When a solid salt is heated with concentrated H_2SO_4 , a...

When a salt containing the chloride anion ($\text{Cl}^{-}$) is heated with concentrated $\text{H}_2\text{SO}_4$, hydrogen chloride ($\text{HCl}$) gas is evolved.

$\text{Cl}^{-} + \text{H}_2\text{SO}_4 \rightarrow \text{HCl} + \text{HSO}_4^{-}$

$\text{HCl}$ is a colourless gas with a pungent smell. When a glass rod dipped in aqueous ammonia ($\text{NH}_4\text{OH}$) is brought near it, dense white fumes of ammonium chloride ($\text{NH}_4\text{Cl}$) are formed.

$\text{HCl} + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl}$ (dense white fumes)

Bromide gives reddish-brown fumes of $\text{Br}_2$, nitrate gives brown fumes of $\text{NO}_2$, and sulphate does not react with concentrated $\text{H}_2\text{SO}_4$ to evolve such gases.

Answer: Chloride, $\text{Cl}^{-}$