Revision notes for AQA GCSE Chemistry Alloys as useful materials. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Alloys as useful materials
What you'll learn
- What an alloy is, and why most everyday metals are alloys rather than pure metals.
- How the particle structure of an alloy can make it harder than a pure metal.
- The key GCSE examples: bronze, brass, gold alloys, steels and aluminium alloys.
- How to use percentages, fractions and ratios to interpret alloy compositions.
Starting point: what properties do metals need?
A metal is an element that usually conducts heat and electricity well, is shiny when freshly cut, and can often be shaped. However, the properties needed depend on the job.
For example, a bridge needs a metal that is strong, a drinks can needs a metal that is light and easily shaped, and cutlery needs a metal that resists corrosion.
Some useful property words:
- Strong means difficult to break under force.
- Hard means difficult to scratch or dent.
- Malleable means can be hammered or pressed into shape.
- Brittle means likely to snap or shatter rather than bend.
- Corrosion means damage caused by chemical reactions with substances in the environment, such as oxygen and water.
- Density means mass per unit volume. A low-density material is light for its size.
Pure metals and alloys
A pure metal contains only one type of metal atom. Pure copper, pure iron and pure gold are examples.
In a pure metal, the atoms are arranged in regular layers. These layers can slide over each other fairly easily, which is why many pure metals are soft and malleable.
Definition
Alloy
An alloy is a mixture of two or more elements, where at least one of the elements is a metal.
Most metals used in everyday life are alloys because alloying can improve properties such as strength, hardness, resistance to corrosion, or density.
Key Idea
Why alloys are useful
Alloys are useful because their properties can be chosen and adjusted by changing which elements are mixed and in what proportions.
Why alloys are often harder
In an alloy, atoms of different elements usually have different sizes. These different-sized atoms distort the regular layers found in a pure metal.
That makes it harder for the layers of atoms to slide over each other. So many alloys are harder and stronger than the pure metal they are based on.
Example
Explaining why an alloy is harder
A student says: “Bronze is harder than pure copper because it contains tin atoms as well as copper atoms.” Explain this using the structure of the metal.
- Pure copper has regular layers of copper atoms, so the layers can slide over each other fairly easily.
- Bronze contains copper atoms and tin atoms, which are different sizes.
- The tin atoms distort the regular arrangement of copper atoms, so the layers cannot slide as easily.
- Therefore bronze is harder than pure copper.
Common Mistake
Saying alloys are compounds
An alloy is a mixture, not a compound. The elements in an alloy are not chemically joined in a fixed formula like NaCl or H₂O.
Bronze and brass
Bronze
Bronze is an alloy of copper and tin.
Bronze is harder than pure copper and resists corrosion well. It has been used for statues, medals, coins, bearings and ship propellers.
Key Idea
Bronze
Bronze = copper + tin. A typical GCSE use to remember is statues or medals.
Brass
Brass is an alloy of copper and zinc.
Brass is harder than pure copper, can resist corrosion, and has an attractive gold-like appearance. It is often used for door handles, taps, screws and musical instruments such as trumpets.
Key Idea
Brass
Brass = copper + zinc. A typical GCSE use to remember is musical instruments or door fittings.
Tip
Bronze vs brass
Both are copper alloys. Remember: bronze has tin, while brass has zinc.
Gold alloys and carats
Pure gold is very soft, so jewellery made from pure gold would scratch and bend too easily. Gold used in jewellery is usually an alloy with metals such as silver, copper and zinc.
The proportion of gold in a gold alloy is measured in carats.
Definition
Carat
For gold alloys, 24 carat means 100% pure gold. So the percentage of gold is found by comparing the carat value with 24.
For example:
- 24 carat gold = 100% gold
- 18 carat gold = 75% gold
- 9 carat gold = 37.5% gold
The calculation is:
percentage of gold=carat value24×100\text{percentage of gold} = \frac{\text{carat value}}{24} \times 100percentage of gold=24carat value×100Example
Calculating the percentage of gold
Calculate the percentage of gold in 18 carat gold.
- Use the carat relationship: 24 carat means 100% gold.
- Write 18 carat as a fraction of 24 carat: 1824\frac{18}{24}2418.
- Convert the fraction to a percentage:
- So 18 carat gold contains 75% gold.
Example
Calculating the mass of gold in jewellery
A ring has a mass of 12 g and is made from 9 carat gold. Calculate the mass of gold in the ring.
- Convert 9 carat into a percentage of gold:
- Convert the percentage into a decimal: 37.5% = 0.375.
- Multiply by the total mass of the ring:
- The ring contains 4.5 g of gold.
Common Mistake
Thinking 18 carat means 18% gold
18 carat does not mean 18% gold. It means 18 parts out of 24 are gold, which is 75% gold.
Steels: alloys of iron
Steel is an alloy based on iron. Steels contain specific amounts of carbon, and some steels also contain other metals.
Changing the amount of carbon changes the properties of the steel.
Low carbon steel
Low carbon steel contains a small amount of carbon.
It is softer than high carbon steel and more easily shaped. This makes it useful for objects that need to be pressed, bent or formed, such as car body panels.
High carbon steel
High carbon steel contains more carbon.
It is strong and hard, but also more brittle. This makes it useful for cutting tools, blades and drill bits, where hardness is important.
Stainless steel
Stainless steels are steels containing chromium and nickel.
They are hard and resistant to corrosion. This makes them useful for cutlery, sinks, kitchen equipment and surgical instruments.
Key Idea
Steels
Steels are alloys of iron. Carbon affects hardness and brittleness, while chromium and nickel make stainless steel resistant to corrosion.
Example
Choosing a steel for a use
A manufacturer needs a metal for a kitchen sink. It must be hard and resist corrosion. Which steel is most suitable?
- A kitchen sink is exposed to water and oxygen, so corrosion resistance is important.
- Stainless steel contains chromium and nickel, which make it resistant to corrosion.
- Low carbon steel is easier to shape, but it would not resist corrosion as well.
- Therefore stainless steel is the best choice.
Common Mistake
Mixing up strong and brittle
High carbon steel is strong, but it is also brittle. Do not write that high carbon steel is “more flexible” or “easy to shape” — that describes low carbon steel better.
Aluminium alloys
Aluminium itself has a low density, meaning it is light for its size. Aluminium alloys keep this useful low-density property, while often being stronger than pure aluminium.
This makes aluminium alloys useful where low mass matters, such as aircraft parts, bicycle frames and transport materials.
Key Idea
Aluminium alloys
Aluminium alloys are useful because they have low density, so they are lightweight.
Interpreting unfamiliar alloys
In the exam, you may be given information about an alloy you have not memorised. You should use the data to connect composition to properties and then to uses.
Composition might be given as:
- percentages, such as 70% copper and 30% zinc
- fractions, such as three-quarters gold
- ratios, such as copper : zinc = 7 : 3
Example
Interpreting an alloy composition
An alloy contains 70% copper and 30% zinc. It is corrosion-resistant and can be shaped into tubes. Suggest a suitable use and justify your answer.
- Identify the alloy from its composition: copper and zinc make brass.
- Match the properties to a use: corrosion resistance is useful where the alloy may contact water or air.
- Tubes and fittings need a material that can be shaped, so the ability to form tubes is relevant.
- A suitable use is taps or plumbing fittings, because brass resists corrosion and can be shaped.
Tip
How to justify a use
A good alloy answer usually links three things: composition → property → use. For example, “stainless steel contains chromium and nickel, so it resists corrosion, so it is suitable for cutlery.”
Summary table
| Alloy | Main elements | Key properties | Example uses |
|---|---|---|---|
| Bronze | Copper and tin | Harder than copper, corrosion-resistant | Statues, medals, coins |
| Brass | Copper and zinc | Hard, corrosion-resistant, attractive appearance | Musical instruments, door handles, taps |
| Gold alloy | Gold with silver, copper and zinc | Harder than pure gold | Jewellery |
| Low carbon steel | Iron with a small amount of carbon | Softer, more easily shaped | Car body panels |
| High carbon steel | Iron with more carbon | Strong and hard, but brittle | Cutting tools, blades |
| Stainless steel | Iron with carbon, chromium and nickel | Hard and corrosion-resistant | Cutlery, sinks, surgical instruments |
| Aluminium alloy | Aluminium with other elements | Low density, often stronger than pure aluminium | Aircraft parts, bicycle frames |
Exam technique
In the exam
- For recall questions, learn the named alloy pairs: bronze = copper + tin, brass = copper + zinc, and stainless steel contains chromium and nickel.
- For explanation questions, mention the distorted layers of atoms in alloys and link this to layers being harder to slide.
- For calculation questions about gold, use 24 carat as 100% gold, then convert using fractions and percentages.
Self review
Check yourself
- Why is bronze harder than pure copper?
- What is the percentage of gold in 12 carat gold?
- Which steel would you choose for cutlery, and why?
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