The solubility of Al(OH)3 with respect to pH is soluble at low pH, insoluble in pH-neutral solution, and soluble at high pH. We can express it as this:
Solubility equilibrium expression for Al(OH)3 is
Al(OH)3 <==> Al3+(aq) + 3OH-(aq)
Initial 0 1.0x10^-3 M
change +s +3s
Remember that solubility is used in chemistry to describe the properties of a solid compound that is mixed with and fully dissolves in a fluid without leaving any undissolved particles.