Hydrated Salts (AQA AS Chemistry): Revision Note

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

  • E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O

• A compound which doesn’t contain water of crystallisation is called an anhydrous compound

  • E.g. anhydrous copper(II) sulfate is CuSO₄

• A compound can be hydrated to different degrees

  • E.g. 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:

Hydrated:        CuSO₄•5H₂O ⇌ CuSO₄ + 5H₂O        :Anhydrous

• The degree of hydration can be calculated from experimental results:

  • The mass of the hydrated salt must be measured before heating

  • The salt is then heated until it reaches a constant mass

  • The two mass values can be used to calculate the number of moles of water in the hydrated salt - known as the water of crystallisation