When dilute $\mathrm{HCl}$ is added to a salt $Q, a$ brisk effervescence is produced and the gas turns lime water milky. When $\mathrm{NH}_4 \mathrm{OH}$ solution is added to the above mixture (after adding dilute $\mathrm{HCl}$ ), it produces a white precipitate which is soluble in excess $\mathrm{NH}_4 \mathrm{OH}$ solution.