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Organic compounds hold nitrogen, sulphur and halogens in covalent bonds, and covalent atoms do not give the usual ionic tests. Lassaigne's test fixes that with one step: fuse the compound with sodium. This post explains what each test detects and why the colour appears, then works through four compounds so you can reason from an observation to an element.
Where does Lassaigne's test appear?
It is section 8.9.2 of the NCERT Class 11 unit on organic chemistry basics. The CBSE Chemistry curriculum for 2026-27 lists "Detection of -Nitrogen, Sulphur, Chlorine in organic compounds" in the Class 11 practicals. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus both include "Detection of extra elements (Nitrogen, sulphur, halogens) in organic compounds", and the JEE Advanced 2026 syllabus has "Detection of elements (N, S, halogens)". Check the current year's documents before your exam.
Why fuse the compound with sodium?
NCERT puts it simply: the elements "are converted from covalent form into the ionic form by fusing the compound with sodium metal". The reactions are:
Na + C + N → NaCN
2Na + S → Na₂S
Na + X → NaX (X = Cl, Br or I)
The fused mass is boiled with distilled water, and the solution is the sodium fusion extract. Every test below is done on this extract. Sodium reacts violently with water, so this step is done only under a teacher's supervision.
How is each element detected?
Nitrogen. Boil the extract with iron(II) sulphate, then acidify with concentrated sulphuric acid. Cyanide forms hexacyanidoferrate(II), and some iron(II) is oxidised to iron(III) on heating. The two combine as Fe₄[Fe(CN)₆]₃, Prussian blue. A blue colour confirms nitrogen.
Sulphur. Acidify with acetic acid and add lead acetate: a black precipitate of PbS shows sulphur. Or add sodium nitroprusside: a violet colour shows sulphur.
Nitrogen and sulphur together. With both present, sodium thiocyanate (NaSCN) can form. It gives a blood red colour with Fe³⁺, and no Prussian blue, because there are no free cyanide ions. NCERT adds that with excess sodium the thiocyanate breaks down to cyanide and sulphide, which then give their usual tests.
Halogens. Acidify with nitric acid and add silver nitrate. NCERT describes a white precipitate soluble in ammonium hydroxide for chlorine, a yellowish precipitate sparingly soluble for bromine, and a yellow precipitate insoluble for iodine.
Four compounds, reasoned through
Chlorobenzene, C₆H₅Cl. No nitrogen or sulphur, so no Prussian blue, no black precipitate. After nitric acid and silver nitrate: a white precipitate that dissolves in ammonium hydroxide. Conclusion: chlorine.
Thiourea, NH₂CSNH₂. It contains carbon, nitrogen and sulphur. With limited sodium, NaSCN forms and the extract turns blood red with Fe³⁺. A student who tests only for Prussian blue may wrongly report "no nitrogen".
4-Chloroaniline, ClC₆H₄NH₂. Nitrogen gives Prussian blue. For chlorine, the extract must be boiled with nitric acid first, because cyanide in the extract would otherwise interfere with the silver nitrate test.
Hydrazine, N₂H₄. It contains nitrogen but no carbon. Look at the first equation: sodium needs carbon and nitrogen together to make NaCN. With no carbon, no cyanide forms and the test for nitrogen fails. This is a favourite reasoning question.
Which mistakes cost marks?
Writing NaCN without carbon. The nitrogen test works through cyanide, so it needs carbon in the compound.
Forgetting the nitric acid step before the halogen test when N or S is present.
Mixing up the reagents. Sulphur: lead acetate or sodium nitroprusside. Nitrogen: iron(II) sulphate then sulphuric acid. Halogens: nitric acid then silver nitrate.
Reading blood red as iron(III) alone. In this test it signals thiocyanate, so both N and S.
Including phosphorus. NCERT detects phosphorus with sodium peroxide and ammonium molybdate, not through the fusion extract.
A practice routine
Write the three fusion equations and the colour for each test from memory, then check them against NCERT.
Make a flow chart: extract → N test → S test → (nitric acid) → halogen test.
Take five compounds, such as urea, sulphanilic acid, bromobenzene, chloroform and thiourea, and predict every observation.
Practise the viva answer: why sodium, why nitric acid, why blood red.
Lassaigne's test is often done in the same session as other organic tests; our guides to tests for organic functional groups and salt analysis in the chemistry practical cover those. If the chemistry behind a colour change is unclear, the Fermi Concept Library covers every concept in chemistry, each with its own lesson.
Before your practical, explain aloud why hydrazine gives no Prussian blue. If you can, you understand the test.
