In Lassaigne's test the organic compound is fused with sodium metal, which converts nitrogen present in the compound into sodium cyanide, NaCN. The fused mass is extracted with water to give the sodium fusion extract. To this extract, freshly prepared ferrous sulphate is added and the solution is boiled; the cyanide ions react with ferrous ions to form sodium ferrocyanide, Na4[Fe(CN)6]. On adding ferric ions (from a little ferric chloride) and acidifying, ferric ferrocyanide, Fe4[Fe(CN)6]3, is produced, which is the Prussian blue colour confirming nitrogen. Sodium thiocyanate is formed when both nitrogen and sulphur are present together, giving a different result, not the blue colour by itself. Ferric thiocyanate gives a blood-red colour and indicates sulphur (with nitrogen), not Prussian blue. Silver cyanide is associated with halogen-type precipitation tests, not nitrogen detection. The formation of Prussian blue is the standard NCERT confirmatory observation for nitrogen. Plausibility check: a blue colour specifically corresponds to the iron ferrocyanide complex, consistent with cyanide originating from the compound's nitrogen.