The five gas tests (method + result)
Every gas-test answer needs both the method and the positive result:
Flame colours and NaOH precipitate colours
Flame test: clean a nichrome wire in concentrated , dip it in the solid, hold it in a blue flame. Colours: red, yellow, lilac, orange-red, blue-green.
With sodium hydroxide solution: → blue precipitate; → green; → orange-brown; → ammonia on warming (damp red litmus blue).
Anion tests: halides, sulfate, carbonate
Halides — add dilute nitric acid then silver nitrate: white (), cream (), yellow ().
Sulfate — add dilute hydrochloric acid then barium chloride: white ().
Carbonate — add dilute acid: fizzing; the gas turns limewater milky.
Acidify first to remove carbonate ions (a false positive).
Drawn from real examiner reports.
Glowing splint used for the hydrogen test
Two errors lose this well-known mark. (1) The hydrogen test uses a lit (burning) splint, not a glowing splint — a glowing splint is the oxygen test. (2) Stating only "squeaky pop" is not enough: you must also say how the test is done. The full answer: a lighted splint burns with a squeaky pop.
June 2023 Paper 2CR examiner report — a significant proportion of candidates used a glowing splint instead of a burning splint, and many gave only "squeaky pop" without describing the test.
Chloride ions treated as chlorine; dry paper
Chloride ions are not the same as chlorine . Chloride ions in solution do not bleach litmus paper; only chlorine gas molecules do. And the litmus/indicator paper must be damp — chlorine has to dissolve in the water on the paper before it can bleach it, so dry paper shows no change.
June 2024 Paper 2C examiner report: chloride ions were wrongly treated as chlorine, and chloride ions do not bleach damp litmus — only chlorine molecules do.
Writing "iodide" where "iodine" is meant
In halogen displacement, name the element iodine that is displaced, not "iodide". The iodide ion is the reactant; the grey/purple iodine is the product. Swapping iodide and iodine (like chloride and chlorine) is repeatedly penalised, and displacement equations only score if correctly balanced.
June 2023 Paper 2CR examiner report: candidates wrote "iodide" where "iodine" was required, and equations only scored if correctly balanced.
Vague or wrong precipitate colours
Precipitate colours must be precise: is green and is orange-brown/red-brown — "brown" alone is too vague. For halides, match white/cream/yellow to in that order. Describe a precipitate as an insoluble solid forming, not a colour change of the solution.
(common Edexcel chemistry error)
Bromine called a liquid, or "orange" gas
Describe the physical state correctly: bromine vapour is a brown gas, not a "brown liquid" or "brown solid". Watch the colour word too — "orange" is bromine water, not bromine gas. When a question asks for a colour change, give both the starting and the final colour.
June 2024 Paper 2C examiner report: bromine gas colour was often wrongly given as "brown liquid/solid" or as "orange" (orange is bromine water).
Ignoring the carbonate false positive
Both (chloride) and (sulfate) are white precipitates — the reagent tells them apart (silver nitrate for a halide, barium chloride for a sulfate). Always add dilute acid first: it removes carbonate ions, which would otherwise form their own white precipitate and give a false positive for chloride or sulfate.
(common Edexcel chemistry error)
Reagent, observation, conclusion — all three
Identification questions need three parts: (1) the reagent and how you add it; (2) the exact observation (e.g. "a cream precipitate forms"); (3) the conclusion — which ion is present. Naming the ion without the evidence loses marks.
Confirm an evolved gas by its own test
The carbonate and ammonium tests need the gas identified by its own test, not just named: pass a carbonate's gas through limewater (milky → ); hold damp red litmus in an ammonium salt's gas (blue → ). Naming the gas alone drops a mark.
Give both colours; use precise words
When a question asks for a colour change, give both the original and final colour, and use precise words — "colourless", not "clear". In a flame test, place the sample in the flame and give the exact colour ( yellow, not "orange").
This topic is almost entirely rote recall of tests, reagents, observations and colours. The marks come from precise wording, so learn the tables below exactly. A precipitate (ppt) is an insoluble solid that forms when two solutions are mixed; state symbols are solid , liquid , gas and aqueous .
Define the term precipitate.
A test tube contains a colourless gas thought to be hydrogen. Describe the test you would carry out and the positive result that would confirm the gas is hydrogen.
