Hydrated & Anhydrous Salts (Cambridge O Level Chemistry)

• When salts are being prepared, some water can be retained within the structure of the salt during the crystallisation process 
• This affects the crystal's shape and colour
• Salts that contain water within their structure are called hydrated salts
• Anhydrous salts are those that contain no water in their structure
• A common example is copper(II) sulfate which crystallises forming the salt hydrated copper(II) sulfate, which is blue
• When it is heated, the water from its structure is removed, forming anhydrous copper(II) sulfate, which is white
• The hydrated salt has been dehydrated to form the anhydrous salt
• This reaction can be reversed by adding water to anhydrous copper(II) sulfate:

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

• Water molecules included in the structure of some salts during the crystallisation process are known as water of crystallisation 
• A compound that contains water of crystallisation is called a hydrated compound
• When writing the chemical formula of hydrated compounds, the water of crystallisation is separated from the main formula by a dot 
  • E.g. hydrated copper(II) sulfate is CuSO₄∙5H₂O
  • Hydrated cobalt(II) chloride is CoCl₂∙6H₂O
• The formula shows the number of moles of water contained within one mole of the hydrated salt
  • E.g. hydrated copper(II) sulfate, CuSO₄∙5H₂O, contains 5 moles of water in 1 mole of hydrated salt
• A compound which doesn’t contain water of crystallisation is called an anhydrous compound
  • E.g. anhydrous copper(II) sulfate is CuSO₄
  • Anhydrous cobalt(II) chloride is CoCl₂
• 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 + 5H₂O 

Diagram showing the dehydration of hydrated copper II) sulfate