Qualitative inorganic analysis (classical wet chemistry) identifies cations and anions in an unknown salt. Cations are separated into groups based on their precipitating reagent. The groups for cations (Sulphide group method): Group I (HCl group): Ag⁺, Pb²⁺, Hg₂²⁺ — precipitate as chlorides with dilute HCl. Group II (H₂S in acid): Cu²⁺, Pb²⁺, Bi³⁺, Cd²⁺, Sn²⁺/Sn⁴⁺, As³⁺/As⁵⁺, Sb³⁺ — precipitate as sulfides in dilute HCl (acidic medium). Group III (NH₄OH): Al³⁺, Fe³⁺, Cr³⁺ — precipitate as hydroxides. Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ are not precipitated (their hydroxides are more soluble). Group IV (H₂S in NH₃): Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺ — precipitate as sulfides in alkaline medium. Group V ((NH₄)₂CO₃): Ba²⁺, Sr²⁺, Ca²⁺ — precipitate as carbonates. Group VI (no reagent): Mg²⁺, Na⁺, K⁺, NH₄⁺ — soluble group, detected by specific flame tests or precipitation reactions.

Reagent: dilute HCl. Precipitates: AgCl (white, curdy, insoluble), PbCl₂ (white, slightly soluble in cold water, dissolves in hot water — used to confirm Pb²⁺), Hg₂Cl₂ (calomel, white, turns black with NH₃ due to disproportionation: Hg₂Cl₂ + 2NH₃ → Hg(NH₂)Cl↓(white) + Hg↓(black) + NH₄Cl). Confirmatory tests: AgCl dissolves in NH₃ (forms [Ag(NH₃)₂]⁺) but PbCl₂ does not → distinguishes Ag⁺ from Pb²⁺. AgCl in NH₃ + acidify → white precipitate returns. AgCl + KI → AgI (yellow) — more insoluble. Ksp values: AgCl = 1.8×10⁻¹⁰, AgBr = 5×10⁻¹³, AgI = 8.5×10⁻¹⁷ (decreasing solubility). PbSO₄ (white) also precipitates with dilute H₂SO₄. Lead stays in both Group I and Group II.

H₂S in water: $H_2S \rightleftharpoons H^+ + HS^- \rightleftharpoons 2H^+ + S^{2-}$. Overall: $K = [H^+]^2[S^{2-}]/[H_2S] \approx 10^{-21}$. At pH = 0 (1M HCl): $[S^{2-}] = 10^{-21}/(1)^2 = 10^{-21}$ M. Only very insoluble sulfides (Ksp < 10⁻²⁰) precipitate → Group II (CuS Ksp = 10⁻³⁶, Bi₂S₃ Ksp = 10⁻⁹⁹). At pH = 9 (ammoniacal): $[S^{2-}] \approx 10^{-21}/(10^{-9})^2 = 10^{-3}$ M. Less insoluble sulfides also precipitate → Group IV (ZnS Ksp = 10⁻²³, NiS Ksp = 10⁻²¹, MnS Ksp = 10⁻¹³). This pH control is the fundamental principle separating Group II from Group IV.

Flame tests identify alkali metal ions and alkaline earth metals (Groups V and VI) because their electrons excited by flame emit characteristic wavelengths. Li⁺: crimson red (670 nm). Na⁺: golden yellow (589 nm, very intense — masks other colours). K⁺: violet/lilac (767 nm). Ca²⁺: brick red (622 nm). Sr²⁺: crimson red (606 nm). Ba²⁺: pale green (554 nm). Cu²⁺: blue-green (513 nm). Pb²⁺: pale blue (not diagnostic). Technique: clean platinum wire (or nichrome), dip in sample dissolved in HCl, hold in non-luminous bunsen flame. View K⁺ through blue cobalt glass (filters yellow Na⁺ light). Spectroscope: identifies elements more precisely than naked eye. Atomisation and electronic excitation in flame → emission spectrum. Modern: ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) does this quantitatively for ppb concentrations.

Dilute H₂SO₄ group: CO₃²⁻ (CO₂, limewater test), S²⁻ (H₂S, smell/lead acetate paper), SO₃²⁻ (SO₂, smell/potassium permanganate decolourise), NO₂⁻ (brown fumes). Concentrated H₂SO₄ group: Cl⁻ (HCl gas, white fumes with NH₃), Br⁻ (HBr + brown Br₂), I⁻ (HI + violet I₂ vapour), NO₃⁻ (brown fumes of NO₂), CH₃COO⁻ (vinegar smell). Silver nitrate test: Cl⁻ → AgCl (white, soluble in NH₃). Br⁻ → AgBr (pale yellow, soluble in conc. NH₃). I⁻ → AgI (yellow, insoluble in NH₃). Barium chloride test: SO₄²⁻ → BaSO₄ (white precipitate, insoluble in HCl). Ferric chloride test: CNS⁻ → blood-red [Fe(CNS)]²⁺. CH₃COO⁻ → iron acetate (reddish brown, fruity smell of ester on heating with H₂SO₄).

Solubility product $K_{sp} = [M^{n+}][A^{m-}]$ for sparingly soluble salt MA. For $M_aA_b$: $K_{sp} = [M]^a[A]^b$. Condition for precipitation: ionic product $Q > K_{sp}$. When $Q = K_{sp}$: saturated solution (equilibrium). $Q < K_{sp}$: unsaturated, no precipitate. Common ion effect: adding a common ion decreases solubility (Le Chatelier). Example: AgCl in 0.1 M NaCl: $[Ag^+] = K_{sp}/[Cl^-] = 1.8\times10^{-10}/0.1 = 1.8\times10^{-9}$ M. Compared to AgCl in pure water: $[Ag^+] = \sqrt{K_{sp}} = 1.34\times10^{-5}$ M. Solubility decreased by factor 7400. Selective precipitation: by controlling anion concentration, separate cations. Fractional precipitation: adding precipitating reagent gradually — less soluble salt precipitates first.

Fe³⁺: KSCN → blood-red [Fe(SCN)]²⁺. K₄[Fe(CN)₆] → Prussian blue Fe₄[Fe(CN)₆]₃. Cu²⁺: NH₃ → deep blue [Cu(NH₃)₄]²⁺. K₄[Fe(CN)₆] → chocolate-brown CuFe(CN)₆ precipitate. Al³⁺: aluminon test → red lake. NH₃ + aluminon solution → Al(OH)₃ adsorbs aluminon. Flame test: not characteristic. Pb²⁺: K₂CrO₄ → yellow PbCrO₄ precipitate. H₂S → black PbS. KI → bright yellow PbI₂. Mn²⁺: sodium bismuthate (NaBiO₃) in HNO₃ → MnO₄⁻ (purple) oxidation confirms Mn. HNO₃ + Mn²⁺ + ammonium persulphate → purple MnO₄⁻. Zn²⁺: K₄[Fe(CN)₆] → white ZnK₂[Fe(CN)₆]. Sodium hydroxide → white Zn(OH)₂ dissolves in excess NaOH (amphoteric → [Zn(OH)₄]²⁻). NH₃ → white Zn(OH)₂ dissolves in excess → [Zn(NH₃)₄]²⁺.

Classical qualitative analysis (wet chemistry): historically important, still taught for conceptual understanding. Time-consuming, requires skilled analyst, uses hazardous H₂S gas. Modern alternatives: ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry): simultaneous determination of 70+ elements at ppb levels in minutes. Sample dissolved, passed through plasma (10,000 K), emitted light analysed. ICP-MS (Mass Spectrometry): ppt sensitivity, isotopic information. XRF (X-ray fluorescence): non-destructive elemental analysis of solid surfaces. AAS (Atomic Absorption Spectroscopy): highly selective, one element at a time, ppm-ppb sensitivity. For NEET: understand the classical group analysis scheme, reagents, chemistry. The conceptual understanding (why H₂S separates Group II from IV, why pH matters, why flame tests work) is more important than memorisation. The group separation scheme also illustrates: Ksp, common ion effect, solubility equilibria, complex ion formation — all important topics.