The equilibrium equation representing the solubility of aluminum hydroxide is,
[tex] Al(OH)_{3}(aq) ---> Al^{3+}(aq) + 3 OH^{-}(aq) [/tex]
When dilute nitric acid is added to it, it would neutralize with hydroxide ions present on the right side of the above equilibrium. So, the concentration of hydroxide ions decreases up on the addition of dilute nitric acid to the aluminum hydroxide solution. This causes the above equilibrium to shift towards the products (right hand side) resulting in the dissolution of more aluminum hydroxide. So. the solubility of aluminum hydroxide in water increases, when dilute nitric acid is added to it
