Chemical Tests

Carbonate ion, CO₃²⁻

• Add a few drops of dilute acid (HCl or HNO₃) to the solid or solution
• Effervescence (fizzing) is seen as carbon dioxide is released:

CO₃²⁻(aq) + 2 H⁺(aq) → CO₂(g) + H₂O(l)

• Confirm the gas is carbon dioxide by passing it through limewater — the limewater turns milky as insoluble calcium carbonate forms:

CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

• The exam-mark wording is "bubble the gas through limewater; if carbonate is present the limewater turns milky" — saying only "limewater turns milky" is the result, not the test

Halide ions, Cl⁻ / Br⁻ / I⁻

• Acidify the sample first with a few drops of dilute nitric acid — this destroys any carbonate impurity that would otherwise give a false-positive precipitate
• Add a few drops of silver nitrate solution, AgNO₃(aq)
• A silver halide precipitate forms whose colour identifies the halide:

• Use nitric acid (not hydrochloric) for the acidification step — HCl introduces chloride ions and would give a false positive
• Memorise the colour order: white → cream → yellow as the halide moves down the group

Sulfate ion, SO₄²⁻

• Acidify the sample with dilute hydrochloric acid — this removes any carbonate impurity that would otherwise precipitate as BaCO₃ and confuse the result
• Add a few drops of barium chloride solution, BaCl₂(aq) (barium nitrate also works)
• A white precipitate of barium sulfate forms if sulfate ions are present:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

• Why HCl this time (not HNO₃): no chloride ion would interfere with the BaSO₄ result, but a carbonate would form BaCO₃ (also white) and give a false positive