Pupil sheet · Redox
Two colour tests — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. state
Supplement. Match each solution to its colour.
Write the number of the matching option.
- brown
- purple
- colourless
- 1. Acidified potassium manganate(VII), before the test: ______
- 2. Acidified potassium manganate(VII), after a reducing agent is added: ______
- 3. Aqueous potassium iodide, before the test: ______
- 4. Aqueous potassium iodide, after an oxidising agent is added: ______
Nudge (only if you are stuck): One test makes a colour disappear; the other makes a colour appear.
2. identify
Supplement. A few drops of colourless solution P are added to acidified potassium manganate(VII). The purple turns colourless. Which statement is correct?
Circle one answer.
- P is a reducing agent: it takes electrons, and manganese is oxidised from +7 to +2
- P is an oxidising agent: it gives electrons, and manganese is reduced from +7 to +2
- P is an oxidising agent: it takes electrons, and manganese is oxidised from +7 to +2
- P is a reducing agent: it gives electrons, and manganese is reduced from +7 to +2
Nudge (only if you are stuck): Manganese goes from +7 in the purple ion to +2 in the colourless one. Is that up or down?
3. deduce
Supplement. A few drops of colourless hydrogen peroxide solution, acidified with dilute sulfuric acid, are added to aqueous potassium iodide, and the solution turns brown. Fill in the gaps. The iodide ions, oxidation number (1) ___, become iodine, oxidation number (2) ___. Each iodide ion (3) ___ an electron, so the iodide ions are (4) ___. Hydrogen peroxide is acting as (5) ___.
(1)
(2)
(3)
(4)
(5)
Nudge (only if you are stuck): Iodine, I2, is an element on its own.
4. identify
Supplement. Four colourless solutions, W, X, Y and Z, are each tested with acidified potassium manganate(VII) and with aqueous potassium iodide. Which solution is an oxidising agent?
| Solution | Acidified potassium manganate(VII) | Aqueous potassium iodide |
|---|---|---|
| W | purple → colourless | stays colourless |
| X | stays purple | colourless → brown |
| Y | purple → paler purple | stays colourless |
| Z | stays purple | stays colourless |
Circle one answer.
- W
- X
- Y
- Z
Nudge (only if you are stuck): Which test shows an oxidising agent: the one where a colour appears, or the one where it goes?
5. explain / describe
Supplement. Solution T is added to acidified potassium manganate(VII), and the purple does not change. A pupil says: "T is unreactive." (a) Explain why the pupil is incorrect. (b) Describe a test to find out whether T is an oxidising agent, and give the result if it is.
(a) Why the pupil is incorrect
(b) The test and its result
Nudge (only if you are stuck): A positive manganate(VII) test shows one kind of agent. Which kind can it miss?
6. deduce / identify
Supplement. Sulfur dioxide gas is bubbled through acidified potassium manganate(VII), and the purple turns colourless. The sulfur ends up in sulfate ions, SO42−. (a) Give the oxidation number of sulfur in SO2 and in SO42−. (b) Is sulfur dioxide oxidised or reduced? Give a reason. (c) Identify the oxidising agent, and give the change in the oxidation number of its manganese.
(a) Oxidation numbers of sulfur
(b) Oxidised or reduced, and why
(c) Oxidising agent, and the change for manganese
Nudge (only if you are stuck): Use the rules for a compound and for an ion of several atoms, with oxygen at −2.