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Hydrolysis is a chemical reaction or process in which a chemical compound reacts with water. This is the type of reaction that is used to break down polymers into many smaller units. In this reaction, water is always added to the chemical compound.
Hydrolysis of metal salts
(As noted above, 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.
This implies that hydrogen chloride is lost in the evaporation of AlCl3 solutions and the residue is a basic salt (in this case an oxychloride) in place of AlCl3. This type of reaction is also seen with other metal chlorides such as ZnCl2, SnCl2, FeCl3 and lanthanide halides such as DyCl3. With some compounds such as TiCl4, the hydrolysis may go to completion and form the pure hydroxide or oxide, in this case TiO2.