Question

Identify the salts $P$ and $Q$ from the observations given below:
When dilute $HCl$ is added to a salt $Q$, a brisk effervescence is produced and the gas turns lime water milky.
When $N{H}_{4}OH$ solution is added to the above mixture (after adding dilute $HCl$), it produces a white precipitate which is soluble in excess $N{H}_{4}OH$ solution.

Answer 1

When dilute $HCl$ is added to a salt $Q$, a brisk effervescence is produced and the gas turns lime water milky.
This indicates presence of carbonate ion.
$C{O}_3^{2+}+2HCl\to H_{2}O+C{O}_2\uparrow +2C{l}^{-}$
$Ca(OH{)}_2+C{O}_2\to CaC{O}_3\downarrow +H_{2}O$
When $N{H}_{4}OH$ solution is added to the above mixture (after adding dilute $HCl$), it produces a white precipitate which is soluble in excess $N{H}_{4}OH$ solution.
This indicates presence of Zn(II) ions.
$Z{n}^{2+}+2N{H}_{4}OH\to Zn(OH{)}_2\downarrow +2N{H}_4^{+}$
$Zn(OH{)}_2+4N{H}_{4}OH\text{(excess)}\to \underset{\text{(water soluble)}}{\left[Zn\right(N{H}_3{)}_4\left]\right(OH{)}_2}+4H_{2}O$
Hence, the salt $Q$ is zinc carbonate $ZnC{O}_3$