CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.2.6 Water of Crystallisation

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.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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