NaCN + HNO₃ $$ \to $$ NaNO₃ + HCN $$ \uparrow $$

Na₂S + HNO₃ $$ \to $$ NaNO₃ + H₂S $$ \uparrow $$

Nilnic acid decomposed NaCN & Na2S, else they precipitate in test & misquite the resolve For testing of halogens, Nitric acid is added to the sodium extract because if CN^– or S^2– are present then they will be oxidised and removed before the test of halides.