AQA A Level Chemistry

Revision Notes

1.2.6 Hydrated Salts

Water of Crystallisation

  • Water of crystallisation is when some compounds can form crystals which have water as part of their structure
  • A compound that contains water of crystallisation is called a hydrated compound
  • The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
    • Eg. hydrated copper(II) sulfate is CuSO45H2O
  • A compound which doesn’t contain water of crystallisation is called an anhydrous compound
    • Eg. anhydrous copper(II) sulfate is CuSO4
  • A compound can be hydrated to different degrees
    • Eg. cobalt(II) chloride can be hydrated by six or two water molecules
    • CoCl2 6H2O or CoCl2 2H2O
  • The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt

Anhydrous to hydrated salt:
CuSO4 + 5H2O → CuSO4∙5H2O

Hydrated to anhydrous salt (by heating):
CuSO4∙5H2O → CuSO4 + 5H2

Exam Tip

To calculate the Mr of hydrated salts, the Mr of the salt and water should be calculated separately and then added together.


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