Qualitative analysis: a mislabelled salt and three manganese oxidation states
A bottle labelled hydrated zinc sulfate that isn't, and three manganese compounds that turn out to be the same element at different oxidation states. Devise your own cation/anion tests on the first, then run a fixed three-test comparison across the second.
WHAT YOU'LL LEARN
- ✓Devising your own test series for a cation, an anion and water of crystallisation from a shared qualitative-analysis reagent kit
- ✓Why 'dropwise then in excess' with both sodium hydroxide and aqueous ammonia is the test that separates zinc from magnesium
- ✓Why a bottle's label is not evidence — only your own observations are
- ✓Thermal decomposition of potassium manganate(VII) and testing the gas evolved
- ✓How hydrogen peroxide, sodium hydroxide and iron(II) sulfate each probe a different manganese oxidation state
- ✓Reading 'no change' as real evidence rather than a non-result
ON YOUR BENCH
- □FA 5 — solid labelled hydrated zinc sulfate (actually hydrated magnesium sulfate, MgSO4·7H2O)
- □FA 6 — solid potassium manganate(VII), KMnO4
- □FA 7 — acidified aqueous potassium manganate(VII) (same solution as FA 2)
- □FA 8 — aqueous manganese(II) sulfate or chloride
- □FA 9 — solid manganese(IV) oxide, MnO2
- □Dilute hydrochloric, nitric and sulfuric acids; aqueous ammonia and sodium hydroxide
- □Aqueous barium chloride/nitrate, limewater, aqueous silver nitrate, acidified dilute potassium manganate(VII)
- □Hydrogen peroxide (10-volume) and aqueous iron(II) sulfate
- □Test-tubes, hard-glass test-tubes, rack, holder, glass rod, spatula, dropping pipettes
- □Filter funnel and paper, Bunsen burner, heatproof mat
- □Red and blue litmus papers, wooden splints, aluminium foil, wash bottle of distilled water
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Devise tests for FA 5
The label claims hydrated zinc sulfate. Using the shared reagent kit, plan and carry out your own tests for water of crystallisation, sulfate and the metal cation — do not assume the label is correct.
- 02
Test for water of crystallisation
Heat a small sample of FA 5 gently in a dry test-tube and watch the cooler part of the glass for condensation.
- 03
Test for sulfate
Dissolve a portion of FA 5 in distilled water, acidify with dilute nitric acid, then add aqueous barium chloride or barium nitrate. A white precipitate that survives the acid is sulfate.
- 04
Test the cation, dropwise then in excess
To separate portions of the FA 5 solution, add aqueous sodium hydroxide dropwise then in excess, and separately aqueous ammonia dropwise then in excess. Zinc hydroxide redissolves in excess of both; if it doesn't, zinc is absent.
- 05
Heat FA 6 to decomposition
Heat a few crystals of FA 6 in a hard-glass test-tube until no further gas is evolved, testing the gas at the mouth of the tube with a glowing splint.
- 06
Dissolve and filter the residue
Add distilled water to the cooled black residue, stir, and filter into a clean test-tube. Record the colour of the filtrate.
- 07
Run Test 1 (hydrogen peroxide) on FA 7, FA 8 and FA 9
Add a few drops of hydrogen peroxide to a portion of each and record what you see, testing any gas evolved with a glowing splint.
- 08
Run Test 2 (sodium hydroxide) on FA 8 and FA 9 only
Add aqueous sodium hydroxide and leave to stand. Record any immediate and any delayed change.
- 09
Run Test 3 (iron(II) sulfate) on FA 7 and FA 8 only
Add aqueous iron(II) sulfate and record what happens to the colour of each.
- 10
Identify the metal and its oxidation states
Use all your observations to name the common metal and its oxidation state in each sample.
The evidence and what it means
- →FA 5 is really hydrated magnesium sulfate, not zinc sulfate: the precipitate from both NaOH and ammonia does not redissolve in excess, which zinc hydroxide would. Sulfate and water of crystallisation are both genuinely present.
- →FA 6 starts purple, decrepitates on heating, leaves a black residue, and relights a glowing splint — 2KMnO4 -> K2MnO4 + MnO2 + O2. The cooled residue dissolves to a dark-green filtrate (manganate(VI)).
- →FA 7 (MnO4-, +7) is decolourised by both hydrogen peroxide and iron(II) sulfate, releasing oxygen with peroxide that relights a splint.
- →FA 8 (Mn2+, +2) shows no change with hydrogen peroxide or iron(II) sulfate, but gives an off-white precipitate with sodium hydroxide that turns brown on standing in air.
- →FA 9 (solid MnO2) shows no change with sodium hydroxide, but catalyses vigorous effervescence with hydrogen peroxide, relighting a glowing splint — the manganese itself is unchanged.
| Test | FA 7 | FA 8 | FA 9 |
|---|---|---|---|
| Test 1: + hydrogen peroxide | Purple to colourless, relights glowing splint | No change | Effervesces, relights glowing splint |
| Test 2: + NaOH, stand | not tested | Off-white ppt, turns brown on standing | No change |
| Test 3: + iron(II) sulfate | Purple to colourless/pale yellow | No change | not tested |
⚠ BEFORE YOU START
- •Dilute acids, aqueous ammonia and sodium hydroxide are all corrosive or irritant — goggles throughout.
- •Hydrogen peroxide can bleach skin and clothing; use the dropper provided.
- •Heating FA 6 releases oxygen, which supports combustion vigorously — keep flammable material away from the mouth of the tube.
- •Acidified potassium manganate(VII) and silver nitrate both stain; wipe up spills immediately.
Try it, then run it for real.
Practise the whole thing on the virtual bench, then book real lab time and do it with your own hands.