Paper 3 Advanced Practical Skills 1
9701/31 · October/November 2025
Question 3 — Qualitative analysis: a mislabelled salt and three manganese oxidation states
Devise your own tests for zinc, sulfate and water of crystallisation on FA 5, then heat solid FA 6 and compare it against FA 7, FA 8 and FA 9 to work out what links them.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 3 — labelled materials
- FA 5(2.0 ± 0.1 g)
- FA 6(0.4 ± 0.1 g)
- FA 7(10 cm³)
- FA 8(10 cm³)
- FA 9(2.0 ± 0.2 g)
- hydrogen peroxide(10 cm³)
- aqueous iron(II) sulfate(10 cm³)
Question 3 — general qualitative-analysis reagents
- dilute hydrochloric acid(10 cm³)
- dilute nitric acid(10 cm³)
- dilute sulfuric acid(10 cm³)
- aqueous ammonia(10 cm³)
- aqueous sodium hydroxide(10 cm³)
- aqueous barium chloride or aqueous barium nitrate(10 cm³)
- limewater(10 cm³)
- aqueous silver nitrate(10 cm³)
- acidified aqueous potassium manganate(VII)(10 cm³)
- apparatus normally used in the centre for testing carbon dioxide with limewater
Question 3 — apparatus
- test-tube(8)
- hard-glass test-tube(2)
- test-tube rack(1)
- test-tube holder(1)
- glass rod(1)
- spatula(1)
- teat/dropping pipette(3)
- funnel(1)
- filter paper(1)
- Bunsen burner and means to light it(1)
- heat-proof mat(1)
- red and blue litmus papers
- wooden splints
- aluminium foil
- wash bottle of distilled water(1)
- pen for labelling glassware(1)
- paper towels
FA 5 — LABELLED AS HYDRATED ZINC SULFATE. IS THE LABEL RIGHT?
Devise your own tests for water of crystallisation, sulfate and the metal cation. Pick a test — you will see exactly what a candidate at the bench would see, and nothing more.
FA 6 — SOLID, HEATED STRONGLY
Heat a few crystals of FA 6 in a hard-glass test-tube until no further gas is evolved, testing the gas with a glowing splint.
FA 7 / FA 8 / FA 9 — THREE TESTS ACROSS THREE MANGANESE SAMPLES
Not every test runs on every sample — Test 2 is FA 8 / FA 9 only, Test 3 is FA 7 / FA 8 only. Pick a test for FA 7.
Why does the label on FA 5 turn out to be wrong?
Zinc hydroxide is amphoteric — it dissolves back into excess sodium hydroxide (as zincate) and into excess aqueous ammonia (as a soluble ammine complex). Magnesium hydroxide does neither: once it precipitates, it stays a precipitate no matter how much more alkali or ammonia you add. That single pair of “dropwise, then in excess” tests is what catches the mislabelling — everything else (water present, sulfate present) is consistent with either salt.
Why three different tests to compare FA 7, FA 8 and FA 9?
Each test probes a different role manganese can play. Hydrogen peroxide can act as either an oxidising or a reducing agent depending on what it meets — against MnO₄⁻ it gets oxidised (releasing O₂), while MnO₂ simply catalyses its decomposition without changing oxidation state itself. Sodium hydroxide only shows anything with a dissolved cation (FA 8), not an insoluble solid (FA 9). Iron(II) sulfate only reacts with an oxidising agent strong enough to take electrons from Fe²⁺ — that rules FA 8 out and confirms FA 7. Together the three tests separate +7, +4-as-catalyst and +2.
Why does FA 8 show 'no change' so often?
Because it is evidence, not a blank. Mn²⁺ is already a low, stable oxidation state for manganese in aqueous solution, so it neither reduces further under H₂O₂ nor gets oxidised by Fe²⁺. Its only reaction here is with sodium hydroxide, which precipitates the dissolved cation and lets you watch it darken in air — that darkening is what tells you manganese can be oxidised beyond +2 at all.
YOUR TASKS · 0/5
- ✓Run every devised test on FA 5
- ✓Conclude whether zinc, sulfate and water are present in FA 5
- ✓Heat FA 6 and dissolve the cooled residue
- ✓Run every applicable test on FA 7, FA 8 and FA 9
- ✓Identify the metal and both oxidation states
IDENTIFICATION
Work through every test on FA 5, then on FA 7/FA 8/FA 9, before drawing conclusions. “No change” is a real observation — record it rather than skipping ahead.
YOUR ANSWERS
A bottle labelled FA 5 is thought to contain hydrated zinc sulfate. Devise and carry out tests to investigate whether zinc ions, sulfate ions and water of crystallisation are present. Record the tests you carry out and the observations you see.
Use your observations in (a)(i) to complete Table 3.1 to show whether Zn²⁺, SO₄²⁻ and H₂O are present in FA 5 (tick if present, cross if not).
Heat a few crystals of FA 6 in a hard-glass test-tube until no further gas is evolved. Record all your observations.
To the cooled residue from (b)(i), add approximately 3 cm depth of distilled water and stir. Filter the solution formed into a test-tube. State the colour of the solution.
You are provided with aqueous FA 7 (the same solution as FA 2), aqueous FA 8 and solid FA 9. Carry out Test 1 (add hydrogen peroxide) on all three; Test 2 (add aqueous sodium hydroxide, then leave to stand) on FA 8 and FA 9 only; and Test 3 (add aqueous iron(II) sulfate) on FA 7 and FA 8 only. Record your observations in Table 3.2.
| FA 7 | FA 8 | FA 9 | |
|---|---|---|---|
| Test 1: add hydrogen peroxide | |||
| Test 2: add aqueous sodium hydroxide, then leave to stand (FA 8, FA 9 only) | |||
| Test 3: add aqueous iron(II) sulfate (FA 7, FA 8 only) |
Suggest the identity of the metal in FA 6/FA 7, FA 8 and FA 9.
Complete Table 3.3 to suggest the oxidation state of the metal in FA 6/FA 7 and FA 8.