Material Matters

Choosing the right material

Chemists choose materials by matching their properties to the job. This module covers three big ideas. Corrosion is when a metal reacts with its surroundings and is slowly eaten away. Iron rusts only when both oxygen and water are present, so we prevent it with a barrier such as paint or grease, or with sacrificial protection where a more reactive metal corrodes instead. Alloys are metals mixed with other elements. The added atoms are a different size, so they distort the layers and stop them sliding, which makes an alloy harder than the pure metal. Finally, ceramics such as glass and clay, polymers, and composites made of fibres set in a binder each suit different tasks.

Materials words

Learn these before you match any material, and notice which one is a way of stopping corrosion.

Thermosoftening against thermosetting

Two kinds of polymer behave very differently when heated, and the difference comes from their structure.

How to explain a material choice

Never just name a material. Tie its behaviour to how it is built. If a metal is being eaten away, ask whether oxygen and water can reach it, then block them or fit a metal that gives up its electrons first. If something must be hard, reach for an alloy and say the odd-sized atoms jam the sliding layers. If it must keep its shape when hot, pick a cross-linked polymer. A top answer names the property, the use, and the structure behind it.

Match the idea to what it is

Why is the alloy harder?

A pure metal is soft, but its alloy is much harder. Which explanation is correct?

• Different-sized atoms distort the layers so they cannot slide.
• The alloy is always much hotter than the pure metal.
• Alloys are harder only because they weigh less.
• Alloys are pure metals with nothing added.

Preventing rust

Select the TWO ways a business can stop iron rusting.

• Coat it with paint to keep out oxygen and water
• Attach a more reactive metal to corrode instead
• Keep it constantly wet with water
• Leave it bare in damp air

Gold in the alloy

A gold ring is made from an alloy that is 75 per cent gold by mass. The ring has a mass of 40 grams. Work out the mass of pure gold in the ring, in grams.

Order how iron rusts

Put the stages of iron rusting in order, earliest first.

• Bare iron is exposed to both oxygen and water
• The iron reacts with the oxygen and water
• Rust, a hydrated iron oxide, forms on the surface
• The rust flakes off, exposing fresh iron to corrode

Complete the facts

A metal that reacts with its surroundings and wears away is undergoing _____. A metal mixed with other elements to make it harder is an _____. A polymer with cross-linked chains that does not melt on heating is a _____ polymer. Fitting a more reactive metal to corrode in place of another is _____ protection.

Match the material to its use

Spot the true materials facts

Tap the TWO statements that are true about these materials.

• Rusting of iron needs both oxygen and water
• An alloy is usually harder than the pure metal
• A thermosetting polymer melts easily and reshapes
• A composite is a single pure element

Choose and explain

Read each case and choose the best response, with a reason.

• A steel ship hull sits in seawater and will rust. Engineers bolt blocks of zinc, a more reactive metal, to the hull. Why does this protect it?
• A designer needs a handle that keeps its shape even when it gets hot near an oven. Which material suits it?
• A pure metal coin bends too easily in a pocket. How could the mint make it harder?

Explain why materials behave as they do

A company builds outdoor railings from steel and wants them to stay hard and rust-free. Explain the chemistry it should use.

• Explain why the steel would rust if left bare
• Explain one barrier method and how sacrificial protection works
• Explain why steel, an alloy, is harder than pure iron
• Use the idea of distorted layers in your explanation
• Finish with a judgement on how to keep the railings strong and protected