn't enough; you need to say "same volume of water" and give an example volume. Also, remember to link the safety precaution directly to the hazard present in the experiment.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A bridge is being designed to span a wide river. The engineers are considering using either steel or aluminium for the main support structure.

(a) State one advantage and one disadvantage of using steel for the bridge support structure. [2]

(b) State one advantage and one disadvantage of using aluminium for the bridge support structure. [2]

(c) The engineers perform tests to compare the tensile strength of two different steel alloys: Alloy A and Alloy B. The tensile strength is the maximum stress a material can withstand before breaking. A sample of each alloy, with a cross-sectional area of $2.0 \times 10^{-4} \ m^2$, is subjected to increasing force until it breaks. Alloy A breaks at a force of $1.2 \times 10^5 \ N$, and Alloy B breaks at a force of $1.5 \times 10^5 \ N$.

Calculate the tensile strength of each alloy. [3]

(d) Based on your calculations in (c) and your knowledge of the properties of steel and aluminium, explain which material (steel or aluminium) and which steel alloy (A or B) you would recommend for the bridge support structure. Justify your answer. [2]

Worked Solution:

(a)

1. Advantage of steel: High tensile strength. Steel can withstand large forces without breaking.

2. Disadvantage of steel: Susceptible to corrosion (rust). Steel can rust if not properly protected, weakening the structure.

How to earn full marks:

• One mark for stating a valid advantage of steel.
• One mark for stating a valid disadvantage of steel.

(b)

1. Advantage of aluminium: Resistant to corrosion. Aluminium forms a protective oxide layer that prevents further corrosion.

2. Disadvantage of aluminium: Lower tensile strength than steel. Aluminium is not as strong as steel and may not be able to withstand as much force.

How to earn full marks:

• One mark for stating a valid advantage of aluminium.
• One mark for stating a valid disadvantage of aluminium.

(c)

1. Tensile strength of Alloy A = Force / Area Tensile strength is defined as force per unit area.

2. Tensile strength of Alloy A = $(1.2 \times 10^5 \ N) / (2.0 \times 10^{-4} \ m^2) = 6.0 \times 10^8 \ N/m^2$ Substituting the given values into the formula.

3. Tensile strength of Alloy B = Force / Area Tensile strength is defined as force per unit area.

4. Tensile strength of Alloy B = $(1.5 \times 10^5 \ N) / (2.0 \times 10^{-4} \ m^2) = 7.5 \times 10^8 \ N/m^2$ Substituting the given values into the formula.

How to earn full marks:

• One mark for stating the correct formula: Tensile strength = Force / Area.
• One mark for calculating the tensile strength of Alloy A: $\boxed{6.0 \times 10^8 \ N/m^2}$.
• One mark for calculating the tensile strength of Alloy B: $\boxed{7.5 \times 10^8 \ N/m^2}$.

(d)

1. I would recommend using steel Alloy B. Steel has a higher tensile strength than aluminium, making it more suitable for a bridge support structure.

2. Alloy B has a higher tensile strength than Alloy A, meaning it can withstand greater forces before breaking. Alloy B is the stronger of the two steel alloys.

How to earn full marks:

• One mark for recommending steel (over aluminium) and Alloy B (over Alloy A).
• One mark for justifying the choice based on higher tensile strength.

Common Pitfall: Remember that tensile strength is force per unit area. Don't just calculate force; divide by the cross-sectional area to get the correct units of $N/m^2$ (Pascals). When justifying your choice of material, make sure to explicitly link the chosen property (tensile strength) to the application (bridge support).