Qualitative analysis of organic compounds

How are the elements in an organic compound detected?

In plain words: many target elements in an organic compound are covalently bound, so ordinary ion tests may not work directly. Ionic organic compounds and easily ionised groups require interpretation rather than this blanket rule. Each method first turns an element into a simple, recognisable product: carbon into carbon dioxide, nitrogen into cyanide, sulphur into sulphide. Then a familiar test with a clear colour or precipitate shows that element is there.

Detecting carbon and hydrogen

The compound is heated with copper(II) oxide. Carbon is oxidised to carbon dioxide, which turns lime water milky, and hydrogen to water, which turns white anhydrous copper sulphate blue (NCERT Section 8.9.1, page 284):

• C + 2CuO → 2Cu + CO₂, then CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O;
• H₂ + CuO → Cu + H₂O (as NCERT writes it; the hydrogen comes from the compound, so 2H + CuO → Cu + H₂O, like the C + 2CuO line, says the same), then 5H₂O + CuSO₄ (white) → CuSO₄·5H₂O (blue).

The same oxidation, with the water and carbon dioxide weighed, is used to estimate carbon and hydrogen.

Lassaigne’s test: the sodium fusion extract

Nitrogen, sulphur and halogens are converted from covalent to ionic form by fusing the compound with sodium metal (NCERT Section 8.9.2, page 284):

• Na + C + N → NaCN (heat);
• 2Na + S → Na₂S;
• Na + X → NaX (X = Cl, Br or I).

C, N, S and X come from the organic compound. The fused mass is boiled with distilled water, which dissolves the sodium cyanide, sulphide and halide: this solution is the sodium fusion extract, also called Lassaigne’s extract.

Test for nitrogen

The extract is boiled with iron(II) sulphate and then acidified with concentrated sulphuric acid. With appropriate reagent blanks and successful fusion, a Prussian blue colour indicates nitrogen (pages 284–285). The cyanide first forms sodium hexacyanidoferrate(II); on heating with concentrated sulphuric acid some iron(II) is oxidised to iron(III), which gives iron(III) hexacyanidoferrate(II) (ferriferrocyanide), the Prussian blue:

• 6CN⁻ + Fe²⁺ → [Fe(CN)₆]⁴⁻;
• 3[Fe(CN)₆]⁴⁻ + 4Fe³⁺ + xH₂O → Fe₄[Fe(CN)₆]₃·xH₂O (Prussian blue).

Test for sulphur

• The extract is acidified with acetic acid and lead acetate is added: a black precipitate of lead sulphide shows sulphur, S²⁻ + Pb²⁺ → PbS.
• With sodium nitroprusside, a violet colour shows sulphur: S²⁻ + [Fe(CN)₅NO]²⁻ → [Fe(CN)₅NOS]⁴⁻ (page 285).

Nitrogen and sulphur together. If both are present, sodium fusion can give sodium thiocyanate, Na + C + N + S → NaSCN. With iron(III) it gives a blood red colour, Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺, rather than Prussian blue if thiocyanate formation has used up the available cyanide. The outcome depends on the fusion conditions and sodium amount. Fusion with excess sodium breaks the thiocyanate up into cyanide and sulphide, NaSCN + 2Na → NaCN + Na₂S, which then give their usual tests (page 285).

Test for halogens

The extract is acidified with nitric acid and treated with silver nitrate, X⁻ + Ag⁺ → AgX (page 285):

Fluorine is not detected this way: silver fluoride is soluble in water, so no precipitate forms.

If nitrogen or sulphur is also present, the extract is first boiled with concentrated nitric acid to decompose the cyanide and sulphide, which would otherwise interfere with the silver nitrate test. The order of the solubilities in ammonia follows the solubility products of the silver halides, AgCl > AgBr > AgI.

Test for phosphorus

NCERT lists phosphorus with Lassaigne’s test in Section 8.9.2, but it is not tested in the sodium fusion extract: a separate sample is heated with an oxidising agent, sodium peroxide, which oxidises phosphorus to phosphate. The solution is boiled with nitric acid and treated with ammonium molybdate; a yellow colour or precipitate of ammonium phosphomolybdate shows phosphorus (NCERT page 285):

• Na₃PO₄ + 3HNO₃ → H₃PO₄ + 3NaNO₃.

Solved examples

Common mistakes to avoid

Quick revision summary

• C and H: heat with CuO; CO₂ turns lime water milky, H₂O turns anhydrous CuSO₄ blue.
• Lassaigne’s test: fuse with Na; extract contains NaCN, Na₂S, NaX.
• N: FeSO₄, then conc. H₂SO₄, Prussian blue Fe₄[Fe(CN)₆]₃.
• S: lead acetate (acetic acid) gives black PbS; nitroprusside gives violet.
• N and S: NaSCN, blood red with Fe³⁺.
• Halogens: HNO₃, then AgNO₃; AgCl white (soluble in NH₃), AgBr cream/pale yellow (dissolves in concentrated ammonia), AgI yellow (not appreciably soluble).
• P: oxidise to phosphate; ammonium molybdate gives yellow ammonium phosphomolybdate.

Questions students ask

Which chapter is Lassaigne’s test in, and is it in the JEE Main 2027 and NEET (UG) 2026 syllabuses?

Lassaigne’s test is in Unit 8 of NCERT Class 11 Chemistry Part II, Organic Chemistry – Some Basic Principles and Techniques, Section 8.9.2 (pages 284–285), and it is in both exam syllabuses. JEE Main 2027 (NTA) and NEET (UG) 2026 (NMC) list ‘Qualitative analysis – detection of nitrogen, sulphur, phosphorus and halogens’ under Unit 13, Purification and Characterisation of Organic Compounds, and the ‘detection of extra elements (nitrogen, sulphur, halogens) in organic compounds’ under Unit 20, Principles Related to Practical Chemistry. In school, the CBSE Class 11 practical syllabus for 2026–27 includes detecting nitrogen, sulphur and chlorine in organic compounds.

How is the sodium fusion extract prepared?

In the supervised laboratory procedure, sodium fusion extract is prepared by heating the small quantity of dry sodium specified by the laboratory manual in a fusion tube until it melts, adding a little of the organic compound, heating the tube to red heat and then plunging it, still red hot, into about 10 mL of distilled water in a china dish, where the tube cracks. The contents are boiled for a few minutes and filtered, and the clear filtrate, which holds the sodium cyanide, sulphide and halide, is the extract used for all the tests. Distilled water is essential, because the chloride in tap water would give a false test for chlorine with silver nitrate.

Why is the sodium fusion extract alkaline?

The sodium fusion extract is alkaline because the excess sodium reacts with the water to form sodium hydroxide, 2Na + 2H₂O → 2NaOH + H₂, and because sodium cyanide and sodium sulphide are salts of the weak acids HCN and H₂S, whose anions hydrolyse, CN⁻ + H₂O ⇌ HCN + OH⁻. The alkali must be neutralised before the silver nitrate test, since OH⁻ would precipitate brown silver oxide, Ag₂O, which is one more reason the extract is acidified with nitric acid first.

Does urea give Lassaigne’s test for nitrogen?

Yes, urea, NH₂CONH₂, gives a positive Lassaigne’s test for nitrogen: it contains carbon as well as nitrogen, so fusion with sodium forms sodium cyanide and the extract gives Prussian blue. Aniline, C₆H₅NH₂, glycine, NH₂CH₂COOH, and nitrobenzene, C₆H₅NO₂, pass the test for the same reason: these compounds supply both carbon and nitrogen for cyanide formation, although decomposition or unsuccessful fusion can cause false negatives.

How do you pronounce Lassaigne’s test?

Lassaigne’s test is said the French way, roughly la-SEN-yuh, with the ‘gn’ like the ‘ny’ in canyon and only a faint final ‘uh’; the name is spelt with a double s and ‘-aigne’ (not Lassagne or Lassaign). It is named after the French chemist Jean Louis Lassaigne, who in 1843 detected nitrogen in organic compounds by fusing them with potassium. Sodium replaced potassium in 1879 and tests for sulphur and halogens were added later, hence the test’s other name, the sodium fusion test.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which colour shows nitrogen in Lassaigne’s test, and what compound causes it (NCERT Exercise 8.37)?

   Show answerHide answer

   Prussian blue, due to iron(III) hexacyanidoferrate(II), Fe₄[Fe(CN)₆]₃ (option (b) in NCERT’s exercise).

2. Q2. A sodium fusion extract, acidified with nitric acid, gives a yellow precipitate with silver nitrate that does not dissolve in ammonia solution. Which halogen is present?

   Show answerHide answer

   Iodine: AgI is yellow and insoluble in ammonia solution.

3. Q3. How are carbon and hydrogen detected in an organic compound?

   Show answerHide answer

   Heat it with copper(II) oxide. Carbon is oxidised to CO₂, which turns lime water milky (CaCO₃); hydrogen is oxidised to water, which turns white anhydrous copper sulphate blue (CuSO₄·5H₂O).