This is the order of increasing solubility of aluminum compounds in water. In general, aluminum compounds become less soluble as the charge on the aluminum ion increases and the size of the anion increases. Aluminum chloride (AlCl3) is the most soluble because it is ionic and has a high charge on the aluminum ion. Aluminum hydroxide (Al(OH)3) is less soluble because it forms insoluble hydroxide complexes in water. Aluminum sulfate (Al2(SO4)3) is even less soluble because it has larger anions that can interfere with the formation of soluble complexes. Finally, aluminum oxide (Al2O3) is the least soluble because it is a highly stable and insoluble compound.