Carbon dioxide gas bubbles if carbonate ions are present. Limewater is used to confirm that the gas is carbon dioxide. It turns from clear to milky when carbon dioxide is bubbled through.
Sulfate ions in solution, SO42-, are detected using barium chloride solution. The test solution is acidified using a few drops of dilute hydrochloric acid, and then a few drops of barium chloride solution are added. A white precipitate of barium sulfate forms if sulfate ions are present.
barium chloride + sodium sulfate → sodium chloride + barium sulfate
BaCl2(aq) + Na2SO4(aq) → 2NaCl(aq) + BaSO4(s)
The hydrochloric acid is added first to remove any carbonate ions that might be present - they would also produce a white precipitate, giving a false positive result.
Barium nitrate solution can be used instead of barium chloride solution. However, nitric acid is added first to acidify the test solution. Sulfuric acid cannot be used because it contains sulfate ions - these would interfere with the second part of the test.
Halide ions in solutions are detected using silver nitrate solutions. The test solution is acidified using a few drops of dilute nitric acid, and then a few drops of silver nitrate solution are added. Different coloured silver halide precipitates form, depending on the halide ions present:
silver nitrate + sodium bromide → sodium nitrate + silver bromide
AgNO3(aq) + NaBr(aq) → NaNO3(aq) + AgBr(s)
One way to remember the colours is to think of ‘milk, cream, butter’ (white, cream, yellow).
The nitric acid is added first to remove any carbonate ions that might be present - they would produce a white precipitate of silver carbonate, giving a false positive result for chloride ions.
Nitrate ions (NO3-) can be detected by reducing them to ammonia. This is done by: