Hydrated & Anhydrous Salts (Cambridge International IGCSE Chemistry)

Spec point — 7.3.3 · Define a hydrated substance as a substance that is chemically combined with water and an anhydrous substance as a substance containing no water

Hydrated & anhydrous salts

• When salts are being prepared, some water can be retained within the structure of the salt during the crystallisation process
• 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 of crystallisation

Spec point — 7.3.5 · Define the term water of crystallisation as the water molecules present in hydrated crystals, including CuSO4•5H2O and CoCl2•6H2O

Extended tier only

• 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:

  • 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:

  • 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:

  • 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

Hydrated copper(II) sulfate and anhydrous copper(II) sulfate

Diagram showing the dehydration of hydrated copper(II) sulfate