Oxides

O Level / IGCSE Chemistry · Acids, bases and salts. 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

Oxides are sorted by how they behave with acids and alkalis. This links to whether the element is a metal or a non-metal.

Most metal oxides are basic: they react with acids to give a salt and water. Most non-metal oxides are acidic: they react with alkalis to give a salt and water, and many dissolve in water to make a solution with a pH below 7.

A few metal oxides are amphoteric. They react with both acids and alkalis, giving a salt and water each time. The two you need to know are aluminium oxide, Al2O3, and zinc oxide, ZnO.

Rules to remember

Common mistake

Students call aluminium oxide and zinc oxide basic because they are metal oxides. They are amphoteric: the test is that they react with an alkali as well as with an acid.

Worked example

Which of these oxides react with aqueous sodium hydroxide: MgO, SO2, ZnO, CuO?

  1. Sodium hydroxide is an alkali, so it reacts with acidic oxides and with amphoteric oxides.
  2. MgO and CuO are basic metal oxides, so they do not react with an alkali.
  3. SO2 is a non-metal oxide, so it is acidic. ZnO is amphoteric.

Answer: SO2 and ZnO react with aqueous sodium hydroxide. MgO and CuO do not.

Practice questions

Try each one, then open the answer.

1. Zinc oxide reacts with dilute hydrochloric acid and also with aqueous sodium hydroxide. What type of oxide is it?

  1. A
    neutral, because it does not react with acids
  2. B
    amphoteric, because it reacts with acids and alkalis
  3. C
    acidic, because it reacts with the alkali
  4. D
    basic, because it reacts with the acid
Show answer

Answer: B. An oxide that reacts with both acids and bases to form a salt and water is amphoteric. Aluminium oxide behaves the same way.

2. Which statement about oxides is generally correct?

  1. A
    metals form acidic oxides and non-metals form basic oxides
  2. B
    non-metals form acidic oxides and metals form basic oxides
  3. C
    all oxides are amphoteric
  4. D
    all oxides are neutral
Show answer

Answer: B. The type of oxide is related to metallic character: metal oxides (CaO, CuO) are basic, non-metal oxides (SO2, CO2) are acidic.

3. Which substances react with aluminium oxide to form a salt and water?

  1. A
    both dilute acids and aqueous alkalis
  2. B
    neither dilute acids nor aqueous alkalis, because it is insoluble
  3. C
    dilute acids only, as it is a basic oxide of a metal
  4. D
    aqueous alkalis only, as it is an oxide of an unreactive metal
Show answer

Answer: A. Aluminium oxide is amphoteric: it shows basic behaviour with acids and acidic behaviour with alkalis.

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