Why is Lassaigne's extract boiled with concentrated before adding for the detection of halogens? — Practical Organic Chemistry and Purification Chemistry Question

💡 Solution & Explanation

If N or S are present, $NaCN$ and $Na_2S$ are formed. These would react with $AgNO_3$ to give white $AgCN$ and black $Ag_2S$ precipitates, interfering with the halogen test. Boiling with concentrated $HNO_3$ decomposes them into volatile $HCN$ and $H_2S$ gases.