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 Class 11 Chemistry, Ch. 8, p. 285Detection Extra Elements
8 MCQs9-step worked example
Source: NCERT Practical and Analytical ChemistryOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Detection Extra Elements, explained for NEET
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 Chapter 8, pages 284–285 (Organic Chemistry — Some Basic Principles and Techniques, section 8.9.2), sets out Lassaigne's test, and the Class 11 laboratory manual runs it as Experiment 7.2 (NCERT Chemistry Lab Manual Class 11, Unit 7, pages 116–120).
Can you answer these Detection Extra Elements 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?
Show answer and why every option is right or wrong
Answer: C. C is correct. Sodium fusion converts covalently bonded N, S, and halogens into ionic forms — NaCN, Na₂S, and NaX respectively — which dissolve in water and can be detected by standard ionic reagents (NCERT Class 11 Chemistry Chapter 8, page 284).
Why A is wrong: A is wrong. Complete oxidation to CO₂ and H₂O is combustion analysis (used for C and H estimation), not the Lassaigne test. Sodium fusion is a reductive process, not oxidative.
Why B is wrong: B is wrong. The purpose is not to isolate free elements. Sodium converts heteroatoms into ionic salts (NaCN, Na₂S, NaX), not into elemental N₂, S, or Cl₂.
Why D is wrong: D is wrong. Sodium acts as a whole atom in this reaction — it doesn't ionise in the ignition tube. It reacts with the organic compound at high temperature to form ionic products.
MCQ 2Easy RecallPractice
Which reagent is used to detect sulphur in the sodium fusion extract?
Show answer and why every option is right or wrong
Answer: D. D is correct. Sodium nitroprusside (Na₂[Fe(CN)₅NO]) reacts with S²⁻ ions in the extract to give a characteristic violet colour, confirming the presence of sulphur.
Why A is wrong: A is wrong. Ferric chloride is used to test for phenolic –OH groups, not for sulphur in Lassaigne's extract.
Why B is wrong: B is wrong. Barium chloride is used to detect sulphate ions (BaSO₄ precipitate) in inorganic qualitative analysis, not sulphide from Lassaigne's extract.
Why C is wrong: C is wrong. Silver nitrate in dilute HNO₃ is the test for halogens (AgCl, AgBr, AgI precipitates), not sulphur.
MCQ 3Easy RecallPractice
In the silver halide test from Lassaigne's extract, which silver halide precipitate is insoluble in ammonium hydroxide?
Show answer and why every option is right or wrong
Answer: C. C is correct. AgI (yellow precipitate) is insoluble in ammonium hydroxide due to the very low solubility product of AgI. AgCl dissolves readily in NH₄OH forming [Ag(NH₃)₂]⁺, and AgBr is sparingly soluble.
Why A is wrong: A is wrong. AgCl (white curdy precipitate) dissolves in NH₄OH to form the soluble diamminesilver(I) complex [Ag(NH₃)₂]Cl. This solubility is a key distinguishing feature of chloride.
Why B is wrong: B is wrong. AgBr (pale yellow precipitate) is sparingly soluble in NH₄OH — it partially dissolves. It is not the halide that is truly insoluble in NH₄OH.
Why D is wrong: D is wrong. They differ in solubility: AgCl is soluble, AgBr is sparingly soluble, and AgI is insoluble in NH₄OH. This graduated solubility is a standard distinguishing test.
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?
Show answer and why every option is right or wrong
Answer: B. B is correct. Prussian blue confirms nitrogen (as CN⁻ → Fe₄[Fe(CN)₆]₃). Violet colour with sodium nitroprusside confirms sulphur (as S²⁻). Black precipitate with lead acetate (PbS) further confirms sulphur. Both N and S are present.
Why A is wrong: A is wrong. While Prussian blue confirms nitrogen, the positive nitroprusside test (violet) and lead acetate test (black PbS) both independently confirm sulphur is also present. Ignoring these results misses sulphur.
Why C is wrong: C is wrong. This accounts for the sulphur tests but ignores the Prussian blue result, which specifically indicates the presence of CN⁻ from nitrogen.
Why D is wrong: D is wrong. No halogen test was described as positive here. The silver nitrate + dilute HNO₃ test (for halogens) was not mentioned. The violet colour and black precipitate both point to sulphur, not halogens.
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?
Show answer and why every option is right or wrong
Answer: A. A is correct. When both N and S are present, Na₂S + NaCN → NaSCN during sodium fusion. The thiocyanate ion does not give Prussian blue. Instead the extract gives a blood-red colour with Fe³⁺, and that colour itself shows both nitrogen and sulphur are present (NCERT Class 11 Chemistry Chapter 8, page 285). With excess sodium in the fusion, the thiocyanate decomposes back to cyanide and sulphide.
Why B is wrong: B is wrong. The question states the compound contains both nitrogen and sulphur. The failure is not due to absence of nitrogen but due to the formation of NaSCN, which sequesters the cyanide.
Why C is wrong: C is wrong. FeSO₄ followed by NaOH and then acidification with H₂SO₄ is the standard reagent sequence for the Prussian blue test for nitrogen. The reagent choice is correct; the issue is the chemical form of nitrogen in the extract.
Why D is wrong: D is wrong. While excessively high temperatures can cause problems, the specific and well-documented reason for Prussian blue failure when S is co-present is NaSCN formation. This is the expected answer in NEET-level practical chemistry.
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?
Show answer and why every option is right or wrong
Answer: A. A is correct. AgBr is a pale yellow precipitate that is sparingly soluble in NH₄OH. AgCl (white, soluble in NH₄OH) and AgI (yellow, insoluble in NH₄OH) are eliminated by colour and solubility characteristics.
Why B is wrong: B is wrong. AgCl is a white curdy precipitate (not pale yellow) and is readily soluble in NH₄OH, not sparingly soluble. The colour and solubility both rule out chlorine.
Why C is wrong: C is wrong. AgI gives a yellow (not pale yellow) precipitate that is insoluble in NH₄OH. The question specifies sparingly soluble, which points to bromine, not iodine.
Why D is wrong: D is wrong. Fluorine cannot be detected by the silver nitrate test because AgF is soluble in water and does not form a precipitate. Fluorine detection requires separate methods (e.g., heating with concentrated H₂SO₄ and etching glass).
MCQ 7Concept TrapPractice
Why must dilute HNO₃ (not dilute HCl or H₂SO₄) be used when testing Lassaigne's extract with AgNO₃ for halogens?
Show answer and why every option is right or wrong
Answer: B. B is correct. HCl introduces extraneous Cl⁻ which would form AgCl regardless of whether chlorine was in the original compound — a false positive. H₂SO₄ produces Ag₂SO₄ (sparingly soluble), which could be confused with a halide precipitate. Dilute HNO₃ acidifies without introducing interfering anions.
Why A is wrong: A is wrong. HNO₃ does not enhance precipitate colour. Silver halide colours are intrinsic properties of the salts (AgCl white, AgBr pale yellow, AgI yellow) and are independent of which acid was used for acidification.
Why C is wrong: C is wrong. The purpose of the acid is to provide an acidic medium and prevent interference from carbonate or sulphide ions, not to dissolve organic matter. The organic compound was already decomposed during sodium fusion.
Why D is wrong: D is wrong. Ag⁺ reduction to metallic silver is not a concern under these mild conditions. The real issue is anion interference: HCl adds Cl⁻ (false positive for chlorine) and H₂SO₄ forms sparingly soluble Ag₂SO₄.
MCQ 8Concept TrapPractice
A compound containing nitrogen, sulphur and chlorine is subjected to Lassaigne's test. Before testing the sodium fusion extract for chlorine with silver nitrate, the student should:
Show answer and why every option is right or wrong
Answer: D. D is correct. NCERT Class 11 Chemistry Chapter 8, page 285: if nitrogen or sulphur is also present, the sodium fusion extract is first boiled with concentrated nitric acid to decompose the cyanide or sulphide of sodium formed during Lassaigne's test. These ions would otherwise interfere with the silver nitrate test for halogens.
Why A is wrong: A is wrong. Cyanide and sulphide in the extract also react with Ag⁺ (AgCN, Ag₂S), so testing directly can give a precipitate that is not silver chloride.
Why B is wrong: B is wrong. FeSO₄ is the reagent for the nitrogen (Prussian blue) test; it does not remove the cyanide and sulphide before the halogen test.
Why C is wrong: C is wrong. Sodium nitroprusside detects sulphide (violet colour); it does not remove it, and it does nothing about the cyanide.
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How do you solve a Detection Extra Elements question? A 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."
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What to remember before solving Detection Extra Elements questions
More in Practical and Analytical Chemistry: 6 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
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