Cation Identification via Precipitation and Ksp-Based Separation
Classic qualitative-analysis cation tests rely on solubility differences that look almost identical at first (a white precipitate, a coloured lake, a sulfide) but resolve cleanly once you know the specific exception each test is built around.
Identifying Pb²⁺ and Al³⁺ from Precipitate and Filtrate Tests is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
PbCl₂ unusual solubility behaviour, and the alizarin test for aluminium
Adding dilute HCl to a cation mixture, PbCl₂ is the one common chloride that precipitates in the cold but then DISSOLVES on heating in water (unlike AgCl or Hg₂Cl₂, which stay insoluble even in hot water) — so a white precipitate that redissolves in hot water is the signature of Pb²⁺. The filtrate left behind, if it gives a positive test with alkaline alizarin (forming a characteristic red/pink lake, alizarin red S dye complexing with the metal hydroxide), confirms Al³⁺ specifically — this is the standard confirmatory test for aluminium in qualitative schemes, distinct from any other common cation.
Using Ksp values to predict which cation precipitates under which pH
When two metal sulfides have very different solubility products, the one with the SMALLER Ksp needs only a very LOW sulfide ion concentration to reach saturation and precipitate — achievable even in ACIDIC solution, where H₂S ionisation (and hence S²⁻ concentration) is suppressed by the excess H⁺ present. The metal sulfide with a LARGER Ksp needs a much HIGHER sulfide ion concentration to precipitate, which is only reached in ALKALINE solution (removing H⁺ shifts the H₂S ionisation equilibrium to release much more S₂⁻). So comparing two Ksp values (for example 1x10⁻⁴⁴ versus 1.4x10⁻²⁴) directly predicts which cation precipitates first, under acidic conditions, and which needs alkaline conditions to precipitate second — this Ksp-driven, pH-controlled sequential precipitation is exactly how H₂S is used to separate cations in classical qualitative analysis schemes.
Reading the anion clues alongside the cation Ksp logic
In the same mixed-salt analysis, the sodium carbonate extract treated with AgNO₃ giving a precipitate that dissolves with difficulty in NH₄OH specifically identifies bromide (recall AgBr dissolving only partially/with difficulty in concentrated ammonia, distinct from AgCl dissolving readily and AgI not dissolving at all), while a separate brown ring test identifies the other anion as nitrate. A statement claiming the anions present are nitrate and chloride, rather than nitrate and bromide, is therefore the incorrect one to flag in this kind of composite question — always cross-check the specific solubility behaviour described (readily soluble vs soluble with difficulty vs insoluble) against the halide it actually points to, rather than defaulting to chloride as the most familiar option.
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- ✓PbCl₂ precipitates in the cold but dissolves in hot water — a distinguishing behaviour among the common insoluble chlorides
- ✓A red/pink lake with alkaline alizarin confirms Al³⁺ in the filtrate
- ✓A metal sulfide with a much smaller Ksp precipitates under ACIDIC conditions (low S²⁻ suffices); a larger-Ksp sulfide needs ALKALINE conditions (more S²⁻ available)
- ✓A precipitate described as soluble with difficulty in NH₄OH specifically identifies bromide, not chloride
- ✓Cross-check each solubility description (readily soluble, soluble with difficulty, insoluble) against the correct halide rather than assuming chloride by default
Do not assume a silver halide precipitate that dissolves imperfectly in ammonia is chloride just because chloride is the most commonly tested halide. Dissolving readily means chloride, dissolving with difficulty means bromide, and not dissolving at all means iodide — read the exact wording, since it is the entire basis for the identification.
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