Alloys, Ceramics, Polymers and Composites

(Separate Chemistry only) The whole of 4.10.3 (using materials) is assessed on AQA GCSE Chemistry, not on Combined Science.

Most metals in everyday use are not pure. They are alloys: mixtures of a metal with one or more other elements, added on purpose to improve the properties.

The reason comes from how metals are held together. In a pure metal the atoms are all the same size and pack into regular layers. When a force is applied, these layers slide over one another, so a pure metal is relatively soft and easily bent. An alloy contains atoms of a different size mixed in. These larger or smaller atoms distort the regular layers, so the layers can no longer slide over each other easily. The alloy is therefore harder than the pure metal.

An alloy is a mixture, not a compound. The added atoms sit among the metal atoms and disrupt the layers, which is why the alloy is harder.

This single idea, that different-sized atoms stop the layers sliding, explains every alloy in this lesson. Pure copper, pure iron and pure gold are all too soft for most jobs, so they are alloyed to make them useful.

Three copper- and gold-based alloys are named in the specification, and you should be able to give a use of each.

Bronze is an alloy of copper and tin. It is harder than copper and resists corrosion, which is why it is used for statues, medals and ships' propellers.

Brass is an alloy of copper and zinc. It is hard and easy to shape, with a gold-like appearance, so it is used for musical instruments, door fittings and taps.

Pure gold is very soft, so gold jewellery is usually an alloy of gold with silver, copper and zinc. The proportion of gold is measured in carats. Pure gold is 24 carat (100% gold). The fraction of gold in any carat value is that number out of 24.

Alloy	Made from	A use
Bronze	Copper + tin	Statues, medals, propellers
Brass	Copper + zinc	Musical instruments, fittings
Gold (jewellery)	Gold + silver/copper/zinc	Rings and other jewellery

Worked example — the gold content of 18 carat gold.

$\text{proportion of gold}=\frac{18}{24}=0.75=75\%$

So 18 carat gold is 75% gold. Checking two more: 24 carat is $\frac{24}{24}=100\%$, and 9 carat is $\frac{9}{24}=0.375=37.5\%$ gold.

Steels are alloys of iron with carbon and, in some cases, other metals. Small changes in composition give steels with very different properties, which is why steel is the most widely used alloy.

The amount of carbon is the key variable. High-carbon steel is very hard and strong but brittle (it snaps rather than bends). Low-carbon steel is softer and more easily shaped. Adding other metals gives further properties: stainless steel contains chromium and nickel, which make it hard and resistant to corrosion.

Steel	Composition	Properties	A use
High-carbon steel	Iron + more carbon	Very hard, strong, brittle	Cutting tools, blades
Low-carbon steel	Iron + less carbon	Softer, easily shaped	Car body panels
Stainless steel	Iron + chromium + nickel	Hard, corrosion-resistant	Cutlery, sinks, surgical tools

More carbon makes steel harder but more brittle; less carbon makes it softer but easier to shape. Chromium and nickel are what make stainless steel resist rusting.

Aluminium alloys are the other named group. Their key property is low density, so they are strong for their weight. This makes them ideal where lightness matters, such as aircraft bodies.

Ceramics are made by shaping and heating materials from the ground. Two types of glass are named, along with clay ceramics.

Soda-lime glass is the ordinary glass used for windows and bottles. It is made by heating a mixture of sand, sodium carbonate and limestone together until they melt; the molten mixture is then shaped and allowed to cool.

Borosilicate glass is made from sand and boron trioxide. It melts at a higher temperature than soda-lime glass, which means it can be heated strongly without softening. This is why borosilicate glass is used for laboratory glassware and oven-proof kitchenware.

Clay ceramics, such as pottery and bricks, are made by shaping wet clay and then heating it in a furnace. Heating drives off water and permanently hardens the clay into a rigid solid.

Material	Made from	Key point
Soda-lime glass	Sand + sodium carbonate + limestone	Ordinary glass; melts more easily
Borosilicate glass	Sand + boron trioxide	Higher melting point; heat-resistant
Clay ceramics	Wet clay, shaped then heated	Hard, brittle; bricks and pottery

The properties of a polymer depend on the monomer used to make it and on the reaction conditions. A striking example is that two different plastics are made from the same monomer, ethene.

Low density poly(ethene), or LD poly(ethene), is made from ethene using high pressure and a trace of oxygen. Its chains are branched, so they cannot pack closely; this makes it flexible and low density, used for plastic bags and squeezy bottles.

High density poly(ethene), or HD poly(ethene), is made from ethene using a catalyst at a lower temperature and pressure. Its chains are straighter and pack closely, making it more rigid and stronger, used for water pipes and buckets.

The exam point: LD and HD poly(ethene) are both made from ethene. The difference in properties comes from the different catalyst and reaction conditions, which change how the chains are arranged.

Both are addition polymers of the same alkene, yet changing the conditions changes the structure of the product, and therefore its properties and uses.

Polymers fall into two groups by what happens when they are heated, and the difference is explained by their structure.

Thermosoftening polymers are made of individual polymer chains, tangled together but not chemically joined. Only weak intermolecular forces hold the separate chains to each other. When heated, these weak forces are overcome, the chains can move apart, and the polymer melts. It can then be moulded into a new shape and will melt again if reheated.

Thermosetting polymers have their chains joined together by strong covalent bonds called cross-links, forming one giant network. Heating does not break these strong cross-links, so a thermosetting polymer does not melt. Instead it keeps its shape or chars if heated strongly.

Feature	Thermosoftening	Thermosetting
Structure	Separate tangled chains	Chains joined by covalent cross-links
Forces between chains	Weak intermolecular forces	Strong covalent cross-links
On heating	Melts; can be remoulded	Does not melt

Cross-links are the key term. Thermosetting polymers do not melt because covalent cross-links lock the chains into a rigid network; thermosoftening polymers have no such cross-links, only weak forces.

A composite is made of two materials combined so that the mixture performs better than either part alone. Most composites are a matrix (or binder) surrounding fibres or fragments of a reinforcement material.

The reinforcement provides strength, while the matrix holds it together and gives shape. Because you can choose the two parts, composites are designed to have exactly the properties needed for a job.

Composite	Reinforcement	Matrix	Why it is used
Fibreglass	Glass fibres	Plastic (resin)	Strong but light; boats, car bodies
Carbon fibre	Carbon fibres	Plastic (resin)	Very strong and very light; bikes, aircraft
Reinforced concrete	Steel rods	Concrete	Concrete is strong when squashed, steel adds strength when stretched

You should be able to compare the properties of glass, ceramics, polymers, composites and metals quantitatively when given data, and use those properties to choose the right material for a use. For example, a bicycle frame needs to be strong yet light: carbon-fibre composite beats steel on weight for the same strength, which is why racing bikes use it despite the higher cost.

1. Calling an alloy a compound

An alloy is a mixture, not a compound. The atoms are not chemically bonded in fixed ratios; the added atoms simply sit among the metal atoms and disrupt the layers. If asked why an alloy is harder, describe the different-sized atoms stopping the layers sliding.

2. Muddling bronze and brass

Bronze is copper + tin; brass is copper + zinc. A quick way to keep them apart: brass and zinc both contain the letter z-sound only loosely, so learn the pair directly. Both are copper alloys, so name the second element to get the mark.

3. Getting carat fractions wrong

Carats are always out of 24, not 100. So 18 carat is $\frac{18}{24}=75\%$ gold, not 18%. Always divide the carat number by 24.

4. Saying LD and HD poly(ethene) come from different monomers

Both are made from the same monomer, ethene. The difference is the catalyst and reaction conditions, which change the chain structure and therefore the properties.

5. Explaining thermosetting without cross-links

The reason a thermosetting polymer does not melt is the covalent cross-links joining the chains into a network. Do not just say "the bonds are stronger"; name the cross-links and say heating does not break them.

6. Forgetting the two parts of a composite

A composite is a matrix (binder) plus a reinforcement. Naming only "it is strong and light" misses the structure marks. State that fibres or fragments of reinforcement are held within a matrix.