Alloys and their properties

O Level / IGCSE Chemistry · Metals. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

An alloy is a mixture of a metal with other elements. Brass is a mixture of copper and zinc. Stainless steel is a mixture of iron with other elements such as chromium, nickel and carbon. An alloy is a mixture, not a compound: the elements are not chemically joined and the proportions can vary.

In a pure metal the atoms are all the same size and lie in regular layers. When a force is applied the layers slide over each other, so the metal is soft and malleable. In an alloy the atoms of the added element are a different size. They break up the regular layers, so the layers can no longer slide over each other. This makes the alloy harder and stronger.

Rules to remember

Common mistake

Calling an alloy a compound, or saying the added atoms make stronger bonds. An alloy is a mixture, and it is harder because the different sized atoms stop the layers sliding.

Worked example

A stainless steel spoon has a mass of 40 g. The steel contains 74% iron and 18% chromium by mass, and the rest is nickel. What mass of nickel is in the spoon?

  1. Percentage of nickel = 100 − 74 − 18 = 8%.
  2. Mass of nickel = 8 ÷ 100 × 40 = 3.2 g.

Answer: 3.2 g of nickel.

Practice questions

Try each one, then open the answer.

1. Which statement describes an alloy?

  1. A
    a compound formed when two metals are chemically bonded together
  2. B
    a pure metal that has been heated and cooled quickly
  3. C
    an element made of two different types of metal atom
  4. D
    a mixture of a metal with other elements
Show answer

Answer: D. An alloy is a mixture, not a compound: the elements are not chemically combined in a fixed ratio.

2. Why is an alloy usually harder than the pure metal it is made from?

  1. A
    different-sized atoms stop the layers from sliding over each other
  2. B
    the different metals in an alloy form covalent bonds with each other
  3. C
    alloys contain more free electrons, so the layers are held tighter
  4. D
    the metallic bonds in an alloy are stronger than in the pure metal
Show answer

Answer: A. The different-sized atoms distort the lattice, so the layers cannot slip as easily: the alloy is harder and less malleable.

3. How do the properties of brass compare with those of pure copper?

  1. A
    brass is a better conductor of electricity
  2. B
    brass is harder and stronger
  3. C
    brass is softer and more malleable
  4. D
    brass is a compound, so it has a fixed formula
Show answer

Answer: B. Zinc atoms of a different size disrupt the layers of copper atoms, so the layers cannot slide as easily. This makes brass harder.

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