CBSE Class 11 chemistry · organic qualitative tests · Organic qualitative analysis

Source: CBSE Class 11 chemistry practical syllabus · use your school laboratory manual (NCERT Lab Manual where your school uses it).

Detection of N, S and halogen (Lassaigne) (Class 11 chemistry practical)

Covalently bound N, S, and halogens in organic compounds are converted to ionic forms in a sodium fusion (Lassaigne) extract.

The compound sample and fusion procedure follow your laboratory manual — often teacher-demonstrated fusion.

Published practice page. Concentrations, masses, and numerical answers come from your school laboratory manual and your own readings — not invented here. Not yet reviewed by a named chemistry reviewer. Version 1.0.0, updated 2026-10-03.

Safety — read before you start

Safety warning. Practise here. Perform the real experiment only in your school lab, with your teacher present.

Sodium metal — fire and explosion with water.
Organic vapours.
Teacher must supervise fusion.

Wear: Lab coat, Splash goggles, Gloves, as your teacher directs.

Follow your teacher’s disposal instructions. Do not pour leftovers down a household drain.

Loads only after you tap. Use the lab hands to pick up and use equipment (pointer hidden in the scene). Falls back to a flat diagram if WebGL is unavailable.

Aim

To detect the presence of nitrogen, sulphur, and halogens in an organic compound through sodium fusion extract and the confirmatory tests listed in your laboratory manual.

Methods and quantities can differ between schools. Follow your teacher's instructions.

Apparatus and chemicals

Apparatus and chemicals
Item	Note
Fusion tube or dry test tube	Red-hot sodium handling — teacher led.
Platinum wire or iron nail	To hold sodium as manual shows.
Test tubes	For extract and tests.
Filter paper	Filter fused extract after boiling with water.
Organic compound sample (organic sample)	Use the concentration, mass, volume, and sample names printed in your school laboratory manual. This page does not invent numerical results for you.
Sodium metal (Na)	Highly reactive — never handle wet.
Distilled water (H2O)	To extract fused mass.
Test reagents (FeSO₄, FeCl₃, AgNO₃, etc.) (reagents)	For N, S, halogen tests on extract.

Test tubes for Lassaigne’s test for nitrogen, sulphur, and halogen — Sodium fusion needs teacher supervision. Match each confirmatory test to the element your school schedule asks for.

Theory and reaction

Open

Fusion with sodium converts C, N, S, X into NaCN, Na₂S, NaX (simplified). Boiling with water gives Lassaigne extract containing CN^-, S^2-, Cl^-/Br^-/I^-.

Nitrogen: sodium cyanide in extract; prussian blue with FeSO₄ and FeCl₃ (as manual).

Sulphur: S^2- gives black lead sulphide with lead acetate or violet colour with sodium nitroprusside.

Halogens: AgNO₃ after acidifying — AgCl white soluble in NH₄OH (distinguish Br^-, I^- by manual table).

Fusion forms ionic NaCN; Fe tests give Prussian blue for nitrogen.

Procedure

Perform sodium fusion with a small piece of organic compound and sodium as your teacher demonstrates.
Boil the fused mass with distilled water. Filter to obtain Lassaigne extract.
Divide extract into portions. Run nitrogen test (FeSO₄ + FeCl₃ + dil. H₂SO₄) on one portion.
Test another portion for sulphur (lead acetate or nitroprusside test as manual).
Acidify a portion with dil. HNO₃, then add AgNO₃ for halogen; note precipitate colour and behaviour with NH₄OH.

Observation table

Enter the readings from your school lab in the practical file below. This page does not fill them in for you.

Type the readings in your practical file

S. No.	Element tested (N / S / X)	Observation (—)	Present? (yes / no)
1
2
3

Calculation

Qualitative detection only.

Result

Report whether N, S, and halogen are present in your sample based on confirmatory colours and precipitates you observed.

Precautions

Sodium metal reacts violently with water — fusion is teacher-supervised.
Use very small samples.
Acidify before AgNO₃ to decompose CN⁻ and S²⁻ that would otherwise interfere.

Viva questions

Answer one question at a time. The full answers stay closed until you open them.

Question 1 of 4

Lassaigne test converts organic N, S, halogens into…

A. Only CO₂B. Ionic species in aqueous extract (e.g. CN⁻, S²⁻, X⁻)C. Pure metalsD. Water only

Pick an answer first.

Show all questions and answers

Lassaigne test converts organic N, S, halogens into…
Answer: Ionic species in aqueous extract (e.g. CN⁻, S²⁻, X⁻). Reason: Fusion makes ionic forms detectable by wet tests.
Prussian blue colour indicates…
Answer: Nitrogen in the compound. Reason: Iron cyanide complex colour for nitrogen.
Before AgNO₃ test for halogen you should acidify to…
Answer: Destroy CN⁻ and S²⁻ that give interfering precipitates. Reason: CN⁻ and S²⁻ otherwise precipitate silver salts.
Sodium metal must be kept…
Answer: Dry and away from moisture — often under kerosene. Reason: Na reacts violently with water.

Common mistakes

Testing halogen before acidifying extract → false AgCN / Ag₂S precipitates.
Confusing blood-red Fe(SCN)₃ with Prussian blue.
Reporting halogen without noting Cl vs Br vs I distinctions your manual teaches.

Frequently asked questions

Can I do this at home?: No. Use this page to practise the order of steps and viva. Do the real experiment only in your school laboratory, with your teacher present.

Your practical file

Name, school, and roll number are optional and are not sent anywhere.

The practical file opens on this device.

Review

Not yet reviewed by a named chemistry reviewer. Published for practice only — follow your teacher and laboratory manual for quantities and safety.

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