🔔 Bronze
Copper + tin. Bronze is used for statues, medals and bells.
Using Materials » Alloys as Useful Materials
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.10.3.2
- Recall the metals in bronze, brass and gold jewellery, and a use of each
- Work out the percentage of gold from a carat value
- Explain why high carbon, low carbon and stainless steels suit different jobs
- Use data to match an alloy to a use
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Unlock This CourseMost metals are alloys
Look around you. The taps in your kitchen, the knife and fork you eat with and the frame of a plane are all made from alloys. Remember, an alloy is a mixture of a metal with other elements. Most metals in everyday use are alloys, not pure metals, because we can choose the mix to give the properties a job needs.
In this lesson you will learn the alloys the spec names, what they are made from and what they are used for.
Key terms:
- Bronze: an alloy of copper and tin.
- Brass: an alloy of copper and zinc.
- Carat: the unit used to measure the proportion of gold in a gold alloy.
- Steel: an alloy of iron that contains specific amounts of carbon and other metals.
Bronze and brass
Brass is copper mixed with zinc, giving trumpets that warm golden shine
Both of these alloys have copper as the main metal. What changes is the second metal.
🎺 Brass
Copper + zinc. Brass is used for musical instruments, door handles and taps.
Remember it
Bronze has tin. Brass has zinc. Both start with copper.
Gold jewellery and carats
Pure 24 carat gold is too soft, so rings like these are alloyed with silver, copper and zinc - 18 carat is 75% gold
Pure gold is very soft, so it bends and scratches easily. Gold used as jewellery is usually an alloy of gold with silver, copper and zinc. This makes it harder wearing.
The proportion of gold in the alloy is measured in carats.
- 24 carat is 100% gold. This is pure gold.
- 18 carat is 75% gold.
The scale has 24 parts, so each carat is 1/24 of the total. To find the percentage of gold:
percentage of gold = (carats ÷ 24) × 100
Worked example
A ring is 9 carat gold. What percentage of it is gold?
(9 ÷ 24) × 100 = 0.375 × 100 = 37.5%
So 62.5% of the ring is other metals such as silver, copper and zinc.
Check your answer
18 carat should come out at 75%. (18 ÷ 24) = 0.75, and 0.75 × 100 = 75%. The higher the carat number, the more gold the alloy contains.
Steels
Steels are alloys of iron. They contain specific amounts of carbon and sometimes other metals. Changing the amount of carbon, or adding other metals, gives steels with very different properties.
🔩 High carbon steel
Strong but brittle. Used where strength matters, such as cutting tools.
🚗 Low carbon steel
Softer and more easily shaped. Used for car body panels and nails.
🍴 Stainless steel
Contains chromium and nickel. Hard and resistant to corrosion. Used for cutlery and sinks.
Brittle means that something breaks or snaps instead of bending. Ordinary iron rusts, as you saw in the lesson on corrosion. Adding chromium and nickel to make stainless steel is why cutlery can be washed again and again without rusting.
Aluminium alloys
Aluminium alloys have a low density. This means they are light for their size. That is useful when a heavy material would be a problem, for example in aircraft and bicycle frames.
Choosing an alloy from data
In the exam you may be given the composition and properties of an alloy you have not met before. Do not panic. Follow these steps:
- Read the composition. Which metals are in it, and in what proportions?
- Read the properties. Is it hard, soft, light, or resistant to corrosion?
- Match the property to the job. A light alloy suits aircraft. A corrosion resistant alloy suits a sink.
Worked example
Alloy X contains 90% copper and 10% tin. It is hard and does not corrode easily. What could it be used for?
Copper and tin make it a bronze. Because it is hard and does not corrode easily, it could be used for a ship's bell or a statue.
Common mistakes
Mixing up bronze and brass. Saying 18 carat gold is 18% gold (it is 75%). Saying high carbon steel is softer than low carbon steel. It is the other way round: high carbon steel is strong but brittle.
Exam-style question
A jeweller makes a necklace from 18 carat gold. (a) What percentage of the necklace is gold? (b) Give one reason why the jeweller does not use 24 carat gold. (c) Name the metals that can be mixed with gold in jewellery.
Model answer
(a) 75%.
(b) 24 carat gold is pure gold, which is too soft for jewellery. Mixing it into an alloy makes it harder wearing.
(c) Silver, copper and zinc.
Exam tip
For carat questions, always divide by 24 first. Write your working down, even if you can do it in your head.