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 Chemistry Lab Manual Class 12, Unit 7, p. 70Cation Qualitative Analysis
8 MCQs9-step worked example
Source: NCERT Practical and Analytical ChemistryPYQ coverage: NEET 2020, 2024, 2025Official key: NTA-verifiedLast updated: 27 Sep 2026
Cation Qualitative Analysis, explained for NEET
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 Chemistry Lab Manual Class 12, Unit 7, page 63):
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.
Can you answer these Cation Qualitative Analysis 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?
Show answer and why every option is right or wrong
Answer: B. Pb²⁺ forms PbCl₂ (white ppt, Group I with dilute HCl) and PbS (black ppt, Group II with H₂S). This dual-group behaviour is documented in NCERT Chemistry Lab Manual Class 12, Unit 7, pages 71–72.
Why A is wrong: A is wrong because Cu²⁺ does not precipitate with dilute HCl — it belongs exclusively to Group II (CuS, black ppt with H₂S in acidic medium). (trap: cation group overlap)
Why C is wrong: C is wrong because Ag⁺ precipitates only in Group I as AgCl (white ppt, insoluble in hot water). It does not form a sulphide precipitate under Group II conditions. (trap: cation group overlap)
Why D is wrong: D is wrong because Fe³⁺ precipitates in Group III as Fe(OH)₃ (brown ppt) with NH₄OH + NH₄Cl. It does not appear in Groups I or II. (trap: cation group overlap)
MCQ 2Easy RecallPractice
The group reagent for Group I cations in qualitative analysis is:
Show answer and why every option is right or wrong
Answer: A. Group I cations (Pb²⁺, Ag⁺, Hg₂²⁺) are precipitated as insoluble chlorides by adding dilute HCl, per NCERT Chemistry Lab Manual Class 12, Unit 7, page 69.
Why B is wrong: B is wrong because H₂S in dilute HCl is the Group II reagent — it precipitates sulphides of Cu²⁺, Pb²⁺, As³⁺ etc. under acidic conditions. (trap: cation group overlap — confusing Group I and Group II reagents)
Why C is wrong: C is wrong because NH₄OH + NH₄Cl is the Group III reagent — it precipitates hydroxides of Fe³⁺, Al³⁺, Cr³⁺. (trap: cation group overlap — reagent–group mis-assignment)
Why D is wrong: D is wrong because (NH₄)₂CO₃ is the Group V reagent — it precipitates carbonates of Ba²⁺, Sr²⁺, Ca²⁺. (trap: cation group overlap — reagent–group mis-assignment)
MCQ 3Direct ApplicationPractice
In qualitative analysis, PbCl₂ precipitate (Group I) is distinguished from AgCl by:
Show answer and why every option is right or wrong
Answer: A. PbCl₂ dissolves in hot water (its solubility increases sharply with temperature), whereas AgCl remains insoluble. The hot-water filtrate is then tested with K₂CrO₄ to confirm Pb²⁺ (yellow PbCrO₄ precipitate). NCERT Chemistry Lab Manual Class 12, Unit 7, page 72.
Why B is wrong: B is wrong because excess NH₄OH dissolves AgCl (forming [Ag(NH₃)₂]⁺), not PbCl₂. Confusing the solubility of the two chlorides in different reagents leads to incorrect group assignment. (trap: cation group overlap)
Why C is wrong: C is wrong because K₂CrO₄ is added to the hot-water filtrate after separating PbCl₂ from AgCl — adding it directly to the mixed precipitate doesn't confirm which chloride dissolved. (trap: cation group overlap — skipping the separation step)
Why D is wrong: D is wrong because concentrated H₂SO₄ forms white PbSO₄, not a yellow product. The yellow confirmatory product is PbCrO₄ (from K₂CrO₄), not a sulphate. (trap: cation group overlap — confusing confirmatory reagents)
MCQ 4CalculationPractice
A student adds dilute HCl to a solution and obtains a white precipitate. The precipitate is insoluble in hot water, and on adding aqueous NH₄OH it dissolves completely to a clear solution. The cation is most likely:
Show answer and why every option is right or wrong
Answer: D. Among Group I chlorides, AgCl is white and insoluble in hot water. PbCl₂ is white but dissolves in hot water. Hg₂Cl₂ is white but turns black with NH₄OH (distinguishing test). The hot-water insolubility points specifically to Ag⁺. NCERT Chemistry Lab Manual Class 12, Unit 7, page 72.
Why A is wrong: A is wrong because PbCl₂ is white but dissolves in hot water — the stem specifies insolubility in hot water, ruling out Pb²⁺. (trap: cation group overlap — failing to apply hot-water solubility test)
Why B is wrong: B is wrong because Ba²⁺ does not precipitate with dilute HCl — BaCl₂ is soluble. Ba²⁺ belongs to Group V (precipitated by (NH₄)₂CO₃). (trap: cation group overlap — assigning a Group V cation to Group I)
Why C is wrong: C is wrong, and it is the one the hot-water test cannot rule out: Hg₂Cl₂ is also white and also insoluble in hot water. It fails at the NEXT step. With NH₄OH it turns black (a mixture of Hg and HgNH₂Cl), whereas AgCl dissolves as the diamminesilver(I) complex — and the stem says the precipitate dissolved. (trap: cation group overlap — stopping at the hot-water test)
MCQ 5Easy RecallPractice
Which Group III cation gives a brown precipitate with NH₄OH in the presence of NH₄Cl?
Show answer and why every option is right or wrong
Answer: C. Fe(OH)₃ is a reddish-brown precipitate formed when Fe³⁺ reacts with NH₄OH (buffered by NH₄Cl). Al(OH)₃ is white, Cr(OH)₃ is green. NCERT Chemistry Lab Manual Class 12, Unit 7, page 65.
Why A is wrong: A is wrong because Al³⁺ gives Al(OH)₃, which is a white gelatinous precipitate — not brown. (trap: cation group overlap — colour confusion among Group III hydroxides)
Why B is wrong: B is wrong because Cr³⁺ gives Cr(OH)₃, which is green — not brown. (trap: cation group overlap — colour confusion among Group III hydroxides)
Why D is wrong: D is wrong because Zn²⁺ belongs to Group IV (precipitated as ZnS with H₂S in ammoniacal medium), not Group III. Zn(OH)₂ is white and dissolves in excess NH₄OH. (trap: cation group overlap — group mis-assignment)
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?
Show answer and why every option is right or wrong
Answer: D. Ag⁺ should have been completely removed in Group I as AgCl. If Group I separation was done correctly, Ag⁺ cannot remain in the filtrate to form Ag₂S in Group II. The black precipitate is due to PbS, CuS, or Bi₂S₃. NCERT Chemistry Lab Manual Class 12, Unit 7, page 83.
Why A is wrong: A is wrong because Pb²⁺ can appear in Group II as PbS (black ppt) — not all Pb²⁺ is removed in Group I due to the relatively higher solubility of PbCl₂. This is exactly the group-overlap trap. (trap: cation group overlap)
Why B is wrong: B is wrong because Cu²⁺ forms CuS (black ppt) in Group II — it does not precipitate in Group I. Cu²⁺ is a standard Group II cation. (trap: cation group overlap)
Why C is wrong: C is wrong because Bi³⁺ forms Bi₂S₃ (black ppt) in Group II — it is a valid Group II cation. (trap: cation group overlap)
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:
Show answer and why every option is right or wrong
Answer: B. ZnS is white (or colourless). NiS is black, CoS is black, and MnS is buff/flesh-coloured (pale pink). A white sulphide precipitate in Group IV strongly indicates Zn²⁺. NCERT Chemistry Lab Manual Class 12, Unit 7, page 69.
Why A is wrong: A is wrong because NiS is black — not white. Confusing sulphide colours across Group IV cations leads to incorrect identification. (trap: cation group overlap — colour-based mis-identification)
Why C is wrong: C is wrong because CoS is black — not white. (trap: cation group overlap — colour-based mis-identification)
Why D is wrong: D is wrong because MnS is flesh-coloured (buff/pale pink) — not white. (trap: cation group overlap — colour-based mis-identification)
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?
Show answer and why every option is right or wrong
Answer: C. If Zn²⁺ remains in solution after incomplete Group IV separation, it reacts with (NH₄)₂CO₃ to form ZnCO₃ (white precipitate), which could be confused with BaCO₃, SrCO₃, or CaCO₃. This is why sequential and complete group separation is essential. NCERT Chemistry Lab Manual Class 12, Unit 7, page 80.
Why A is wrong: A is wrong because by Group V stage, the H₂S step is already past — ZnS would have precipitated in Group IV, not contaminate Group V as a sulphide. The interference from incomplete separation is Zn²⁺ ions in solution, not ZnS particles. (trap: cation group overlap — confusing precipitate carryover vs ionic carryover)
Why B is wrong: B is wrong because Ba²⁺ does not form BaS under these conditions — BaS is highly soluble and H₂S does not precipitate Group V cations. The interference is the reverse: Group IV ions contaminating Group V, not Group V ions reacting with a Group IV reagent. (trap: cation group overlap)
Why D is wrong: D is wrong because Ca²⁺ cannot be oxidised by H₂S — Ca²⁺ is already in its highest common oxidation state (+2) and H₂S is a reducing agent, not an oxidising agent. (trap: cation group overlap — inventing a non-existent redox reaction)
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How do you solve a Cation Qualitative Analysis question? A 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.)
---
What to remember before solving Cation Qualitative Analysis questions
Where do students lose marks on Cation Qualitative Analysis?
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.
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.
More in Practical and Analytical Chemistry: 4 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
Cation Qualitative Analysis questions from past NEET papers
3 questions from NEET 2020, 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
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)
NEET 2020
1CuCO3 ⋅Cu(OH)2
2CuSO4
3[Cu(NH3 )4 ] 2+
4Cu(OH)2
NTA Answer: Option 3(final)
All 7 past-paper questions from Practical and Analytical Chemistry →
Sources
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