Fuse organic compound with sodium → soluble inorganic salts. Filtrate (Lassaigne extract) tested: (a) N: FeSO4 + dil. H2SO4 → blue/green Prussian-blue colour confirms NaCN → Fe4[Fe(CN)6]3. (b) S: sodium nitroprusside → violet (Na2S). (c) Halogens: AgNO3 + dil. HNO3 → white (Cl), cream (Br), yellow (I) ppt.
-- NCERT, p. 286Detection Extra Elements
8 MCQs9-step worked example
Source: NCERT Unit 20PYQ coverage: NEET 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
Detection of Nitrogen, Sulphur, and Halogens in Organic Compounds
Organic compounds built only from C, H, and O burn cleanly. But the moment nitrogen, sulphur, or a halogen is present, specific chemical tests can expose them — and NEET expects you to know the sequence cold.
The Lassaigne test (sodium fusion test) is the gateway. A small piece of the organic compound is fused with metallic sodium in an ignition tube. Sodium converts covalently bonded extra elements into ionic form:
- N → NaCN (sodium cyanide)
- S → Na₂S (sodium sulphide)
- Halogen (Cl, Br, I) → NaX (sodium halide)
- If both N and S are present → NaSCN (sodium thiocyanate)
The fused mass is extracted in distilled water — this is the sodium fusion extract (Lassaigne's extract).
Testing the extract:
| Element | Reagent added to extract | Positive result |
|---|---|---|
| N | FeSO₄ + NaOH, then H₂SO₄ | Prussian blue colour (Fe₄[Fe(CN)₆]₃) |
| S | Sodium nitroprusside | Violet colour |
| S (alternative) | Lead acetate + acetic acid | Black PbS precipitate |
| Cl | AgNO₃ + dilute HNO₃ | White curdy precipitate (AgCl), soluble in NH₄OH |
| Br | AgNO₃ + dilute HNO₃ | Pale yellow precipitate (AgBr), sparingly soluble in NH₄OH |
| I | AgNO₃ + dilute HNO₃ | Yellow precipitate (AgI), insoluble in NH₄OH |
The high-frequency NEET trap: When both N and S are present, simple NaCN and Na₂S react with each other during fusion to form NaSCN. The standard Prussian blue test for nitrogen fails because no free CN⁻ remains. You must first decompose the thiocyanate — boil the extract with concentrated HCl to release HCN, then proceed with the iron(II) sulfate test.
Halogen interference with nitrogen detection: If halogens are also present alongside nitrogen, adding FeSO₄ can produce coloured precipitates of iron halides that mask the Prussian blue. Acidify with dilute H₂SO₄ first to remove halide interference before testing for N.
NCERT Class 11 Chemistry (Organic Chemistry — Some Basic Principles and Techniques), page 286, covers detection of these elements in the exercise section.
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
In Lassaigne's test, an organic compound is fused with metallic sodium. What is the purpose of this fusion?
MCQ 2Easy RecallPractice
Which reagent is used to detect sulphur in the sodium fusion extract?
MCQ 3Easy RecallPractice
In the silver halide test from Lassaigne's extract, which silver halide precipitate is insoluble in ammonium hydroxide?
MCQ 4Direct ApplicationPractice
An organic compound on sodium fusion gives a Lassaigne's extract that produces Prussian blue colour with FeSO₄ and NaOH followed by H₂SO₄, and also gives a violet colour with sodium nitroprusside. When the same extract is tested with lead acetate and acetic acid, a black precipitate forms. Which elements are confirmed present?
MCQ 5Direct ApplicationPractice
An organic compound contains both nitrogen and sulphur. On performing the Lassaigne test, a student directly tests the extract with FeSO₄ and NaOH but fails to get the Prussian blue colour. What is the most likely reason?
MCQ 6Direct ApplicationPractice
A student tests Lassaigne's extract with AgNO₃ and dilute HNO₃ and obtains a pale yellow precipitate that is sparingly soluble in ammonium hydroxide. Which halogen is indicated?
MCQ 7Concept TrapPractice
Why must dilute HNO₃ (not dilute HCl or H₂SO₄) be used when testing Lassaigne's extract with AgNO₃ for halogens?
MCQ 8Concept TrapPractice
A compound containing nitrogen, sulphur, and chlorine is subjected to Lassaigne's test. In what order should the student test the sodium fusion extract to avoid interference?
Worked Example
- 1
Given
- Portion 1: positive nitroprusside test (violet) - Portion 2: negative Prussian blue test - Portion 3: white curdy precipitate with AgNO₃, soluble in NH₄OH
- 2
Required
Identify which extra elements (N, S, Cl, Br, I) are present. Explain the anomalous Portion 2 result.
- 3
Concept
Lassaigne's test converts covalent heteroatoms to ionic forms via sodium fusion. Each ion has a specific confirmatory test. When N and S co-exist, NaSCN forms and masks the standard CN⁻ test.
- 4
Analysis of each portion
- **Portion 1:** Violet with nitroprusside → **S²⁻ present → sulphur confirmed.** - **Portion 3:** White curdy precipitate with AgNO₃ in dilute HNO₃, soluble in NH₄OH → **AgCl → chlorine confirmed.** - **Portion 2:** No Prussian blue. But does this mean nitrogen is absent?
- 5
Reasoning for Portion 2 failure
Sulphur is confirmed (Portion 1). If nitrogen is also present, during fusion: Na + C/N/S (organic) → NaCN + Na₂S → NaSCN NaSCN consumes the free CN⁻. No free CN⁻ → no Prussian blue. The negative result does NOT rule out nitrogen. It signals that the NaSCN interference pathway is active.
- 6
Corrective procedure
Boil a fresh portion of the extract with concentrated HCl: NaSCN + HCl → NaCl + HSCN → HCN↑ + S Re-test the treated extract with FeSO₄/NaOH/H₂SO₄. If Prussian blue now appears, nitrogen is confirmed.
- 7
Final answer
**Elements detected:** Sulphur (confirmed by nitroprusside), Chlorine (confirmed by AgCl test). **Nitrogen: suspected** (NaSCN interference explains the false negative; confirmatory re-test after HCl decomposition required).
- 8
Common trap
Students see "no Prussian blue" and conclude nitrogen is absent. In NEET, when the question states both S and N are present, always consider the NaSCN interference. The absence of Prussian blue in the direct test is not evidence of absence of nitrogen — it is evidence that the thiocyanate pathway consumed the cyanide.
- 9
Similar NEET-style question
"An organic compound containing C, H, N, S, and Cl is fused with sodium. Describe the sequence of tests you would perform on the Lassaigne's extract and the expected observations for each element." ---
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.
Test yourself on this topic with real past-paper questions:
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