Qualitative analysis: identifying the ions in two unknown solids
Two labelled solids, a rack of reagents and no answers. Work through the standard test series, record what you actually see, and name the ions from the evidence.
WHAT YOU'LL LEARN
- ✓The standard cation tests: sodium hydroxide and aqueous ammonia, dropwise then in excess
- ✓The standard anion tests: barium nitrate for sulfate, silver nitrate for halides
- ✓Why 'dropwise then in excess' is two observations and two marks, not one
- ✓Writing observations that describe what you saw, not what you concluded
ON YOUR BENCH
- □Solid E and solid F in stoppered boiling tubes
- □Rack of 3 test-tubes, 3 boiling tubes, test-tube holder
- □Aqueous sodium hydroxide and aqueous ammonia, 1 mol/dm³
- □Dilute nitric acid, 1 mol/dm³
- □Aqueous silver nitrate and aqueous barium nitrate
- □Acidified aqueous potassium manganate(VII)
- □Red and blue litmus papers, wooden splints, limewater
- □Bunsen burner, heatproof mat, flame-test apparatus
- □Dropping pipettes, distilled water
The protocol, step by step
The same guide is printed and waiting at your bench.
- 01
Dissolve each solid
Add about 3 cm depth of distilled water to the boiling tube, replace the stopper and shake until it dissolves. Divide the solution into three roughly equal portions — you get one shot at each test.
- 02
Test with sodium hydroxide, then warm it
Add sodium hydroxide to the first portion and look carefully — 'no change' is a real, creditable observation. Then warm it gently and hold damp red litmus in the mouth of the tube. Litmus turning blue means ammonia, which means an ammonium salt.
- 03
Test for sulfate
Dilute nitric acid first, then barium nitrate. The acid removes carbonate, which would also give a white precipitate and fool you. A white precipitate that survives the acid is sulfate.
- 04
Test with acidified manganate(VII)
A few drops into the third portion. If it stays pink, nothing has reduced it. If it decolourises, you have a reducing agent — that difference is the whole test.
- 05
Flame test the second solid
Clean loop, then the solid at the edge of a blue flame. An intense yellow is sodium — and sodium is so easy to pick up by accident that a clean loop matters more here than anywhere.
- 06
Test for halide
Nitric acid then silver nitrate. White is chloride, cream is bromide, yellow is iodide. Say which yellow you mean — 'precipitate' alone earns nothing.
- 07
Sodium hydroxide and ammonia, dropwise then in excess
Add a few drops, record what forms, then keep going and record what happens to it. A white precipitate that redissolves in both reagents points at zinc; one that stays put does not.
- 08
Name the ions
Collect your observations and identify every ion you have evidence for. Solid F is a mixture, so expect more than two.
The evidence and what it means
- →Solid E is ammonium sulfate: no change with cold alkali, ammonia on warming, and a white precipitate with acidified barium nitrate.
- →Solid F is a mixture — a yellow flame for sodium, a yellow precipitate with silver nitrate for iodide, and a white precipitate that dissolves in excess of both sodium hydroxide and ammonia for zinc.
- →'Dissolves in excess' is what separates zinc from most other white precipitates. Aluminium redissolves in excess sodium hydroxide but not in excess ammonia, so testing both is what pins it down.
- →Write what you saw. 'White precipitate, then colourless solution' scores; 'zinc present' in the observation box does not.
| Test | Observation | Conclusion |
|---|---|---|
| NaOH, cold | No change | No cation that precipitates with alkali |
| NaOH, warmed | Red litmus → blue | Ammonium, NH₄⁺ |
| HNO₃ then Ba(NO₃)₂ | White precipitate | Sulfate, SO₄²⁻ |
| Flame test | Yellow flame | Sodium, Na⁺ |
| HNO₃ then AgNO₃ | Yellow precipitate | Iodide, I⁻ |
| NaOH dropwise → excess | White precipitate → dissolves | Zinc, Zn²⁺ |
| NH₃ dropwise → excess | White precipitate → dissolves | Zinc, Zn²⁺ confirmed |
⚠ BEFORE YOU START
- •Warming alkali releases ammonia, which is toxic and irritating — keep the lab ventilated and never put your nose over the tube.
- •Silver nitrate stains skin black and the stain takes days to go. Use the dropper.
- •Dilute nitric acid and 1 mol/dm³ sodium hydroxide are both corrosive. Goggles throughout.
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.