CIE A Level Chemistry复习笔记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 CuSO4∙5H2O
  • 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∙5H2OHydrated to anhydrous salt (by heating):

CuSO4∙5H2O → CuSO+ 5H2O

Exam Tip

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