NOTES · CHEMISTRY
Qualitative analysis in O-Level Chemistry: every test, and how to word it
Method & Mark ·
Qualitative analysis is where Chemistry marks are won or lost on wording. The syllabus prints a short set of notes: five anion tests, seven cation tests and six gas tests. A student who knows those tables exactly, and writes observations in their words, has most of the marks already. Below are the tables as the 2026 O-Level syllabus (6092) gives them, then the places students go wrong.
Tests for anions
| Anion | Test | Result |
|---|---|---|
| carbonate, CO32− | add dilute acid | effervescence, carbon dioxide produced |
| chloride, Cl− | acidify with dilute nitric acid, then add aqueous silver nitrate | white ppt. |
| iodide, I− | acidify with dilute nitric acid, then add aqueous silver nitrate | yellow ppt. |
| nitrate, NO3− | add aqueous sodium hydroxide, then aluminium foil; warm carefully | ammonia produced |
| sulfate, SO42− | acidify with dilute nitric acid, then add aqueous barium nitrate | white ppt. |
The chloride, iodide, nitrate and sulfate tests are done on the anion in solution.
Tests for aqueous cations
| Cation | Aqueous sodium hydroxide | Aqueous ammonia |
|---|---|---|
| aluminium, Al3+ | white ppt., soluble in excess giving a colourless solution | white ppt., insoluble in excess |
| ammonium, NH4+ | ammonia produced on warming | – |
| calcium, Ca2+ | white ppt., insoluble in excess | no ppt. |
| copper(II), Cu2+ | light blue ppt., insoluble in excess | light blue ppt., soluble in excess giving a dark blue solution |
| iron(II), Fe2+ | green ppt., insoluble in excess | green ppt., insoluble in excess |
| iron(III), Fe3+ | red-brown ppt., insoluble in excess | red-brown ppt., insoluble in excess |
| zinc, Zn2+ | white ppt., soluble in excess giving a colourless solution | white ppt., soluble in excess giving a colourless solution |
Tests for gases
| Gas | Test and result |
|---|---|
| ammonia, NH3 | turns damp red litmus paper blue |
| carbon dioxide, CO2 | gives white ppt. with limewater (ppt. dissolves with excess CO2) |
| chlorine, Cl2 | bleaches damp litmus paper |
| hydrogen, H2 | ‘pops’ with a lighted splint |
| oxygen, O2 | relights a glowing splint |
| sulfur dioxide, SO2 | turns aqueous acidified potassium manganate(VII) from purple to colourless |
The pairs students mix up
- Al3+ or Zn2+
- Both give a white precipitate that dissolves in excess sodium hydroxide. Aqueous ammonia separates them: the zinc precipitate dissolves in excess, the aluminium one does not.
- Ca2+ or Al3+
- Both give a white precipitate with sodium hydroxide, but the calcium one stays in excess. With aqueous ammonia, calcium gives no precipitate at all.
- Fe2+ or Fe3+
- Colour decides it: green for iron(II), red-brown for iron(III), with either reagent.
- Cu2+
- The only cation here whose precipitate dissolves in excess aqueous ammonia to give a dark blue solution.
- NH4+ or NO3−
- Both end with ammonia turning damp red litmus blue. Nitrate needs aluminium foil as well as sodium hydroxide; ammonium needs only sodium hydroxide and warming.
Where the marks go
- Write both halves of a cation result. “White ppt.” alone is incomplete. The scheme wants what happens in excess too: “white ppt., soluble in excess giving a colourless solution”.
- Name the gas and its test. “Effervescence” earns the observation. Carbon dioxide is identified only when the gas is tested: it gives a white precipitate with limewater.
- Acidify with nitric acid. Hydrochloric acid would add the chloride you are testing for, and sulfuric acid the sulfate. The notes say dilute nitric acid for all three silver and barium tests.
- Use the scheme’s colours. Light blue, dark blue, green, red-brown. “Brown” for iron(III) or “blue” for the copper(II) precipitate can cost the mark.
- Say “damp” litmus. Ammonia and chlorine are tested with damp litmus paper; a gas cannot change dry litmus.
Source: the Notes for Qualitative Analysis in the SEAB O-Level Chemistry 6092 syllabus (2026). For how the pure and combined chemistry papers differ, see Pure or combined science.