Hydrated Salts (AQA A Level Chemistry): Revision Note

Water of Crystallisation

• Water of crystallisation refers to water molecules that are chemically incorporated into the crystal structure of a compound

• A compound that contains water of crystallisation is called a hydrated compound

• In the chemical formula, the water molecules are shown after a dot

  • For example, hydrated copper(II) sulfate is written as: CuSO₄∙5H₂O

• A compound that does not contain water of crystallisation is called an anhydrous compound

  • For example: CuSO₄

• Some compounds can exist in different hydrated forms

  • For example, cobalt(II) chloride can exist as:

CoCl₂ ∙6H₂O or CoCl₂ ∙2H₂O

• Hydrated salts can be converted to anhydrous salts by heating, which removes the water of crystallisation:

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

• The reverse process occurs when water is added to the anhydrous salt

• The degree of hydration can be determined experimentally

  • First, the mass of the hydrated salt is measured

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

  • The decrease in mass corresponds to the mass of water lost.

  • From these values, the number of moles of water and the mole ratio of water to salt can be calculated to determine the formula of the hydrated compound