Hydrated & Anhydrous Salts (Cambridge (CIE) IGCSE Chemistry)

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

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