Question

What do you understand by amphoteric oxide? Give the balanced equations for the reaction with three different amphoteric oxides with a caustic alkali. Write your observation if any.

Answer 1

The metal oxides which react with both acid and base to produce salt and water are known as amphoteric oxides. The oxides of zinc, aluminium, lead, etc. are amphoteric in nature.
The reaction of these three different amphoteric oxides with caustic soda are as follows:

1. Zinc oxide reacts with caustic soda to form colourless solution of sodium zincate.

$$\begin{gathered} \mathrm{ZnO}+2\mathrm{NaOH}\stackrel{}{\to }{\mathrm{Na}}_2{\mathrm{ZnO}}_2+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Zinc}\mathrm{Oxide}\mathrm{Caustic}\mathrm{soda}\mathrm{Sodium}\mathrm{Zincate}\mathrm{Water} \\ \left(\mathrm{White}\right)(\mathrm{colourless},\mathrm{soluble}) \end{gathered}$$

2. Aluminium oxide reacts with caustic soda to form white precipitate of sodium aluminate.

$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\to 2{\mathrm{NaAlO}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Aluminium}\mathrm{oxide}\mathrm{Caustic}\mathrm{soda}\mathrm{Sodium}\mathrm{aluminate} \\ \left(\mathrm{White}\right)\left(\mathrm{white}\right) \end{gathered}$$

3. Lead oxide reacts with caustic soda to form colourless solution of sodium plumbite.

$$\begin{gathered} \mathrm{PbO}\left(\mathrm{s}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\to {\mathrm{Na}}_2{\mathrm{PbO}}_2\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Lead}\mathrm{oxide}\mathrm{Caustic}\mathrm{soda}\mathrm{Sodium}\mathrm{Plumbite} \\ \left(\mathrm{yellow}\right)(\mathrm{colourless},\mathrm{soluble}) \end{gathered}$$