This reference gives the colours you use to identify ions in qualitative analysis: the colour of common ions in solution, the colour of hydroxide and halide precipitates, and the flame colour of metal ions. Read them to name an unknown from what you observe.
Qualitative analysis is really a matter of matching what you see to a known colour, so a reliable colour reference is exactly what you need beside a Paper 3 practical. When you add a reagent and a coloured precipitate forms, or a metal salt colours a flame, the colour names the ion. This reference collects the standard SPM colours in three tables, arranged as the DSKP expects, so you can identify a cation or anion from the observation.
Scope note
The colours below are the standard KSSM reference used in SPM Chemistry. They cover the ions the syllabus asks you to identify, the hydroxide precipitates formed with sodium hydroxide and ammonia solution, the silver and lead halide precipitates, and the flame colours of common metal ions. Describe the colour in the exam using the standard word, because the marking scheme looks for it.
Colours of common ions in solution
| Ion | Formula | Colour in solution |
|---|---|---|
| Copper(II) | Cu2+ | Blue |
| Iron(II) | Fe2+ | Pale green |
| Iron(III) | Fe3+ | Brown (yellow-brown) |
| Manganate(VII) | MnO4- | Purple |
| Dichromate(VI) | Cr2O7 2- | Orange |
| Most other ions (Na+, K+, Ca2+, Zn2+, Al3+, Pb2+, Mg2+, NH4+) | , | Colourless |
Colours of precipitates
| Precipitate | Formula | Colour |
|---|---|---|
| Copper(II) hydroxide | Cu(OH)2 | Blue |
| Iron(II) hydroxide | Fe(OH)2 | Green |
| Iron(III) hydroxide | Fe(OH)3 | Brown |
| Lead(II) hydroxide | Pb(OH)2 | White |
| Zinc hydroxide | Zn(OH)2 | White (dissolves in excess NaOH) |
| Aluminium hydroxide | Al(OH)3 | White (dissolves in excess NaOH) |
| Magnesium / calcium hydroxide | Mg(OH)2 / Ca(OH)2 | White |
| Silver chloride | AgCl | White |
| Silver bromide | AgBr | Cream |
| Silver iodide | AgI | Yellow |
| Lead(II) iodide | PbI2 | Yellow |
Flame test colours
| Metal ion | Formula | Flame colour |
|---|---|---|
| Lithium | Li+ | Red (crimson) |
| Sodium | Na+ | Golden yellow |
| Potassium | K+ | Lilac (purple) |
| Calcium | Ca2+ | Brick red (orange-red) |
| Barium | Ba2+ | Green |
| Copper(II) | Cu2+ | Blue-green |
How to use the colours
Work from the observation to the ion. A blue solution suggests copper(II); a pale green solution suggests iron(II); a brown solution suggests iron(III). When you add sodium hydroxide solution, the colour of the precipitate names the metal: a blue precipitate is copper(II), a green one is iron(II), a brown one is iron(III), and a white one that dissolves in excess alkali is zinc or aluminium. For a halide, add dilute nitric acid then silver nitrate solution: a white precipitate is chloride, cream is bromide and yellow is iodide. For the flame test, dip a clean wire in the salt and hold it in a blue flame.
Building a full identification
A strong answer usually combines two or three of these colour observations rather than relying on one. Suppose an unknown green solution is given. The pale green already points to iron(II); adding sodium hydroxide solution then produces a green precipitate of iron(II) hydroxide, confirming it. If, on standing, that green precipitate slowly turns brown at the surface, you are watching iron(II) hydroxide oxidise to iron(III) hydroxide, which is itself a useful piece of evidence. In the same way, a colourless solution that gives a white precipitate dissolving in excess sodium hydroxide, and a metal that colours a flame, can be cross-checked against both the precipitate table and the flame table until only one ion fits every clue. Training yourself to line up several observations, and to write each colour with the precise word the tables use, is what turns a good guess into a secure identification.
Common mistakes to avoid
- Calling every white precipitate the same. Zinc and aluminium hydroxides dissolve in excess sodium hydroxide; magnesium and calcium do not.
- Confusing the pale green of iron(II) with a colourless solution.
- Writing “yellow” for a sodium flame when the mark scheme wants “golden yellow”, or reversing the potassium and calcium flames.
- Forgetting to acidify before adding silver nitrate in a halide test.
How we help
Our online one-to-one teachers rehearse these observations with you until the colour instantly names the ion, which is exactly the speed a practical question rewards. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Because qualitative analysis threads through Paper 2 and Paper 3, a firm command of these colours turns identification questions into quick, confident marks.
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