RP 4: Test-tube identification of cations & anions

Hazards

Method

1. Group 2 cations: NaOH, then H₂SO₄Add NaOH(aq) dropwise then in excess: Mg²⁺ gives a white precipitate insoluble in excess; Ca²⁺ a slight white precipitate; Ba²⁺ stays clear. Then with dilute H₂SO₄: Ba²⁺ gives a dense white precipitate of BaSO₄, Ca²⁺ a slight one, Mg²⁺ none — the two solubility trends run in opposite directions down the group.
2. Ammonium: warm with NaOHWarm the unknown gently with NaOH(aq) and hold damp red litmus at the mouth of the tube. Ammonia turns it blue — the only common gas that does. Warm, don't boil, or splashes of alkali reach the litmus and lie to you.
3. Carbonate: acid, then limewaterAdd dilute HCl — effervescence suggests CO₃²⁻. Bubble the gas through limewater: turning milky (a cloudy white precipitate of CaCO₃) confirms CO₂.
4. Sulfate: acidified barium chlorideAcidify with dilute HCl first (to destroy any carbonate, which would also give a white precipitate), then add BaCl₂(aq): a white precipitate of BaSO₄ confirms sulfate.
5. Halides: acidified silver nitrateAcidify with dilute HNO₃ — never HCl, or you add the very ion you're testing for — then add AgNO₃(aq): white precipitate = chloride, cream = bromide, yellow = iodide.
6. Resolve the colours with ammoniaAgCl dissolves in dilute NH₃; AgBr only in concentrated NH₃; AgI in neither — the three-rung ladder that separates ambiguous precipitate colours.
7. Hydroxide, and how to record everythingOH⁻(aq) turns red litmus blue (pH 13–14 on pH paper). In every observation give colour AND state: 'white precipitate', 'effervescence', 'no visible change'.

Mistakes examiners see every year

What to expect

A complete grid: Mg²⁺ white ppt with NaOH insoluble in excess, no sulfate ppt; Ba²⁺ no NaOH ppt but dense white BaSO₄; NH₄⁺ releases ammonia on warming with alkali; halides white/cream/yellow with AgNO₃, resolved by dilute vs concentrated ammonia.