Practical chemistry · salt analysis
Qualitative Salt Analysis
Identify the basic radical (cation) by systematic group separation, confirm it, and test for the acidic radical (anion). Follow the separation scheme, look up any test, then try a mystery salt.
Separation scheme · cations
One reagent at a time, filtrate passed on
Tap a group to see its reagent, why it works, the precipitate colours and the confirmatory tests.
Reference
Every test: reagent → observation
Search an ion, reagent or colour. The dot shows the colour you should see.
Practice
Mystery salt
Reveal clues one by one, as in the practical exam: appearance, anion tests, then the group test and a confirmatory test. Name both radicals with as few clues as you can.
Exam notes
Why the scheme works
Common-ion effect: Group II versus Group IV sulphides
H₂S ⇌ 2H⁺ + S²⁻. In dilute HCl the extra H⁺ pushes this equilibrium left, so [S²⁻] is very small and only sulphides with very low Ksp (CuS, PbS, HgS, CdS, Bi₂S₃, As₂S₃, SnS) exceed their solubility product. In ammoniacal solution OH⁻ removes H⁺, [S²⁻] rises, and the more soluble sulphides of Group IV (ZnS, MnS, NiS, CoS) precipitate.
Why NH₄Cl is added with NH₄OH in Group III
NH₄Cl supplies NH₄⁺, which suppresses the ionisation of NH₄OH (common-ion effect). [OH⁻] stays low, so only the least soluble hydroxides, Fe(OH)₃, Al(OH)₃ and Cr(OH)₃, precipitate. Without it, Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺ and Mg²⁺ hydroxides could also come down. H₂S is boiled off first so that Group IV sulphides do not precipitate here, and conc. HNO₃ oxidises Fe²⁺ to Fe³⁺ because Fe(OH)₂ is too soluble to precipitate completely.
Why NH₄⁺ is tested on the original salt
Ammonium salts (NH₄Cl, NH₄OH, (NH₄)₂CO₃) are added as reagents during the group analysis, so a test for NH₄⁺ later on would always be positive. Warm the original salt with NaOH: NH₃ turns moist red litmus blue and gives dense white fumes with a glass rod dipped in conc. HCl.
Pb²⁺ appears in two groups
PbCl₂ is fairly soluble in cold water, so Pb²⁺ is not removed completely by dilute HCl. The remainder precipitates as black PbS in Group II. That is why lead is listed in both Group I and Group II.
Flame colours
| Na⁺ | persistent golden yellow |
| K⁺ | lilac (violet); view through blue cobalt glass to cut out sodium’s yellow |
| Ca²⁺ | brick red |
| Sr²⁺ | crimson red |
| Ba²⁺ | apple (grassy) green |
| Cu²⁺ | bluish green |
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