Choosing the Right Material

Introduction

Why are bridges made of steel and concrete rather than wood or glass? Why are drink bottles plastic or glass but not iron? Choosing a material means matching its properties to the demands of the job. This page brings together all the properties studied in this unit and shows how scientists and engineers use them to make, and justify, sensible choices.

Core explanation

Step 1 — list the requirements. Every use has demands. A frying pan must conduct heat, withstand high temperatures and not react with food. A window must be transparent, hard enough to resist scratching and weatherproof.

Step 2 — match properties. Compare candidate materials against each requirement: physical properties (density, strength, conductivity, melting point, transparency) and chemical properties (resistance to corrosion, flammability, reactivity with contents).

Step 3 — consider practical factors. - Cost: gold is an excellent, unreactive conductor but far too expensive for household wiring. - Availability and ease of shaping: some materials are hard to mould, cut or join. - Safety: toxic materials such as lead are avoided in water pipes and paints. - Environmental impact: energy needed to extract and process the material, and whether it can be recycled.

Step 4 — accept trade-offs. No material is perfect. Aluminium is lighter than steel and does not rust, but it is weaker and more expensive; aircraft use aluminium alloys and composites because low density outweighs cost. Car bodies usually use steel because strength and low cost matter more there.

Sustainability. Life-cycle thinking considers extraction, manufacture, use and disposal. Aluminium requires a lot of energy to extract from its ore, but recycling it uses only about 5% of that energy, so recyclable aluminium cans can be a sustainable choice when they are actually collected and recycled.

Step-by-step reasoning

Choosing a material for the handle of a saucepan:

1. Requirements: poor thermal conductor, withstands about 200 °C, strong, not slippery, safe to touch. 2. Candidates: steel, wood, heat-resistant polymer (Bakelite-type thermoset), glass. 3. Steel conducts heat too well; glass is brittle. 4. Wood and heat-resistant polymers are insulators and strong enough. 5. Choose a heat-resistant thermoset polymer (mouldable, durable, easy to clean) or wood.

Visual explanation

A decision matrix for a drinks container:

Requirement Glass PET plastic Aluminium --- --- --- --- Unreactive with drink ✓ ✓ ✓ (with inner coating) Low density (light) ✗ ✓ ✓ Resists breaking ✗ ✓ ✓ Transparent ✓ ✓ ✗ Recyclable ✓ ✓ ✓

Each material has strengths; the best choice depends on which requirements matter most.

Real-world analogy

Choosing a material is like picking a player for a sports team. The fastest runner might be a poor defender, and the tallest player might be slow. The coach lists what the position needs and chooses the player with the best overall fit, accepting some weaknesses.

Real-world example

Modern mobile phone screens use toughened glass, a ceramic chosen because it is transparent and very hard (resisting scratches from keys and sand), treated to make it less brittle. Plastic screens are cheaper and less likely to crack but scratch easily. Manufacturers accept the higher cost of glass because customers value a scratch-free display.

Why?

Why is copper, not silver, used for most electrical wiring even though silver conducts slightly better? The requirement is good conductivity at reasonable cost. Copper's conductivity is only about 6% lower than silver's, but it is far cheaper and more abundant, so it gives the best overall fit.

Common misconception

"The best material is the one with the best single property." Choices depend on the whole set of requirements, including cost, safety and environmental impact. The strongest material may be too heavy or expensive; the best conductor may be too costly.

Worked example

Question: Suggest a material for the cables of a suspension bridge and justify it.

Reasoning: Requirements: very high tensile strength, flexibility, durability outdoors, affordable in large quantities. Steel wire has very high tensile strength and toughness, is relatively cheap and can be protected from corrosion by galvanising (coating with zinc).

Answer: Galvanised steel wire, because it combines high tensile strength and toughness at reasonable cost, with the zinc coating preventing rust.

Quick check

1. Why is lead no longer used for household water pipes? Answer: Lead is toxic, and small amounts can dissolve into drinking water.

Exam focus

"Suggest and explain" questions reward specific properties linked to the use ("aluminium is used for aircraft because it has a low density and resists corrosion"). Include at least two properties and, where relevant, cost or environmental factors. Avoid vague terms such as "strong and good".

Advanced insight

Engineers use material property charts (often called Ashby charts) that plot one property against another — for example strength against density — for thousands of materials. Lines on the chart show the best materials for requirements such as "light but stiff", allowing choices to be made systematically rather than by guesswork.

Summary

To choose a material, list the requirements of the job, compare candidate materials' physical and chemical properties, and weigh cost, safety and environmental impact. Trade-offs are unavoidable. Good choices are justified with specific properties linked directly to the use.

Practice questions

1. Why are aircraft bodies made mainly from aluminium alloys and composites rather than steel? Answer: They have a much lower density, saving fuel, while still being strong and corrosion-resistant. 2. Suggest two properties needed in a material for oven gloves. Answer: Poor thermal conductor (insulator) and flexible (also heat-resistant or non-flammable). 3. Give one environmental factor to consider when choosing a material for drinks cans. Answer: Whether it can be recycled (or the energy needed to extract and process it). 4. Explain why toughened glass is used for phone screens despite being more expensive than plastic. Answer: It is transparent and much harder, so it resists scratching; the treatment makes it less likely to shatter.