Chemistry Help: Common Ion Effect?

(a)
The solubilizing of Al(OH)₃ is a reversible reaction with the equation.
Al(OH)₃(s) ⇌ Al³⁺(aq) + 3OH⁻(aq)

When Al(NO₃)₃ solution is added slowly, the concentration of Al³⁺ ions increases. This leads to the shift of equilibrium position to the left. Hence, when a certain amount of Al(NO₃)₃ solution has been added, the white precipitate of Al(OH)₃(s) is formed.


(b)
The solubilizing of Al(OH)₃ is a reversible reaction with the equation.
Al(OH)₃(s) ⇌ Al³⁺(aq) + 3OH⁻(aq)

When KOH solution is added slowly, the concentration of OH⁻ ions increases. This leads to the shift of equilibrium position to the left. Hence, when a certain amount of KOH solution has been added, the white precipitate of Al(OH)₃ is formed.

However, when excess KOH is added to the reaction mixture, soluble aluminate ions are formed and thus the white precipitate of Al(OH)₃ is re-dissolved to give a colorless solution.