Test for Anions

Test for Carbonates

Carbonates react with acids to form carbon dioxide gas. Dilute nitric acid is added to the sample. Bubbles of gas will be seen if it is a carbonate.Na₂CO₃(aq) + 2HNO₃(aq) → 2NaNO₃(aq) + CO₂(g) + H₂O(l)The gas when bubbled through limewater will turn it cloudy.CO₂ + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Test for Sulfates

Most sulfates are soluble in water. Exceptions are calcium, strontium barium, lead and silver. Aqueous barium chloride or barium nitrate is added to the sample. The formation of a white precipitate indicates the presence of a sulfate. Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)Usually barium nitrate is used if a halide test is also to be carried out. The correct sequence is carbonate, then sulfate then halide. Barium carbonate is white and insoluble so this can be ruled out first with the carbonate test. Silver sulfate and silver carbonate are both insoluble so this can be ruled out with the carbonate and sulfate tests first.