Anion tests: acid, then a metal-ion solution
Carbonate: add dilute acid; the gas turns limewater milky. Chloride, bromide, iodide: acidify with dilute nitric acid, then add aqueous silver nitrate — white, cream and yellow precipitates respectively. Nitrate: add aluminium foil and aqueous sodium hydroxide, warm gently; nitrate is reduced to ammonia, turning damp red litmus blue. Sulfate: acidify with dilute nitric acid, then add aqueous barium nitrate — white precipitate.
Cation tests: sodium hydroxide and ammonia
Six cations are identified by adding aqueous sodium hydroxide or ammonia dropwise, then in excess. Ammonium: no precipitate, but warming releases ammonia. Calcium: white, insoluble in excess. Zinc: white, dissolves in excess to a colourless solution. Copper(II): light blue, dissolving in excess ammonia to a deep blue solution but not in excess sodium hydroxide. Iron(II): green. Iron(III): red-brown. Formulas of the complex ions are not required.
Gas tests and flame tests
Gases: ammonia turns damp red litmus blue; carbon dioxide turns limewater milky; chlorine bleaches damp litmus; hydrogen pops with a lighted splint; oxygen relights a glowing splint. Flame tests: dip a clean flame-test (nichrome) wire into the solid or solution and hold it in a blue Bunsen flame — lithium red, sodium yellow, potassium lilac, copper(II) blue-green.
Drawn from real examiner reports.
Silver nitrate is not barium nitrate
Silver nitrate identifies the halides (chloride, bromide, iodide); barium nitrate identifies sulfate. Both give a white precipitate with their own target ion, so "a white precipitate formed" is an incomplete answer — always name which solution was added. Swapping the two reagents loses the mark even when the rest of the method is right.
Flagged Jun 2022 P51 Q4; Nov 2022 P43 Q8b
Acidify with nitric acid only
The halide and sulfate tests are acidified with dilute nitric acid. Hydrochloric acid would add extra chloride ions and sulfuric acid extra sulfate ions, each giving a false-positive white precipitate. The acid also reacts away any carbonate present, which would otherwise precipitate with silver or barium ions and mask a genuine result.
Limewater milky, not a burning splint
Carbon dioxide turns limewater milky — a white precipitate of calcium carbonate. It does not burn a splint, and "cloudy emulsion" is not credited. The solid formed is calcium carbonate, not calcium oxide, and the gas given off when dilute acid meets a carbonate is carbon dioxide, not oxygen or hydrogen.
Flagged Jun 2022 P53 Q3d; Nov 2023 P41 Q5cii
Cream is bromide, yellow is iodide
With acidified silver nitrate, chloride gives a white precipitate, bromide a cream one and iodide a yellow one. Cream and yellow are the pair that gets swapped, because both are pale and far less familiar than the clearly white chloride precipitate. Learn them in order: white, then cream, then yellow.
Precipitate colour alone is not enough
A cation result has two parts: the colour on first adding the reagent, and what happens when it is added in excess. Calcium and zinc both give a white precipitate with sodium hydroxide — only excess separates them, since zinc hydroxide dissolves to a colourless solution while calcium hydroxide does not. Stopping after the first addition costs the mark.
Flagged Jun 2022 P51 Q4; Nov 2022 P43 Q8b
Flame colours are widely unfamiliar
Lithium = red, sodium = yellow, potassium = lilac, copper(II) = blue-green (the examiner reports record copper(II) simply as green). Lithium and potassium are the pair most often swapped. Use the mark-scheme colour word — "pink" for lithium and "purple" for potassium are imprecise — and name the cation identified, not just "a coloured flame".
Flagged Jun 2022 P11 Q20, P42 Q8c
Reagent first, then observation
Answer in a fixed order: name the exact reagent and any condition (acidify first, warm gently, add in excess), then give the observation with a colour word and a state word — precipitate forms, gas released, litmus turns. "It changes colour" earns nothing.
Decide anion or cation first
Check what is being identified before reaching for a reagent. Anions need an acid plus a metal-ion solution; cations need aqueous sodium hydroxide or aqueous ammonia, or a flame test. That one decision fixes the reagent at once and stops halide and cation tests being mixed up.
Balance the charges, then bracket
Balance charge before writing a precipitate formula: needs three hydroxide ions, giving and not . Bracket the hydroxide group — "ZnOH2" is wrong. If an ionic equation is asked for, give only the reacting ions with charges.
Cambridge 0654 spec reference: Section C12 "Experimental techniques and chemical analysis", sub-topic C12.5 "Identification of ions and gases". All four numbered statements are Core: (1) anion tests for carbonate, chloride/bromide/iodide, nitrate, sulfate; (2) cation tests using aqueous sodium hydroxide and aqueous ammonia for ammonium, calcium, copper(II), iron(II), iron(III), zinc; (3) gas tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen; (4) flame tests for lithium, sodium, potassium, copper(II).
| Term | Mark-scheme-precise meaning |
|---|---|
| Precipitate | An insoluble solid that forms and separates out of a solution when two aqueous solutions react |
| Flame test | A test in which a sample is heated in a Bunsen flame; the colour of the flame identifies the metal cation present |
| Anion test | A test that identifies a negatively charged ion using an added acid and/or a metal-ion solution |
| Excess reagent | Adding much more of a reagent than needed to react with all of the substance present, used to see whether an initial precipitate then dissolves |
State the test for chloride, bromide and iodide ions, and the precipitate colour each gives.
A student adds dilute acid to a white solid suspected to contain carbonate ions. The gas produced is bubbled through limewater, , and the limewater turns milky.
Write the balanced symbol equation for the reaction between the gas produced and limewater. (2 marks)
