Whenever water reacts with another chemical compound, the process is called hydrolysis. Hydrolysis differs somewhat from hydration, although the two can occur together. Hydration is the bonding of whole water molecules to an ion (a charged atom or molecule), usually a metal ion. Hydrolysis, on the other hand, involves an actual chemical reaction of the water molecule itself with another reactant. Aluminum ion, for example, can bond with six water molecules to form the hydrated aluminum ion. In water, however, the hydrated ion can undergo hydrolysis; some of the hydrated molecules contribute a hydrogen ion to the solution, making the solution acidic.
Solutions of non-hydrated ions often become either acidic or basic because of hydrolysis, too. In general, negative ions (anions) form basic solutions if they hydrolyze, because the negative charge on the ion attracts the positively charged hydrogen ion (H+) away from water, leaving the basic hydroxide ion (OH-) behind. Similarly, positive ions (cations) form acidic solutions if they hydrolyze, because the positive charge on the ion attracts the negatively charged hydroxide ion away from water, leaving the acidic hydrogen ion behind. Hydrolysis of these ions only occurs, however, if the ion originally came from a weak acid or base, or the salt of a weak acid or base. (A salt is an ionic chemical compound derived from an acid or base, often as the result of a neutralization reaction.) Ions do not hydrolyze if they are from strong acids or bases—such as chloride ion from hydrochloric acid or sodium ion from sodium hydroxide (a base)—or their salts.
In biochemistry, hydrolysis often involves the decomposition of a larger molecule. If a fat undergoes hydrolysis, for example, it reacts with water and decomposes to glycerol and a collection of fatty acids. Similarly, complex sugars can hydrolyze to smaller sugars, and nucleotides can hydrolyze to a five carbon sugar, a nitrogenous base, and phosphoric acid.