A salt on warming with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$ gives brown fumes, and a filter paper dipped in starch-iodide solution turns blue in these fumes. The anion is

Nitrite gives $\mathrm{HNO}{}_2$, which decomposes to NO; NO is oxidised by air to brown $\mathrm{NO}{}_2$ which liberates iodine from iodide and turns starch blue. Acetate gives a vinegar smell, carbonate gives $\mathrm{CO}{}_2$, and sulphate gives nothing.