Question

Name a metallic hydroxide which is insoluble in caustic soda but soluble in excess of ammonium hydroxide.

Answer 1

• Copper hydroxide is a metallic hydroxide which is insoluble in caustic soda but soluble in excess of ammonium hydroxide.
• Due to the formation of a soluble complex product, copper hydroxide is soluble in excess ammonium hydroxide.

$$\begin{gathered} \mathrm{Cu}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+4{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to [\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4]{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right) \\ \mathrm{Copper}\mathrm{Ammonium}\mathrm{Tetraamine}\mathrm{copper}\mathrm{complex} \\ \mathrm{hydroxide}\mathrm{hydroxide} \end{gathered}$$

• But in the case of Caustic soda $\left(\mathrm{NaOH}\right)$, no soluble products are formed, hence it is insoluble in caustic soda..