Making soluble salts (except sodium, potassium and ammonium salts)
4 marks · NormalDefine the term 'hydrated salt'.
Explain, with reference to water of crystallisation, how heating a hydrated salt to dryness changes its formula.
How can you tell if a sample of copper(II) sulfate is hydrated or anhydrous? State what you would observe on heating.
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(a) A salt that contains water of crystallisation (chemically bound water molecules).
(b) Heating removes the water of crystallisation from the hydrated salt, changing its formula from, e.g., CuSO4·5H2O to CuSO4. The compound becomes anhydrous.
(c) The hydrated salt is blue. On heating it turns white as the water is driven off, showing it's anhydrous.
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Marking points
B1 (a) · 1 mark · Defines hydrated salt as a salt with water of crystallisation
E1 (b) · 1 mark · Describes loss of water of crystallisation on heating
E2 (b) · 1 mark · Explains formula changes to anhydrous form (e.g., CuSO4·5H2O becomes CuSO4)
B2 (c) · 1 mark · States colour change on heating: blue to white
Worked solution
a) A hydrated salt contains water of crystallisation bound in its crystal structure.
b) Heating a hydrated salt removes the water of crystallisation. For example, heating CuSO4·5H2O gives CuSO4, which is anhydrous.
c) The hydrated form is blue and turns white when heated as water is lost.