Hydrated & Anhydrous Salts (Cambridge (CIE) O Level Chemistry): Revision Note

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:

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