What does 'hydrated' mean concerning a compound?

When referring to a compound as 'hydrated,' it indicates that the compound contains water molecules integrated into its structure. This can mean that water molecules are chemically bonded to the compound, often forming hydrates, which are solid compounds consisting of water and the substance in question. For instance, in hydrated copper(II) sulfate (CuSO4·5H2O), the "5H2O" signifies that five water molecules are part of the structure.

This definition emphasizes the structural role of water in the compound, which is critical in fields like crystallography and material science. Hydration can significantly influence the physical properties of a compound, including its solubility, stability, and color. Understanding hydration is crucial, especially in chemistry where reactions are typically carried out in aqueous solutions, and where compounds' states can change with the loss or gain of water.