Qualitative analysis of anions is a staple of NEET's practical chemistry section. The core task: given an unknown salt, identify the anion present using systematic wet tests. Where aspirants lose marks is not in forgetting the tests themselves, but in confusing anions that produce similar observations — gases with overlapping smell or colour, precipitates with similar appearance, or confirmatory tests that share a reagent.
The systematic approach. Anion analysis follows a fixed sequence: preliminary tests (heating, flame test, borax bead), then group reagent tests with dilute H₂SO₄ and concentrated H₂SO₄, followed by confirmatory (wet) tests specific to each anion.
Group A — Anions that react with dilute H₂SO₄. CO₃²⁻ gives brisk effervescence with CO₂ (turns lime water milky). S²⁻ gives H₂S (rotten-egg smell, turns lead acetate paper black). SO₃²⁻ gives SO₂ (suffocating smell, turns acidified K₂Cr₂O₇ green). NO₂⁻ gives brown fumes of NO₂. CH₃COO⁻ gives vinegar smell.
Group B — Anions that react only with concentrated H₂SO₄. Cl⁻ gives white fumes of HCl (dense white fumes with NH₃). Br⁻ gives reddish-brown vapour of Br₂. I⁻ gives violet vapour of I₂. NO₃⁻ gives brown fumes (ring test confirms — brown ring of FeSO₄·NO at junction). C₂O₄²⁻ gives CO₂ on heating with conc. H₂SO₄ plus MnO₂.
Confirmatory tests matter. The preliminary observation narrows candidates, but the confirmatory test clinches the identity. For example, both CO₃²⁻ and HCO₃⁻ give CO₂ with acid — the distinguishing test uses MgSO₄ solution (white precipitate only with CO₃²⁻, not HCO₃⁻). SO₄²⁻ is confirmed with BaCl₂ (white precipitate of BaSO₄, insoluble in conc. HCl), while SO₃²⁻ gives BaSO₃ that dissolves in dilute HCl. PO₄³⁻ is confirmed by ammonium molybdate (canary-yellow precipitate on warming).
Watch-out for NEET: questions often list four anions and ask which gives a specific observation — they rely on you confusing SO₃²⁻ with SO₄²⁻, or CO₃²⁻ with HCO₃⁻, or Cl⁻ with Br⁻. The discriminating detail is always in the confirmatory test, not the preliminary observation.
(Ref: NCERT Class 12 Chemistry Chapter 4, page 124 — exercise on qualitative analysis.)
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which anion, on treatment with dilute H₂SO₄, produces a gas that turns lime water milky?
The confirmatory test for SO₄²⁻ involves adding BaCl₂ solution. What is observed?
When a salt containing CH₃COO⁻ is warmed with dilute H₂SO₄, the observation is:
A student adds dilute H₂SO₄ to an unknown salt and observes a gas with a suffocating smell that turns acidified potassium dichromate solution green. The anion present is:
Two salts, X and Y, both produce effervescence with dilute HCl. Salt X gives a white precipitate with MgSO₄ solution, but salt Y does not. The anions in X and Y are, respectively:
A salt is treated with concentrated H₂SO₄. Dense white fumes are produced, which give thick white fumes when a glass rod dipped in NH₃ solution is brought near. The anion present is:
A student performs the ring test on a salt solution. A brown ring forms at the junction of two layers. Which anion does this confirm?
An unknown salt gives (i) no reaction with dilute H₂SO₄, (ii) a canary-yellow precipitate when warmed with ammonium molybdate and concentrated HNO₃, and (iii) a white precipitate with magnesia mixture (MgCl₂ + NH₄Cl + NH₄OH). The anion is:
Given
- No reaction with dilute H₂SO₄. - Reddish-brown vapour with concentrated H₂SO₄. - Pale-yellow precipitate with AgNO₃, sparingly soluble in NH₃.
Required
Identify the anion present.
Concept
Anion analysis follows a two-stage approach: (a) preliminary test with dilute then concentrated H₂SO₄ to narrow the group, (b) confirmatory wet test to identify the specific anion. The colour and solubility of the silver halide precipitate distinguishes Cl⁻, Br⁻, and I⁻.
Formula / Rule
- Group A anions (CO₃²⁻, S²⁻, SO₃²⁻, NO₂⁻, CH₃COO⁻) react with dilute H₂SO₄ → eliminated. - Group B: Cl⁻ → white fumes of HCl; Br⁻ → reddish-brown vapour of Br₂; I⁻ → violet vapour of I₂; NO₃⁻ → brown fumes of NO₂. - AgNO₃ confirmatory: AgCl = white, soluble in dilute NH₃; AgBr = pale yellow, sparingly soluble in NH₃; AgI = yellow, insoluble in NH₃.
Substitution / Application
- No gas with dilute H₂SO₄ → anion is NOT in Group A. - Reddish-brown vapour with concentrated H₂SO₄ → candidate is Br⁻ (not Cl⁻ which gives white fumes, not I⁻ which gives violet vapour). - Pale-yellow precipitate with AgNO₃, sparingly soluble in NH₃ → matches AgBr.
Reasoning
Both observations converge on Br⁻. The reddish-brown vapour test narrows to Br⁻, and the AgNO₃ confirmatory test (pale yellow, sparingly soluble in NH₃) clinches it. If the precipitate were white and freely soluble in dilute NH₃, the anion would be Cl⁻; if deep yellow and insoluble, it would be I⁻.
Final answer
The anion is **Br⁻ (bromide)**.
Common trap
Confusing the colour of AgBr (pale yellow) with AgI (yellow). Also confusing the NH₃ solubility: AgCl dissolves freely, AgBr dissolves sparingly, AgI is insoluble. Misremembering this solubility order causes mis-identification.
Similar NEET-style question
"An unknown salt gives violet vapour with concentrated H₂SO₄ and a yellow precipitate with AgNO₃ that is insoluble in NH₃. Identify the anion." (Answer: I⁻ — violet vapour = I₂, yellow AgI insoluble in NH₃.) ---
Carbonate (CO3²⁻): dil. HCl → effervescence + lime water turns milky. Sulphate (SO4²⁻): BaCl2 → white ppt insoluble in dil. HCl. Sulphite (SO3²⁻): dil. HCl → SO2 (smell) + decolourised KMnO4. Halides (Cl⁻/Br⁻/I⁻): AgNO3 → white/cream/yellow ppt; Cl⁻ soluble in NH4OH. Nitrate (NO3⁻): brown-ring test (FeSO4 + conc. H2SO4 layered).
-- NCERT Class 12 Chemistry, Ch. 4, p. 124Use when normality is awkward (e.g., diprotic acids). Stoichiometric coefficients from balanced equation.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| V | volume | L |
| n | coefficient | - |
Equivalents of acid = equivalents of base at end-point. Or for redox: equivalents of oxidant = equivalents of reductant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | normality | eq/L |
| V | volume | mL or L |
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Cations like Pb²⁺ precipitate in BOTH Group I (with HCl) and Group II (with H2S) — assigning to only one group misses the redundancy.
Cation that appears in two analytical groups, e.g. Pb²⁺ (Group I + Group II) or Hg²⁺ vs Hg2²⁺.
Apply confirmatory tests for each candidate group; do not assume mutual exclusivity.
Category: Overthinking
Continuing to add titrant past the first persistent colour change because the colour seemed to fade after a swirl.
Question describes 'colour faded after swirling' or 'persistent colour' — distinguishes transient vs end-point.
End-point = first PERSISTENT colour change (lasts ≥30 s). Transient fades back to original on swirling.
Category: Similar Terms
Phenolphthalein (pH 8.2–10) and methyl orange (pH 3.1–4.4) only mark equivalence when the eq-pt pH falls within their range; using the wrong indicator gives an end-point that disagrees with the actual equivalence point.
Titration prompt mentions a specific weak/strong combination but asks which indicator is suitable.
Match the indicator's pH-change range to the equivalence-point pH: phenolphthalein for eq-pt > 7, methyl orange for eq-pt < 7.
Root cause: concept gap
Pb²⁺ appears in both Group I (PbCl2 white ppt, soluble in hot water) and Group II (PbS black ppt). Confirm with hot-water solubility test.
Root cause: concept gap
Strong-acid + strong-base: any indicator (eq pt = 7). Weak-acid + strong-base: phenolphthalein (eq pt > 7). Strong-acid + weak-base: methyl orange (eq pt < 7).
Root cause: rushed under time pressure
Slow drop-wise addition near the end-point; first persistent colour change is the end-point. Re-do if overshot.
3 questions from NEET 2024, 2025. Answers verified against NTA official keys.
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
wrong zero error direction
Sign convention is easily flipped.
Test yourself on this topic with real past-paper questions:
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