What happens when aluminum ions are treated with aqueous sodium hydroxide?

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

Answer: (A)

The key idea is that aluminum hydroxide is amphoteric. When Al3+ ions meet hydroxide, they form a white solid, Al(OH)3. But this solid can react with excess base to form a soluble complex, [Al(OH)4]−, which is colourless in solution. So you get a white precipitate initially, and in excess NaOH it dissolves to give a colourless solution.