An unknown salt on heating with concentrated H_2SO_4 produces a...

The evolution of a brown gas upon heating a salt with concentrated $\text{H}_2\text{SO}_4$, which is further intensified by the addition of copper turnings, is a characteristic confirmatory test for the nitrate ($\text{NO}_3^-$) ion. The copper turnings reduce the formed nitric acid to nitrogen dioxide ($\text{NO}_2$), a brown gas.

The aqueous solution of a nitrate salt gives a positive brown ring test when treated with freshly prepared $\text{FeSO}_4$ and concentrated $\text{H}_2\text{SO}_4$. In this test, $\text{Fe}^{2+}$ reduces nitrate to nitric oxide ($\text{NO}$), which then coordinates with the remaining $\text{Fe}^{2+}$ to form the brown ring complex.

The chemical formula of the brown ring complex is $[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+}$.

Answer: $\text{NO}_3^-$ and $[\text{Fe}(\text{H}_2\text{O})_5(\text{NO})]^{2+}$