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.
Reading it isn't running it.
Collect the apparatus, build the setup and take your own readings — as many times as it takes.