Group I (dilute HCl): Pb²⁺, Ag⁺, Hg2²⁺ — white ppt. Group II (H2S, dil. HCl): Cu²⁺, Cd²⁺, Hg²⁺, Bi³⁺, Sn — coloured sulfide ppts. Group III (NH4Cl + NH4OH): Fe³⁺ (red-brown), Al³⁺ (gel), Cr³⁺ (green) hydroxides. Group IV (H2S, NH4OH): Zn²⁺ (white ZnS), Mn²⁺ (pink), Co²⁺, Ni²⁺. Group V (NH4)2CO3): Ca²⁺, Sr²⁺, Ba²⁺. Group VI: Mg²⁺, Na⁺, K⁺ (flame test).
-- NCERT Class 12 Chemistry, Ch. 4, p. 121Cation Qualitative Analysis
8 MCQs9-step worked example
Source: NCERT Unit 20PYQ coverage: NEET 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The most frequent trap in qualitative analysis of cations is group overlap — assigning a cation to only one analytical group when it precipitates in two. Pb²⁺ is the classic offender: it forms PbCl₂ (white precipitate) in Group I with dilute HCl, yet also forms PbS (black precipitate) in Group II with H₂S in acidic medium. Students who memorise groups as mutually exclusive lists lose marks here.
The systematic scheme (NCERT Class 12 Chemistry Chapter 4, page 121):
Cations are separated into five analytical groups by selective precipitation using group reagents applied in a fixed order:
| Group | Reagent | Precipitating ion/condition | Example cations |
|---|---|---|---|
| I | Dilute HCl | Cl⁻ in acidic medium | Pb²⁺, Ag⁺, Hg₂²⁺ |
| II | H₂S in dilute HCl | S²⁻ at low pH | Pb²⁺, Cu²⁺, As³⁺, Bi³⁺ |
| III | NH₄OH + NH₄Cl | OH⁻ (buffered) | Fe³⁺, Al³⁺, Cr³⁺ |
| IV | H₂S in NH₄OH/NH₄Cl | S²⁻ at high pH | Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺ |
| V | (NH₄)₂CO₃ | CO₃²⁻ | Ba²⁺, Sr²⁺, Ca²⁺ |
Why order matters for NEET: The scheme is sequential — each group reagent is added only after the previous group's precipitate is filtered off. If you skip filtration or apply reagents out of order, overlapping cations contaminate downstream groups.
Confirmatory tests resolve overlap. For Pb²⁺: dissolve the Group I precipitate in hot water (PbCl₂ is soluble in hot water, AgCl is not) → add K₂CrO₄ → yellow PbCrO₄ confirms lead. This hot-water solubility distinction is a high-frequency NEET distractor target.
Watch out: Hg₂²⁺ (mercurous) vs Hg²⁺ (mercuric) is another overlap pair — mercurous appears in Group I, mercuric in Group II. The oxidation state determines the group, not the element.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following cations precipitates in both Group I and Group II of qualitative analysis?
MCQ 2Easy RecallPractice
The group reagent for Group I cations in qualitative analysis is:
MCQ 3Direct ApplicationPractice
In qualitative analysis, PbCl₂ precipitate (Group I) is distinguished from AgCl by:
MCQ 4Direct ApplicationPractice
A student adds dilute HCl to a solution and obtains a white precipitate. The precipitate is insoluble in hot water. The cation is most likely:
MCQ 5Easy RecallPractice
Which Group III cation gives a brown precipitate with NH₄OH in the presence of NH₄Cl?
MCQ 6Direct ApplicationPractice
During qualitative analysis, a student filters off the Group I precipitate and then passes H₂S through the acidic filtrate. A black precipitate forms. Which of the following is NOT a possible cation?
MCQ 7Direct ApplicationPractice
A solution gives no precipitate with dilute HCl (Group I absent). On passing H₂S in ammoniacal medium (Group IV conditions), a white precipitate forms. The cation is most likely:
MCQ 8Concept TrapPractice
In the qualitative analysis scheme, Group V cations (Ba²⁺, Sr²⁺, Ca²⁺) are precipitated using (NH₄)₂CO₃. Before adding this reagent, all previous groups must be removed. If Group IV separation is incomplete, which interference is most likely?
Worked Example
- 1
Given
- White precipitate with dilute HCl → Group I cation present - Precipitate dissolves in hot water - Hot-water filtrate + K₂CrO₄ → yellow precipitate
- 2
Required
Identify the Group I cation.
- 3
Concept
Group I cations (Pb²⁺, Ag⁺, Hg₂²⁺) form insoluble chlorides with dilute HCl. These chlorides have different solubilities in hot water: PbCl₂ is soluble, AgCl and Hg₂Cl₂ are not.
- 4
Formula / Principle
- PbCl₂ → soluble in hot water (Ksp increases with temperature) - Hot filtrate + K₂CrO₄ → PbCrO₄ (yellow, insoluble) confirms Pb²⁺
- 5
Substitution / Application
- White ppt with dil. HCl: could be PbCl₂, AgCl, or Hg₂Cl₂ - Dissolves in hot water: eliminates AgCl (insoluble) and Hg₂Cl₂ (insoluble) → must be PbCl₂ - Yellow ppt with K₂CrO₄: confirms Pb²⁺ (PbCrO₄ is the standard confirmatory product)
- 6
Calculation
No numerical calculation required — this is a logic-based identification. Each observation eliminates candidates until one remains.
- 7
Final answer
The cation is **Pb²⁺** (lead ion).
- 8
Common trap
Students who memorise Pb²⁺ as "only a Group II cation" (because PbS forms with H₂S) may reject lead as a Group I answer. Remember: Pb²⁺ appears in BOTH Group I and Group II — the hot-water solubility of PbCl₂ is the distinguishing confirmatory test (trap: cation group overlap).
- 9
Similar NEET-style question
"A white precipitate obtained with dilute HCl turns black when treated with H₂S solution. The precipitate was initially soluble in hot water. Identify the cation and the black compound formed." Answer: Cation = Pb²⁺; black compound = PbS. (Tests the same group-overlap concept from the H₂S direction.) ---
Before solving, remember these
Formulas
Molarity-stoichiometry titration
Use 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 | - |
Valid when
- Balanced equation known
- Same end-point
Normality equation in titration
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 |
Valid when
- Same titration end-point
- Equivalent factors known
Exam Traps & Common Mistakes
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.
When it triggers
Cation that appears in two analytical groups, e.g. Pb²⁺ (Group I + Group II) or Hg²⁺ vs Hg2²⁺.
How to avoid
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.
When it triggers
Question describes 'colour faded after swirling' or 'persistent colour' — distinguishes transient vs end-point.
How to avoid
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.
When it triggers
Titration prompt mentions a specific weak/strong combination but asks which indicator is suitable.
How to avoid
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
Correction
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
Correction
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
Correction
Slow drop-wise addition near the end-point; first persistent colour change is the end-point. Re-do if overshot.
Past Year Questions
3 questions from NEET 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1A-III, B-II, C-I, D-IV
2A-III, B-IV, C-II, D-I
3A-III, B-IV, C-I, D-II
4A-III, B-II, C-IV, D-I
NTA Answer: Option 3(final)
NEET 2024Revised key
1dilute hydrochloric acid
2concentrated sulphuric acid
3dilute nitric acid
4dilute sulphuric acid
NTA Answer: Option 4(revised_final)
NEET 2024Revised key
1B, A, D, C, E
2B, C, A, D, E
3E, C, D, B, A
4E, A, B, C, D
NTA Answer: Option 1(revised_final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Practical chemistry — titrimetric end-point selection, indicator–pH matching, qualitative-analysis cation/anion identification, organic functional-group tests.
Direct ApplicationMedium
Common distractors
wrong zero error direction
Sign convention is easily flipped.
Sources
NCERT refs: Class 12 Chemistry Chapter 4, p.121
Test yourself on this topic with real past-paper questions:
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