Cation tests: NaOH and ammonia precipitates
With sodium hydroxide: copper(II) gives a light blue precipitate, iron(II) green, iron(III) red-brown — all insoluble in excess. Lead(II) and zinc give a white precipitate that re-dissolves in excess NaOH, their hydroxides being amphoteric. Ammonium gives no precipitate but releases ammonia on warming. With aqueous ammonia the diagnostic is copper(II): its blue precipitate dissolves in excess to a deep blue solution; zinc's white dissolves to colourless.
Anion tests: reagent and observation
Carbonate: add dilute acid — it fizzes and the gas turns limewater milky. Halides: acidify with dilute nitric acid, then silver nitrate — chloride white, bromide cream, iodide yellow. Sulfate: acidify, then barium chloride or nitrate — white barium sulfate. Acidify first to destroy carbonate and sulfite that would else give false precipitates. (Extended) Nitrate: warm with aluminium and NaOH to give ammonia; sulfite decolourises acidified manganate(VII).
Gas tests: reagent then observation
Each gas needs its reagent and the exact change. Ammonia turns damp red litmus blue. Carbon dioxide turns limewater milky. Chlorine bleaches damp litmus white — not "turns blue", which is ammonia. Hydrogen gives a squeaky pop with a lit (burning) splint; oxygen relights a glowing splint. The splint type is marked — burning for hydrogen, glowing for oxygen — and the litmus must be damp.
Drawn from real examiner reports.
Chloride ion test vs chlorine gas test
Different species, different tests — both widely failed. For chlorine gas, avoid "silver nitrate gives a white precipitate" (that is the chloride-ion test) — chlorine bleaches damp litmus white. For chloride ions, avoid the bleaching test or "add HCl" (HCl already has chloride); acidify with dilute nitric acid, then add silver nitrate for a white precipitate.
chloride ions W22 P31 Q5(c); W22 P32 Q5(c); chlorine gas S23 P31 Q2(d); S22 P31 Q2(b)
(Extended) Potassium flame is lilac, not red
Flame-test colours are poorly recalled. Potassium is lilac/violet — not lithium's red, sodium's yellow, or "brown". Lithium is crimson/red, calcium orange-red, barium green. Candidates also pick the wrong test, adding silver nitrate or ammonia for potassium; only a flame test identifies a Group I metal (clean wire in concentrated HCl, then into a blue flame).
W22 P32 Q5(c)(iii); W22 P41 Q2(c)(i)
Observation, not the product name
When a question asks what you observe, give the visible change, not the product's name. "Iron(III) hydroxide is formed" or "carbon dioxide is given off" score zero. Credit goes to "a red-brown precipitate forms", "a white precipitate forms" and "the limewater turns milky" — the most persistent error in qualitative analysis every sitting.
W22 P31 Q6(a); W22 P42 Q4(b); S23 P31 Q4(d)(ii)
Iron(II) green, iron(III) red-brown
The two iron precipitates with sodium hydroxide come up most years. Iron(II) gives a green precipitate that darkens to rust-brown on standing as it oxidises in air; iron(III) gives a red-brown precipitate straight away. Swapping the colours, or giving one colour for both, loses the mark. Fe(II) is the lower oxidation state, oxidised further to Fe(III), the higher state.
Glowing splint for oxygen, lit for hydrogen
The splint tests are marked on the splint type, and candidates mix them up. Oxygen relights a glowing (not flaming) splint. Hydrogen gives a squeaky pop when a burning/lit splint is applied. A glowing splint for hydrogen, or a burning splint as the oxygen test, loses the mark even if the pop or relighting is described. Match them: glowing to oxygen, burning to hydrogen.
White precipitate: halide or sulfate?
Two anions both give a white precipitate, so the reagent decides which. A white precipitate with acidified silver nitrate is chloride (silver chloride). A white precipitate with acidified barium chloride or nitrate is sulfate (barium sulfate). Name the reagent before concluding. Carbonate also whitens barium solution, which is why the sulfate test is acidified first.
Answer in two parts: reagent, then result
Cambridge marks each test in two halves, so give (1) the reagent applied and (2) the observable result. "Add acidified silver nitrate; a white precipitate forms." "Hold damp red litmus in the gas; it turns blue." Giving only one half drops half the marks.
Say why you acidify before the reagent
"Why acidify?" wants the reason. Before silver nitrate, add dilute nitric acid to remove carbonate and sulfite, which else give a false positive with silver — nitric, not hydrochloric (which adds chloride). Before barium, acidify to remove those that also whiten with it.
Use excess reagent to confirm the cation
Excess reagent separates look-alike ions. A white precipitate that re-dissolves in excess sodium hydroxide is an amphoteric hydroxide — lead(II) or zinc. For copper(II), excess aqueous ammonia turns the light blue precipitate deep blue, a decisive confirmation.
Chemical analysis in Cambridge IGCSE Chemistry (0620) uses specific reagents to identify ions and gases. Every identification test has two parts: the reagent applied, and the observable result. Product names are never observations — "a white precipitate" is an observation; "silver chloride" is not.
Precipitate: an insoluble solid formed when two solutions are mixed. The colour, solubility in excess reagent, and the reagent used all carry marks.
Amphoteric: a substance that can react with both acids and bases. and are amphoteric — they dissolve in excess because the hydroxide itself acts as an acid.
Write the ionic equation for the reaction that forms the precipitate when sodium hydroxide solution is added to a solution of iron(III) chloride.
Sodium hydroxide solution is added separately to three solutions: copper(II) sulfate, iron(II) sulfate and iron(III) chloride.
(a) State the colour of the precipitate formed in each case.
(b) Write an ionic equation, including state symbols, for the reaction between copper(II) ions and hydroxide ions.
(c) State what happens to the precipitate formed with copper(II) ions when excess ammonia solution is added.
