When zinc ions are treated with aqueous sodium hydroxide, what is observed?

• A. White precipitate soluble in excess to form a colourless solution.
• B. Red-brown precipitate insoluble in excess.
• C. Green precipitate.
• D. No reaction.

Answer: (A)

Zinc ions reacting with a strong base first form zinc hydroxide, which is a white precipitate because Zn(OH)2 is only sparingly soluble in water. But zinc hydroxide is amphoteric, so in excess sodium hydroxide it reacts further to form a soluble zincate complex [Zn(OH)4]2−. This makes the solid dissolve and the solution becomes colourless. So you observe a white precipitate that dissolves in excess base to give a colourless solution.