Which test is used to detect nitrogen, sulfur, and halogens?

The test used to detect nitrogen, sulfur, and halogens is Lassaigne's test

The Lassaigne's test also known as sodium fusion test  is used in elemental analysis for the qualitative measurements of the presence of elements such as halogens, nitrogen, and sulphur, in any  organic compound. It was developed by J. L. Lassaigne. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are taken up in aqueous solution and can be detected by simple chemical tests taken in laboratory.

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