In chemistry, hydration is the condition of being combined with water.

Hydration can create a hydrate from which water can be reextracted. When hydration occurs in a chemical reaction it is called a hydration reaction, in which water is permanently and chemically combined with a reactant in a way that it can no longer be reextracted.

Hydration can cause hydrolysis.

Examples of hydrate synthesis

1. Hydration of a carbonyl group.