Suggest why aluminium hydroxide is insoluble in water?

The lattice enthalpy of the ionic salt (Al(OH)3) is much higher than the enthalpy of hydration of the individual ions. However, if you look more carefully at the problem you will realize that there is a huge change in entropy when the hydrated water (can be as many as 20 water molecules per ion) is released upon precipitation. Therefore, the insolubility of this complex is a result of entropic effects, which are more positive when the salt remains undissolved.