Question 1

What is Lassaigne’s test used to detect?

Accepted Answer

Lassaigne’s test detects nitrogen, sulphur and halogens in an organic sample. Sodium fusion converts these elements into water-extractable sodium salts, which are then identified using characteristic precipitation reactions.

Question 2

Why is the fused mixture dissolved in water?

Accepted Answer

Water dissolves the sodium salts formed during sodium fusion, including NaCN, Na2S and sodium halides. This produces the aqueous extract used for confirmatory tests, while any insoluble residue is not treated as the sodium-salt test solution.

Question 3

Which precipitate confirms nitrogen?

Accepted Answer

Nitrogen is confirmed by a deep Prussian blue precipitate. The sodium extract is first treated with ferrous sulphate solution and then acidified with dilute hydrochloric acid, allowing the ferrocyanide chemistry to produce Prussian blue.

Question 4

Why is dilute nitric acid used before silver nitrate in the halogen test?

Accepted Answer

Dilute nitric acid destroys interfering CN−, CO3^2− and SO3^2− ions that could otherwise form precipitates with Ag+. It does not add a halide ion. Hydrochloric acid is not used because it introduces Cl− and can create a false silver chloride precipitate.

Question 5

How can silver chloride, silver bromide and silver iodide be distinguished?

Accepted Answer

Their precipitates have different colours: AgCl is white, AgBr is cream or pale yellow, and AgI is yellow. Their ammonia behaviour also differs: AgCl dissolves in dilute aqueous ammonia, AgBr in concentrated aqueous ammonia, while AgI remains insoluble.

Lassaigne Test Bench — Na Fusion, Then Five Test Tubes of Evidence

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