2019 O Level Chem P1 Q4 — Qualitative Analysis / Observations and Colour Changes Solution

Answer: D (experiments in the order 3, 1, 2). Match each colour change to its experiment.

Experiment 1, sulfur dioxide with acidified potassium manganate(VII): SO₂ is a reducing agent and reduces the purple manganate(VII) ion to colourless Mn²⁺, so acidified KMnO₄ goes purple to colourless.

Experiment 2, Universal Indicator added to a weak acid: a weak acid gives a pH around 4–6, turning the indicator from green towards orange, so green to orange.

Experiment 3, aqueous potassium iodide and dilute nitric acid, then aqueous silver nitrate: silver ions react with iodide ions to form a yellow precipitate of silver iodide, AgI, so a colourless solution gives a yellow precipitate.

So 'colourless to yellow precipitate' is experiment 3, 'purple to colourless' is experiment 1 and 'green to orange' is experiment 2 — the order 3, 1, 2, which is row D.