JEE Main · 2021 · 16 Mar 2021 · Shift 2 · Practical and Analytical Chemistry

Fusing the compound with sodium turns any nitrogen, sulphur, phosphorus or halogen present into the corresponding sodium salt, so Lassaigne's test is the single preparation that opens the way to all four of these elements. Heating the compound with copper(II) oxide burns its carbon to carbon dioxide, which turns lime water milky, and so detects carbon. Silver nitrate added to the fusion extract acidified with nitric acid throws down a silver halide, which detects halogen specifically. Acetic acid and lead acetate added to the fusion extract give a black precipitate of lead sulphide, which shows sulphur. The matching is Lassaigne's test with nitrogen, sulphur, phosphorus and halogen, copper(II) oxide with carbon, silver nitrate with halogen specifically, and the lead acetate test with sulphur.