WHEN SODIUM HYDROXIDE SOLUTION IS ADDED TO A SOLUTION OF ZINC SALT, THE WHITE PRECIPITATE FORMED RE-DISSOLVE IN EXCESS SODIUM HYDROXIDE BECAUSE

• A. sodium hydroxide is a strong alkali
• B. sodium is more reactive than zinc
• C. zinc is more reactive than sodium
• D. zinc hydroxide is amphoteric ✓
• E. zinc hydroxide is unstable

The answer to the question is: D. zinc hydroxide is amphoteric

When sodium hydroxide solution is added to a solution of zinc salt, a white precipitate of zinc hydroxide is formed. However, this white precipitate re-dissolves in excess sodium hydroxide due to the formation of a soluble complex ion. The reaction can be explained as follows:

Zn^2+ (aq) + 2OH^- (aq) → Zn(OH)2 (s)

Zn(OH)2 (s) + 4OH^- (aq) → [Zn(OH)4]^2- (aq)

The first equation represents the formation of the white precipitate of zinc hydroxide, while the second equation shows the re-dissolution of the precipitate to form a soluble complex ion.

The re-dissolution occurs because zinc hydroxide is amphoteric, meaning it can act as both an acid and a base. In the presence of excess hydroxide ions, zinc hydroxide behaves as a base and forms a soluble complex ion with the hydroxide ions.

This phenomenon is an example of complexation reactions, where complex ions are formed through the interaction of metal ions with ligands. In this case, the ligand is the hydroxide ion, and the complex ion formed is soluble in excess sodium hydroxide.

Overall, the re-dissolution of the white precipitate of zinc hydroxide in excess sodium hydroxide is due to the formation of a soluble complex ion through a complexation reaction.