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Sequential precipitation exploiting Ksp differences. Group 0 — NH4+: Heat with NaOH → NH3 gas (turns moist red litmus blue). Test first, before adding group reagents. Group I — Dilute HCl: PbCl2 (white, soluble in hot water), AgCl (white, soluble in NH3), Hg2Cl2 (white, turns black with NH3). Confirmatory: Pb2+ → yellow PbCrO4 or yellow PbI2. Ag+ → [Ag(NH3)2]+ (dissolved AgCl). Group II — H2S in acid: CuS (black), CdS (yellow), HgS (black), As2S3 (yellow), Sb2S3 (orange), SnS (brown). Low [S2-] in acid precipitates only very insoluble sulfides. Group III — NH4Cl + NH4OH: Fe(OH)3 (brown), Al(OH)3 (white, gelatinous), Cr(OH)3 (green). NH4Cl controls [OH-] via common ion effect. Confirmatory: Fe3+ → blood red with KSCN. Al3+ → aluminon lake. Cr3+ → green, amphoteric in NaOH. Group IV — H2S in base: NiS (black), CoS (black), MnS (pink), ZnS (white). Higher [S2-] in base precipitates more soluble sulfides. Group V — (NH4)2CO3: BaCO3, SrCO3, CaCO3 (all white). Flame: Ba = green, Sr = crimson, Ca = brick red. Group VI — Na+, K+, Mg2+ (no group reagent): Na = yellow flame, K = violet flame (cobalt blue glass), Mg = magneson reagent.