Qualitative Analysis

Concept overview

Qualitative analysis is the systematic identification of the cations and anions present in an unknown inorganic sample. It's a workflow-driven chapter — memorising the six cation groups and the reagents that separate them is the entire challenge. JEE tests this both as direct-recall questions and as short chain-of-reactions problems.

The six cation groups, in order of Group Reagent addition: - Group I: Ag⁺, Pb²⁺, Hg₂²⁺ precipitate with dilute HCl as insoluble chlorides. - Group II: Cu²⁺, Cd²⁺, Bi³⁺, Hg²⁺, As³⁺/⁵⁺, Sb³⁺/⁵⁺, Sn²⁺/⁴⁺ precipitate with H₂S in acidic medium as sulfides. - Group III: Al³⁺, Fe³⁺, Cr³⁺ precipitate with NH₄OH (in NH₄Cl) as hydroxides. - Group IV: Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺ precipitate with H₂S in basic medium as sulfides. - Group V: Ba²⁺, Sr²⁺, Ca²⁺ precipitate with (NH₄)₂CO₃ as carbonates. - Group VI: Mg²⁺, Na⁺, K⁺, NH₄⁺ — soluble, identified by specific flame or precipitation tests.

Once a cation is placed in its group, specific tests confirm identity. Fe³⁺ gives a blood-red colour with KSCN. Cu²⁺ gives a deep blue solution with NH₃ (Schweitzer's reagent). Ni²⁺ gives a rosy-red precipitate with dimethylglyoxime. Pb²⁺ gives yellow PbI₂ (soluble in hot water, reprecipitating on cooling — the 'golden shower' test). These specific confirmatory tests are the JEE-favourite direct-recall questions.

Anion identification uses characteristic reactions: sulfate with BaCl₂ gives white insoluble BaSO₄; carbonate with dilute acid gives brisk effervescence of CO₂; halides with AgNO₃ give characteristic coloured silver halide precipitates (AgCl white, AgBr pale yellow, AgI yellow). Nitrate is confirmed by the brown-ring test with FeSO₄ and conc. H₂SO₄.

Flame tests provide a quick screen for group I and V metals: Na gives golden-yellow, K gives lilac (viewed through cobalt-blue glass to filter Na), Ca gives brick-red, Sr crimson, Ba apple-green.

Key formulas & concepts

• Group I reagent: Dilute HCl → precipitates AgCl, PbCl₂, Hg₂Cl₂
• Group II reagent: H₂S in acidic → sulfides of Cu, Cd, Bi, Hg, As, Sb, Sn
• Group III reagent: NH₄OH + NH₄Cl → hydroxides of Al, Fe, Cr
• Group IV reagent: H₂S in basic → sulfides of Zn (white), Ni (black), Co (black), Mn (buff)
• Group V reagent: (NH₄)₂CO₃ → carbonates of Ba, Sr, Ca
• Brown ring (nitrate): Fe²⁺ + NO₃⁻ + conc. H₂SO₄ → [Fe(NO)]²⁺ ring at interface
• Cu²⁺ + NH₃ excess: Deep blue [Cu(NH₃)₄]²⁺ (Schweitzer's reagent)

Common mistakes to avoid

• Confusing Group II sulfides (acidic, coloured) with Group IV sulfides (basic, mostly black except ZnS white) — the medium determines the group.
• Assuming Group I has all cations that give chlorides — only Ag⁺, Pb²⁺, Hg₂²⁺ give insoluble chlorides; others like Ca²⁺ remain in solution.
• Skipping the cobalt-blue-glass step for K⁺ flame test — without the filter, Na⁺ swamps the lilac K⁺ colour.
• Confusing PbSO₄ (soluble in NH₄Ac) with BaSO₄ (insoluble in most reagents) — this distinguishes Pb²⁺ from Ba²⁺ in the workflow.

JEE Main relevance

JEE Main asks 1 qualitative analysis question every paper — usually a chain: given a test result, identify the ion; or given an ion, predict the test result. These are direct-recall.

JEE Advanced relevance

Advanced problems chain three or four tests — for instance, an unknown sample gives white precipitate with dilute HCl, dissolves in hot water, and gives yellow precipitate with KI. Identifying the cation from this chain requires knowledge of the group workflow and specific confirmatory tests.

Previous-year questions (PYQ)

Filter the PYQ bank by 'Qualitative Analysis'. Memorise the six group reagents and 20 confirmatory tests — this alone answers 90% of JEE Main questions from this chapter.