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15.2 Useful Alloys

GCSE Chemistry · AQA · Chapter 15: Using Our Resources

A free GCSE Chemistry lesson for AQA (8462) from OBA School: why most metals we use are alloys: bronze, brass, gold, steels and aluminium alloys. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain why alloys are harder than pure metals, using the arrangement of atoms
  • Recall the main metals in bronze and brass, and their uses
  • Use carats to work out the proportion and mass of gold in an alloy
  • Compare low-carbon, high-carbon and stainless steels, link hardness to carbon content and link properties to uses
  • Interpret data on the composition and properties of alloys to choose one for a job

Key words

  • Alloy: A mixture of a metal with one or more other elements, usually other metals or carbon.
  • Bronze: An alloy of copper and tin.
  • Brass: An alloy of copper and zinc.
  • Carat: A measure of the purity of gold. Pure gold is 24 carat; 18 carat gold is 18 parts gold out of 24.
  • Steel: An alloy of iron with a small, controlled amount of carbon, and sometimes other metals.
  • Stainless steel: Steel containing chromium and nickel. It is hard and resists corrosion.

Why use alloys?

Most pure metals are too soft for everyday use. In a pure metal all the atoms are the same size and sit in regular layers, so the layers can slide over each other easily when a force is applied.

In an alloy, atoms of other elements have a different size. They distort the layers, so it is much harder for the layers to slide. This makes alloys harder (and often stronger) than the pure metal.

Key fact: An alloy is a mixture, not a compound. The metals are not chemically bonded together in fixed proportions, so the composition of an alloy can be changed to suit the job.

Iron straight from the blast furnace is about 96% iron. The other 4% is mainly carbon, which makes it very brittle, so it is turned into steels with carefully controlled amounts of carbon and other metals.

Copper alloys: bronze and brass

  • Bronze; Made from copper and tin; Useful properties harder than copper, tough, resists corrosion (even in sea water); Uses statues, medals, bells, ship propellers
  • Brass; Made from copper and zinc; Useful properties harder than copper but still easy to shape; gold coloured; does not corrode easily; Uses musical instruments, door handles, plumbing fittings, electrical plug pins

Worked example: percentage composition

A 400 g sample of brass contains 260 g of copper. The rest is zinc.
Mass of zinc = 400 − 260 = 140 g
Percentage of copper = 260 ÷ 400 × 100 = 65%; percentage of zinc = 100 − 65 = 35%

Answer: 65% copper, 35% zinc

Gold alloys and carats

Pure gold is very soft, so gold used in jewellery is usually alloyed with silver, copper and zinc. This makes it harder wearing and can change its colour (rose gold contains more copper). The purity of gold is measured in carats.

percentage of gold = (number of carats ÷ 24) × 100

mass of gold = mass of item × (number of carats ÷ 24)

Worked example: mass of gold in a ring

An 18 carat gold ring has a mass of 6.0 g.
Fraction of gold = 18 ÷ 24 = 0.75 (75%)
Mass of gold = 6.0 × 0.75 = 4.5 g

Answer: 4.5 g of gold

Steels

Iron straight from the blast furnace (cast iron) contains about 4% carbon, so it is very hard but brittle, while pure iron is too soft for most uses. Steel-making removes most of the carbon and then adds back carefully controlled amounts of carbon and other metals. Changing the composition changes the properties.

  • Low-carbon (mild) steel; Composition iron with about 0.1–0.3% carbon; Properties softer, easily shaped, does not crack when bent; Uses car body panels, wire, nails, girders
  • High-carbon steel; Composition iron with about 0.6–1.5% carbon; Properties very strong and hard, but brittle; Uses cutting tools, drill bits, chisels, railway lines
  • Stainless steel; Composition iron with chromium (about 18%) and nickel (about 8%); Properties hard and very resistant to corrosion; Uses cutlery, sinks, saucepans, surgical instruments, chemical reaction vessels
  • Other alloy steels; Composition iron with a few % of another metal, e.g. nickel, tungsten, manganese; Properties each metal gives a special property, e.g. tungsten steel stays hard when very hot; Uses high-speed cutting tools, heavy-duty springs, armour plate

Cost: Low-carbon steels and high-carbon steels are the cheapest steels. Alloy steels cost more because of the added metals, and stainless steel costs most, so it is only chosen when resistance to corrosion is worth the extra cost.

Aluminium alloys and choosing an alloy

Pure aluminium has a low density but is quite soft and weak. Adding small amounts of metals such as copper, magnesium or zinc gives aluminium alloys that are much stronger but still light. There are hundreds of aluminium alloys with very different properties. They are used for aircraft bodies (an alloy with copper and magnesium is a classic aircraft metal), bicycle frames, car wheels and drinks cans.

  • Pure aluminium; Density (g/cm³) 2.7; Relative strength low; Resists corrosion? yes
  • Aluminium alloy; Density (g/cm³) 2.8; Relative strength high; Resists corrosion? yes
  • Low-carbon steel; Density (g/cm³) 7.9; Relative strength high; Resists corrosion? no (rusts)
  • Stainless steel; Density (g/cm³) 8.0; Relative strength high; Resists corrosion? yes

Exam tip: When choosing from data, quote the figures. For example: “the aluminium alloy is best for an aircraft panel because it is strong but its density (2.8 g/cm³) is about a third of that of steel (7.9 g/cm³), so the plane is lighter and uses less fuel.”

Common mistakes

  • Calling an alloy a compound. It is a mixture, so its composition can vary.
  • Saying alloys are harder because the bonds are stronger. They are harder because different-sized atoms distort the layers, so the layers cannot slide easily.
  • Thinking carats measure mass. Carats measure purity: 24 carat is pure gold whatever the mass.
  • Mixing up bronze (copper + tin) and brass (copper + zinc).
  • Saying high-carbon steel is best for everything because it is strongest. It is brittle, so it is not used where the metal must bend.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the percentage of gold in 18 carat gold?

Answer: 75 %

Working: 18 ÷ 24 × 100 = 75%.

Question 2. Why are alloys harder than the pure metals they are made from? (Choose from: Different-sized atoms distort the layers, so the layers cannot slide easily, The atoms in an alloy are joined by stronger covalent bonds, Alloys have no delocalised electrons, The atoms in an alloy are all the same size, so they pack tightly.)

Answer: Different-sized atoms distort the layers, so the layers cannot slide easily

Working: In a pure metal, regular layers slide easily. Atoms of a different size distort the layers, making sliding harder.

Question 3. Which steel is best for making a wood chisel? (Choose from: High-carbon steel, Low-carbon steel, Pure iron, Brass.)

Answer: High-carbon steel

Working: High-carbon steel is very hard and strong (though brittle), so it keeps a sharp cutting edge.

Related lessons

Continue in the full course

This lesson is section 15.2 (AGCCH1502) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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