Inorganic Chemistry · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 8% of your exam marks.
Ion tests, flame tests and gas tests all appear regularly; expect to recall observations.
| Cation | Ionic equation | Precipitate colour |
|---|---|---|
| Fe²⁺ (iron(II)) | Fe²⁺(aq) + 2 OH⁻(aq) → Fe(OH)2(s) | Pale green |
| Fe³⁺ (iron(III)) | Fe³⁺(aq) + 3 OH⁻(aq) → Fe(OH)3(s) | Orange-brown (rust colour) |
| Cu²⁺ (copper(II)) | Cu²⁺(aq) + 2 OH⁻(aq) → Cu(OH)2(s) | Light blue |
NH4+(aq) + OH⁻(aq) → NH3(g) + H2O(l)
Identifying Fe²⁺, Fe³⁺ and NH₄⁺ with sodium hydroxide
What comes up: Describe how to distinguish iron(II) from iron(III), or how to confirm an ammonium ion is present.
Write (for iron ions, two marks): (1) Add sodium hydroxide solution to the sample. (2) A green precipitate identifies iron(II); a brown (or red-brown/orange-brown) precipitate identifies iron(III).
Write (for ammonium ion, three marks): (1) Add sodium hydroxide solution and warm the mixture. (2) Test the gas produced with damp red litmus paper held at the mouth of the tube. (3) The litmus paper turns blue, confirming ammonia and therefore ammonium ions are present.
Watch out: For the ammonium test, the litmus paper must be held at the mouth of the tube — not dipped into the solution. The mark scheme does not award the observation marks if you test the solution directly with litmus. Also note: for iron(III), orange-brown and red-brown are accepted alongside brown, but make sure to distinguish it clearly from the pale green of iron(II).