An unknown salt on heating with concentrated H ₂ SO ₄ produces a brown gas. The evolution of this gas is significantly enhanced when copper turnings are added to the reaction mixture. Furthermore, the aqueous solution of the salt gives a brown ring at the junction of the liquids when treated with freshly prepared FeSO ₄ solution followed by the careful addition of concentrated H ₂ SO ₄ along the sides of the test tub

Step-by-step solution

The evolution of a brown gas upon heating a salt with concentrated H ₂ SO ₄ , which is further intensified by the addition of copper turnings, is a characteristic confirmatory test for the nitrate ( NO ₃^- ) ion. The copper turnings reduce the formed nitric acid to nitrogen dioxide ( NO ₂ ), a brown gas. The aqueous solution of a nitrate salt gives a positive brown ring test when treated with freshly prepared FeSO ₄ and concentrated H ₂ SO ₄ . In this test, Fe ²⁺ reduces nitrate to nitric oxide ( NO ), which then c