Identification of Ions and Gases

This guide covers subtopic 12.5, Identification of ions and gases, for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

This is fully shared content. Unlike bonding or the mole, subtopic 12.5 sits entirely in the Core column for 0620 — there is no Extended-only material here at all — and the O Level 5070 wording is effectively identical. Every candidate, on either qualification, is expected to know every test on this page.

Qualitative analysis is tested by giving you an unknown substance or solution and asking what you would do and what you would see. Examiners mark on three things: the correct reagent, the correct observation, and the correct conclusion. Missing any one of the three loses marks, even if the other two are right.

Testing for anions

Anion	Test	Observation
Carbonate, CO₃²⁻	Add dilute acid	Effervescence; the gas turns limewater milky (carbon dioxide)
Chloride, Cl⁻	Acidify with dilute nitric acid, then add aqueous silver nitrate	White precipitate
Bromide, Br⁻	Acidify with dilute nitric acid, then add aqueous silver nitrate	Cream precipitate
Iodide, I⁻	Acidify with dilute nitric acid, then add aqueous silver nitrate	Yellow precipitate
Nitrate, NO₃⁻	Reduce with aluminium foil and aqueous sodium hydroxide, then warm	Ammonia gas is released (turns damp red litmus paper blue)
Sulfate, SO₄²⁻	Acidify with dilute nitric acid, then add aqueous barium nitrate	White precipitate
Sulfite, SO₃²⁻	Add acidified aqueous potassium manganate(VII)	The purple solution decolourises

Two pairs are easy to confuse. Silver nitrate identifies the halide (Cl⁻/Br⁻/I⁻) by precipitate colour — you need acidified dilute nitric acid first, so any carbonate present doesn’t also precipitate and confuse the result. Barium nitrate identifies sulfate the same way, again after acidifying with nitric acid.

Testing for cations with sodium hydroxide and ammonia

Add the aqueous alkali a few drops at a time, then in excess, and record both stages.

Cation	With NaOH (dropwise)	With NaOH (excess)	With aqueous ammonia (dropwise)	With ammonia (excess)
Aluminium, Al³⁺	White precipitate	Dissolves — colourless solution	White precipitate	Insoluble
Ammonium, NH₄⁺	No precipitate; warming releases ammonia gas	—	No precipitate	—
Calcium, Ca²⁺	White precipitate	Insoluble	White precipitate	Insoluble
Chromium(III), Cr³⁺	Grey-green precipitate	Dissolves — dark green solution	Grey-green precipitate	Insoluble
Copper(II), Cu²⁺	Pale blue precipitate	Insoluble	Pale blue precipitate	Dissolves — deep (royal) blue solution
Iron(II), Fe²⁺	Green precipitate	Insoluble	Green precipitate	Insoluble
Iron(III), Fe³⁺	Red-brown precipitate	Insoluble	Red-brown precipitate	Insoluble
Zinc, Zn²⁺	White precipitate	Dissolves — colourless solution	White precipitate	Dissolves — colourless solution

The pattern worth memorising: aluminium and zinc both give a white precipitate with NaOH that redissolves in excess to a colourless solution — the way to distinguish them is that zinc also redissolves in excess ammonia, while aluminium does not. Calcium and iron(II)/iron(III) precipitates stay insoluble in excess of either reagent, and are told apart by colour alone.

Testing for gases

Gas	Test	Positive result
Ammonia, NH₃	Hold damp red litmus paper in the gas	Turns blue
Carbon dioxide, CO₂	Bubble the gas through limewater	Limewater turns milky
Chlorine, Cl₂	Hold damp litmus paper in the gas	Bleaches — turns white (may briefly redden first)
Hydrogen, H₂	Apply a lighted splint	A squeaky pop
Oxygen, O₂	Apply a glowing splint	Relights the splint
Sulfur dioxide, SO₂	Add to acidified aqueous potassium manganate(VII)	The purple solution decolourises

Sulfite ions and sulfur dioxide gas give the same positive result — decolourising acidified potassium manganate(VII) — because sulfite reacting with acid is what produces the sulfur dioxide in the first place. If a question gives you a solid and asks for a gas test, you are testing for the gas that reaction gives off, not testing the solid directly.

Flame tests

A flame test identifies certain metal cations from the colour they give a Bunsen flame, usually using a clean nichrome or platinum wire dipped in the solid or solution.

Cation	Flame colour
Lithium, Li⁺	Red
Sodium, Na⁺	Yellow
Potassium, K⁺	Lilac
Calcium, Ca²⁺	Orange-red
Barium, Ba²⁺	Green
Copper(II), Cu²⁺	Blue-green

Flame tests are a separate technique from the sodium hydroxide/ammonia tests above — useful because sodium and potassium compounds don’t form informative precipitates with either reagent, so a flame test is often the only practical way to identify them.

Common mistakes

• Forgetting to acidify before testing for halide or sulfate ions. Adding silver nitrate or barium nitrate straight to an untreated solution risks a false positive from a carbonate also present.
• Reporting only the reagent, not the observation. “Add silver nitrate” is not a complete answer — the mark is for stating what you would see.
• Mixing up chloride, bromide and iodide precipitate colours. White, cream and yellow, in that order, and they get progressively less white as you go down Group VII.
• Confusing aluminium and zinc. Both redissolve in excess sodium hydroxide; only zinc also redissolves in excess ammonia.
• Assuming a coloured precipitate must be a transition metal you haven’t memorised. The eight cations above are the full syllabus list — if it isn’t one of these, look again at the question.

Quick revision checklist

All candidates (0620 and 5070):

• Anion tests: carbonate, chloride, bromide, iodide, nitrate, sulfate, sulfite
• Cation tests using sodium hydroxide and ammonia, dropwise and in excess: aluminium, ammonium, calcium, chromium(III), copper(II), iron(II), iron(III), zinc
• Gas tests: ammonia, carbon dioxide, chlorine, hydrogen, oxygen, sulfur dioxide
• Flame tests: lithium, sodium, potassium, calcium, barium, copper(II)

There is no Core/Extended split to track here — every test on this page is examinable for every candidate on both qualifications.

Related resources

• Ionic, Covalent and Metallic Bonding — ionic charges behind the formulae used above
• The Periodic Table: Groups and Trends — Group I and Group VII properties referenced in several tests
• Redox Reactions — the manganate(VII) decolourisation test is a redox reaction
• Group 17: The Halogens — the AS Level continuation of the halide precipitate/ammonia tests (9701), not required for IGCSE or O Level
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.