Final answer:
A compound with water molecules in its crystalline structure is called a hydrate. Hydrates are identifiable by their distinct colors, and they can be reverted to their anhydrous forms by heating to remove the water.
Step-by-step explanation:
A compound that contains water molecules in its crystalline structure is called a hydrate. Ionic compounds that contain water molecules as an integral part of their crystal structure are known as hydrates. The water is not simply trapped mechanically in the pores of the crystalline material but is chemically bonded to the host compound. Notably, the color of a hydrate is often distinct from its anhydrous form. For example, anhydrous cobalt(II) chloride is blue, but when it forms a hydrate, it takes on a distinctive magenta color.
This distinction is particularly important because it provides a means to identify various forms of a compound. By applying heat, one can drive the water off and convert a hydrate back into an anhydrous form, a process which is often used in chemical analysis and reactions.