In organic compounds, the elements nitrogen, sulphur, and halogens are present in the covalent form. These elements cannot be directly tested using standard aqueous chemical tests because covalent compounds do not dissociate into ions in water.

Therefore, the organic compound is fused with metallic sodium during Lassaigne’s test. This fusion converts the covalent form of these elements into their ionic form.

During this process, the following reactions take place:

$$\text{Na} + \text{C} + \text{N} \xrightarrow{\Delta} \text{NaCN}$$ $$2\text{Na} + \text{S} \xrightarrow{\Delta} \text{Na}_2\text{S}$$

$\text{Na} + \text{X} \xrightarrow{\Delta} \text{NaX}$ (where X = Cl, Br, or I)

The resulting sodium cyanide, sulphide, and halide are ionic and can be extracted by boiling with distilled water to form the sodium fusion extract, which is then used for detecting the respective elements.