Question

# Distinguish between the following pairs of compounds using the tests given within brackets:A lead salt and a zinc salt (using excess ammonium hydroxide)

Hint: Consider the solubility of metal hydroxides in presence of excess ammonium hydroxide.

Both lead salt and zinc salt react with ammonium hydroxide to form white precipitate of ${\rm{M}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}$. Here, M represents lead or zinc metal. But in presence of excess ammonium hydroxide, the precipitate of zinc hydroxide dissolves. But the precipitate of lead hydroxide does not dissolve in presence of excess of ammonium hydroxide.
$\begin{array}{l} {\rm{P}}{{\rm{b}}^{2 + }}{\rm{ + 2 N}}{{\rm{H}}_4}{\rm{OH }} \to {\rm{ Pb}}{\left( {{\rm{OH}}} \right)_{\rm{2}}} \downarrow {\rm{ + 2 NH}}_4^ + {\rm{ }}\\ {\rm{Z}}{{\rm{n}}^{2 + }}{\rm{ + 2 N}}{{\rm{H}}_4}{\rm{OH }} \to {\rm{ Zn}}{\left( {{\rm{OH}}} \right)_{\rm{2}}} \downarrow {\rm{ + 2 NH}}_4^ + {\rm{ }} \end{array}$
Chalky white ${\rm{Pb}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}$ precipitate is insoluble in presence of excess ammonium hydroxide whereas gelatinous white ${\rm{Zn}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}$ precipitate is soluble in presence of excess ammonium hydroxide.