Testing for Aqueous Anions (OCR GCSE Chemistry A (Gateway))

Testing for Carbonate Ions

• Carbonates all contain the carbonate ion, CO₃^2-

• The test for this ion involves adding dilute acid and testing the gas released

• If a carbonate compound is present then 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 (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)

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

Testing for Sulfate Ions

• Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride

• If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

• The test can also be carried out with barium nitrate solution

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

Testing for Halide Ions

• Acidify the sample with dilute nitric acid (HNO₃) followed by the addition of silver nitrate solution, AgNO₃

• The acidification is done to remove carbonate ions that might give a false positive result

• If a halide ion is present it forms a silver halide precipitate:

Ag⁺ (aq) + X^– (aq) → AgX (s)

• Depending on the halide present, a different coloured precipitate is formed, allowing for identification of the halide ion

• Silver chloride precipitate is white, silver bromide precipitate is cream and silver iodide precipitate is yellow

Each silver halide produces a precipitate of a different colour