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.

Tier note: every test on this page is 0620 Core. Unlike bonding or the mole, subtopic 12.5 sits entirely in the Core column of the 0620 syllabus, with no Supplement outcomes. The one item marked (0620 Extended, 5070 required) is a side explanation that uses the idea of a reducing agent (6.4 outcomes 10 and 12). 5070 has no tiers, and its 12.5 wording is effectively identical, so O Level candidates need all of it too.

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	No precipitate (or a very faint trace)	No change
Chromium(III), Cr³⁺	Green precipitate	Dissolves in excess*	Green precipitate	Insoluble
Copper(II), Cu²⁺	Pale blue precipitate	Insoluble	Pale blue precipitate	Dissolves — deep (royal) blue solution
Iron(II), Fe²⁺	Green precipitate, turning brown near the surface on standing	Insoluble	Green precipitate, turning brown near the surface on standing	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. (*The exact colour of chromium(III) hydroxide redissolved in excess NaOH is additional detail beyond the core observation and not required — the issued notes state only the green precipitate.) Calcium’s precipitate stays insoluble in excess NaOH, but this is also the key test that tells calcium apart from aluminium and zinc in the first place: aqueous ammonia is too weak a base to precipitate the more soluble calcium hydroxide in any useful amount, so calcium gives no clear precipitate with ammonia at all — unlike aluminium and zinc, which both do. Iron(II) and iron(III) precipitates stay insoluble in excess of either reagent. Colour is the main distinguishing feature (green vs red-brown), and the iron(II) precipitate also turns brown near the surface on standing as it oxidises in air — an additional feature that can help confirm it.

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). The sulfite test is done on the solution itself: the sulfite ions react with the manganate(VII) directly, and no gas needs to be given off first. (0620 Extended, 5070 required) Both results happen because sulfite ions and sulfur dioxide are reducing agents: they reduce the purple manganate(VII) ions.

If a question describes a solid reacting (for example, a carbonate with acid) and asks for a gas test, you are testing the gas that the 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⁺	Light 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.
• Expecting calcium to behave like aluminium or zinc with ammonia. All three give a white precipitate with sodium hydroxide, but calcium is the odd one out with aqueous ammonia — it gives no clear precipitate at all, because ammonia is too weak a base to precipitate calcium hydroxide in useful amounts.
• 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 for the tests — 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.