WAEC - Chemistry (2000 - No. 37)
When sodium hydroxide solution is added to a solution of zinc salt, the white precipitate formed re-dissolves in excess sodium hydroxide because
sodium is more reactive than zinc
sodium hydroxide is a strong alkali
zinc hydroxide is amphoteric
zinc hydroxide is unstable
Uitleg
Sodium hydroxide is a strong base that can react with certain salts, especially those that contain amphoteric metals. Zinc is an amphoteric metal, meaning it can react with both acids and bases. Like the hydroxides of other metals, such as lead, aluminium, beryllium, tin and chromium, zinc hydroxide (and zinc oxide), is amphoteric. Thus it will dissolve readily in a dilute solution of a strong acid, such as HCl, and also in a solution of an alkali such as sodium hydroxide. The correct answer in option C