Tests for Anions (Edexcel GCSE Chemistry)

Carbonate Ion

• Add dilute acid and test the gas released.
• Effervescence should be seen and the gas produced is CO₂ which forms a white precipitate of calcium carbonate when bubbled through limewater:

CO₃^2- (aq) + 2H⁺ (aq) → CO₂ (g) + H₂O (l)

CO₂ + Ca(OH)₂ → CaCO₃ (s) + H₂O (l)

 

Limewater turns milky in the presence of CO₂ caused by formation of insoluble calcium carbonate

Sulfate Ion

• Acidify with dilute hydrochloric acid and add aqueous barium chloride.
• A white precipitate of barium sulfate is formed:

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Halide Ions

• Acidify with dilute nitric acid (HNO₃) followed by the addition of silver nitrate solution (AgNO₃).
• This forms a silver halide precipitate:

• Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion
• Silver chloride is white, silver bromide is cream and silver iodide is yellow

Each silver halide produces a precipitate of a different colour