While testing the lassaigne’s extract for the presence of halogens, it is first boiled with dilute nitric acid. This is done to decompose NaCN to HCN and Na₂S to H₂S and to expel these gases. That is, if any nitrogen and sulphur are present in the form of NaCN and Na₂S, then they are removed. The chemical equations involved in the reaction are represented as

NaCN + HNO₃ → NaNO₃ + HCN

Na₂S + 2HNO₃ → 2NaNO₃ + H₂S