Hydrated ionic compounds (i.e., hydrates) have a specfic number of water molecules in their chemical formulas. In the solid, these water molecules (also called "waters of hydration") are part of the structure of the compound.

Rule 1. The ionic compound (without the waters of hydration) is named first by using the rules for naming ionic compounds (e.g., Ba(OH)₂•8H₂O = "barium hydroxide").

Rule 2. Greek prefixes are attached to the word "hydrate" to indicate the number of water molecules per formula unit for the compound (e.g., Ba(OH)₂•8H₂O; 8 water molecules = "octahydrate").

Rule 3. When the chemical formula for a hydrated ionic compound is written, the formula for the ionic compound is separated from the waters of hydration by a centered "dot".