Lassaigne's extract is used for the detection of nitrogen and sulphur. In this the organic compound reacts with the sodium thiocyanate followed by the ${\mathrm{Fe}}^{3+}$ cation and formed the $\left[\mathrm{Fe}\right(\mathrm{SCN}){]}^{2+}$ complex which is red blood colour.

For the test of halogens, it is necessary to remove the sodium cyanide and sodium sulphide. So, the compound $\mathrm{X}$ is sodium cyanide and compound $\mathrm{Y}$ is sodium sulphide. These can be removed by the nitric acid.

$$\begin{gathered} \mathrm{NaCN}\left(\mathrm{X}\right) + {\mathrm{HNO}}_3\left(\mathrm{Z}\right)\to {\mathrm{NaNO}}_3 + \mathrm{HCN} \\ {\mathrm{Na}}_2\mathrm{S}\left(\mathrm{Y}\right) + 2{\mathrm{HNO}}_3\left(\mathrm{Z}\right)\to 2{\mathrm{NaNO}}_3 +2{\mathrm{H}}_2\mathrm{S} \end{gathered}$$

Hence, $\mathrm{X}=\mathrm{NaCN};\mathrm{Y}={\mathrm{Na}}_2\mathrm{S} ;\mathrm{Z}=\text{ conc. }{\mathrm{HNO}}_3$.