It is a general test for the detection of halogens, nitrogen and sulphur in an organic compound. These elements are covalently bonded to the organic compounds. In order to detect them, these have to be converted into their ionic forms. This is done by fusing the organic compound with sodium -carbon test.

Lassaigne's test is used for the detection of nitrogen, sulphur, halogens, and phosphorus in an organic compound. It involves the formation of sodium fusion extract by fusing the organic compound with sodium metal, followed by a series of chemical reactions to identify the presence of these elements.

Here is a detailed explanation of Lassaigne's test for the detection of nitrogen, sulphur, halogens, and phosphorus in an organic compound:

1. Sodium Fusion:
- In this step, a small amount of the organic compound is mixed with powdered sodium metal in a fusion tube.
- The mixture is heated strongly, and the organic compound undergoes fusion with sodium metal.
- During the fusion, nitrogen, sulphur, halogens, and phosphorus present in the compound combine with sodium to form sodium cyanide (NaCN), sodium sulphide (Na2S), sodium halide (NaX), and sodium phosphide (Na3P), respectively.

2. Extraction and Filtration:
- After the fusion, the reaction mixture is extracted with water to dissolve the sodium salts formed.
- The extraction is repeated several times to ensure the maximum recovery of the sodium salts.
- The resulting extract is filtered to remove any insoluble impurities.

3. Acidification and Liberation of Gases:
- The filtrate obtained from the extraction is acidified with dilute sulphuric acid (H2SO4).
- This step ensures the conversion of sodium salts into corresponding acids.
- The acidification liberates the gases HCN, H2S, HX (X is the halogen), and PH3 (phosphine) from their respective salts.

4. Testing for Nitrogen:
- To detect the presence of nitrogen, the liberated gas HCN is passed into a test tube containing a solution of sodium hydroxide (NaOH).
- The test tube is gently heated, and the liberated gas reacts with NaOH to form sodium cyanide (NaCN).
- The presence of nitrogen is confirmed by the formation of a Prussian blue precipitate when Fe(III) salt is added to the sodium cyanide solution.

5. Testing for Sulphur:
- To detect the presence of sulphur, the liberated gas H2S is passed into a test tube containing lead acetate solution (Pb(CH3COO)2).
- The formation of a black precipitate of lead sulphide (PbS) confirms the presence of sulphur.

6. Testing for Halogens:
- To detect the presence of halogens, the liberated gas HX (X = Cl, Br, I) is passed into a test tube containing silver nitrate solution (AgNO3).
- The formation of a white precipitate of silver halide (AgX) confirms the presence of halogens.

7. Testing for Phosphorus:
- To detect the presence of phosphorus, the liberated gas PH3 is passed into a test tube containing a filter paper soaked in ammonium molybdate [(NH4)2MoO4] solution.
- The filter paper turns yellow due to the formation of ammonium phosphomolybdate, indicating the presence of phosphorus.

In this way, Lassaigne's test allows for the detection of nitrogen, sulphur, halogens, and phosphorus in an organic compound by the formation of specific compounds and subsequent reactions with specific reagents.

Lassaignes test