A student carries out two separate tests using the standard test reagents for oxidising and reducing agents. Test 1: sulfur dioxide gas, SO2, is bubbled through a few cm^3 of acidified aqueous potassium manganate(VII). Test 2: a few drops of chlorine water are added to a few cm^…

Answer: ((a)) The purple colour disappears (turns colourless). ((b)) Sulfur dioxide is acting as a reducing agent, because it decolourises acidified potassium manganate(VII). ((c)) The colourless solution turns brown; the coloured product is iodine. ((d)) Chlorine is acting as an oxidising agent, because it turns potassium iodide from colourless to brown (forming iodine).. Part (a): Observation in test 1 Acidified potassium manganate(VII) is purple. When sulfur dioxide is bubbled through it, the purple colour disappears and the solution becomes colourless. Part (b): Identifying sulfur dioxide's role Acidifie…

IGCSE Chemistry 0620: Oxidising and Reducing Agent Colour Tests

A student carries out two separate tests using the standard test reagents for oxidising and reducing agents.

Test 1: sulfur dioxide gas, $\text{SO}_2$ , is bubbled through a few $\text{cm}^3$ of acidified aqueous potassium manganate(VII).

Test 2: a few drops of chlorine water are added to a few $\text{cm}^3$ of aqueous potassium iodide.

(a) State the colour change observed in test 1. [1]

(b) State whether sulfur dioxide is acting as an oxidising agent or a reducing agent in test 1, giving a reason based on your answer to (a). [2]

(c) State the colour change observed in test 2, and name the coloured product responsible for this colour change. [2]

(d) State whether chlorine is acting as an oxidising agent or a reducing agent in test 2, giving a reason based on your answer to (c). [2]

Part (a): Observation in test 1

Acidified potassium manganate(VII) is purple. When sulfur dioxide is bubbled through it, the purple colour disappears and the solution becomes colourless.

Part (b): Identifying sulfur dioxide’s role

Acidified potassium manganate(VII) turning from purple to colourless is the standard test for a reducing agent. The manganate(VII) ions are being reduced. Since sulfur dioxide causes this colour change, sulfur dioxide is acting as a reducing agent in test 1.

Part (c): Observation in test 2

Aqueous potassium iodide is colourless. When chlorine water is added, the solution turns brown. The coloured product responsible for this is iodine, formed as chlorine displaces iodine from the iodide ions.

Part (d): Identifying chlorine’s role

Aqueous potassium iodide turning from colourless to brown, as iodine forms, is the standard test for an oxidising agent. The iodide ions are being oxidised. Since chlorine causes this colour change, chlorine is acting as an oxidising agent in test 2.

Final answers

(a) Purple colour disappears (turns colourless).
(b) Sulfur dioxide is a reducing agent, it decolourises acidified potassium manganate(VII).
(c) Colourless to brown; the coloured product is iodine.
(d) Chlorine is an oxidising agent, it turns potassium iodide from colourless to brown.