On adding dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$ to a salt, a pale brown vapour appears at the mouth of the tube. The anion is most likely

Nitrite with dilute acid gives nitrous acid which disproportionates to $\mathrm{NO}$; the NO is oxidised by air to brown $\mathrm{NO}{}_2$. Nitrate needs concentrated acid and heat, chloride gives colourless HCl only with concentrated acid and carbonate gives colourless $\mathrm{CO}{}_2$.