Dehydration synthesis is the formation of a new compound by the removal of oxygen and hydrogen from reactants, while hydrolysis is the breakdown of a compound through a chemical reaction with water. These opposite processes are very common in nature and in life. One common non-biological form of hydrolysis is the dissolution of salt in water, a process which breaks apart the ionic bonds between the sodium and chloride ions.
Hydrogen and oxygen are a part of a vast number of biological and non-biological compounds, and water can easily react with other compounds in the right circumstances to form them. One way this happens is in acid-based reactions. The pH of water falls exactly in the middle of the acid-base scale, and it can act as either an acid, proton donator, or base, a proton acceptor. In the presence of acetic acid, also known as vitamin C, it accepts a proton, or hydrogen nucleus, to become the positive hydronium ion. This is an example of acid hydrolysis. If another base is present, it will often donate the proton to the base in turn.
The opposite happens with the weak base, ammonia. In water, ammonia accepts a hydrogen ion to become ammonium, leaving a hydroxide ion as the other chemical product.