Hydrolysis of metal salts is more commonly known as hydration. Many metal ions are strong Lewis acids, and in water they may undergo hydrolysis to form "basic salts". Such salts contain a hydroxyl group that is directly bound to the metal ion in place of a water ligand. The positive charge on metal ions creates an attraction to water, a Lewis base with a non-binding electron pair on the oxygen atom, and alters the water molecule's electron density. This in turn increases the polarity of the O-H bond, which now acts as a proton donor under Brønsted-Lowry acid-base theory to release the hydrogen as a H+ ion, increasing the acidity of the solution. For example, aluminium chloride undergoes extensive hydrolysis in water with the solution being very acidic.