Chemistry 0620 · syllabus 12.5
Cambridge IGCSE Chemistry 0620 — Tests for ions and gases topical past papers
Identifying anions, cations and gases: silver nitrate, barium nitrate and carbonate tests, precipitates with sodium hydroxide and ammonia, flame tests and gas tests.
- Questions
- 489
- Years
- 2009–2026
- With mark scheme
- 98%
- Questions 2022–2026
- 215
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Tests for ions and gases covers
Qualitative analysis identifies the ions in an unknown compound and the gases it gives off. For anions: a carbonate fizzes with dilute acid, giving carbon dioxide; chloride, bromide and iodide, acidified with dilute nitric acid, give white, cream and yellow precipitates with aqueous silver nitrate; a sulfate, acidified with dilute nitric acid, gives a white precipitate with aqueous barium nitrate; a nitrate gives off ammonia when warmed with aqueous sodium hydroxide and aluminium foil; and a sulfite turns acidified aqueous potassium manganate(VII) from purple to colourless.
Cations are tested with aqueous sodium hydroxide and aqueous ammonia, adding a little and then an excess. Copper(II) gives a light blue precipitate that dissolves in excess ammonia to form a dark blue solution; iron(II) gives a green precipitate and iron(III) a red-brown one, both insoluble in excess; chromium(III) gives a green precipitate that dissolves in excess sodium hydroxide; calcium a white precipitate with sodium hydroxide but little or none with ammonia; and ammonium gives ammonia on warming with sodium hydroxide. Aluminium and zinc both give white precipitates that dissolve in excess sodium hydroxide, and only ammonia tells them apart: zinc's precipitate dissolves in excess, aluminium's does not. Flame tests identify lithium (red), sodium (yellow), potassium (lilac), calcium (orange-red), barium (light green) and copper(II) (blue-green).
Gases have tests of their own: ammonia turns damp red litmus paper blue, carbon dioxide turns limewater milky, chlorine bleaches damp litmus paper, hydrogen 'pops' with a lighted splint, oxygen relights a glowing splint, and sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless. All of these tests are Core content. Notes summarising them are printed in Papers 5 and 6, but not in the multiple-choice or theory papers, so they have to be learned for those.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 12.5 Identification of ions and gases
What you should be able to do
- Describe the tests for carbonate, chloride, bromide, iodide, sulfate, sulfite and nitrate ions and their results
- Identify Al³⁺, NH₄⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺ using aqueous sodium hydroxide and aqueous ammonia
- Describe flame tests for Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺ and Cu²⁺
- Describe the tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide
- Work out the identity of a compound from a set of observations
- Write ionic equations for the precipitates formed in the tests (Extended)
Key ideas
- Acidify with dilute nitric acid before the halide and sulfate tests, so that carbonate ions cannot give a false precipitate
- Silver halides: AgCl white, AgBr cream, AgI yellow; BaSO₄ white
- Zn²⁺: white precipitate soluble in excess of both reagents; Al³⁺: white precipitate soluble in excess sodium hydroxide only
- Cu²⁺: light blue precipitate, dark blue solution in excess ammonia; Fe²⁺: green precipitate; Fe³⁺: red-brown precipitate
- Ag⁺(aq) + Cl⁻(aq) → AgCl(s); Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
How Tests for ions and gases has been examined
From 2022 to 2026, Tests for ions and gases was on Paper 6 · Alternative to Practical in 13 of 14 exam series, worth about 43% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 60 | 5% | 13 of 14 |
| Paper 2 · Multiple Choice (Extended) | 38 | 3% | 14 of 14 |
| Paper 3 · Theory (Core) | 36 | 14% | 11 of 14 |
| Paper 4 · Theory (Extended) | 23 | 13% | 11 of 14 |
| Paper 6 · Alternative to Practical | 58 | 43% | 13 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 6.
Filed in Momentum under: In the Lab (chemical Test& Salt Analysis).
A worked example
A pale green solid dissolves in water. With aqueous sodium hydroxide the solution gives a green precipitate, insoluble in excess, that turns brown near the surface on standing. A second sample, acidified with dilute nitric acid, gives a white precipitate with aqueous barium nitrate. Identify the compound and write ionic equations for the two precipitates.
- A green precipitate with sodium hydroxide that does not dissolve in excess shows iron(II) ions, Fe²⁺; chromium(III) also gives a green precipitate, but it dissolves in excess
- The surface turns brown because oxygen in the air oxidises the iron(II) hydroxide to an iron(III) compound
- A white precipitate with barium nitrate, after acidifying with nitric acid, shows sulfate ions, SO₄²⁻
- So the compound is iron(II) sulfate, FeSO₄
- Precipitates: Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) and Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Answer: Iron(II) sulfate, FeSO₄; Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) and Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Acidifying with hydrochloric acid before a chloride test, or sulfuric acid before a sulfate test — the acid adds the very ion being tested for, so dilute nitric acid is used
- Stopping after one reagent for a white precipitate — aluminium and zinc look the same with sodium hydroxide, and only excess aqueous ammonia tells them apart
- Mixing up the splints — hydrogen 'pops' with a lighted splint, while oxygen relights a glowing one
- Writing 'the solution turns blue' for copper(II) with sodium hydroxide — the result is a light blue precipitate; only excess ammonia gives a dark blue solution
- Leaving out 'damp' in litmus tests for gases — the gas has to dissolve in water on the paper to change its colour
Exam technique
- Record observations in the words of the qualitative analysis notes: the colour, the word 'precipitate', and 'soluble' or 'insoluble in excess'
- Before naming a compound, check that every result supports it, with at least one test for the cation and one for the anion
- For gas tests give the test and the result together: 'turns damp red litmus paper blue', not just 'litmus'
- When describing a procedure, add the reagent a few drops at a time and then in excess, so that both observations can be made
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.