A hydrated crystal is a crystal with water molecules bonded to its ions. Although it contains water, it is still a solid. It is created from a highly ionic substance that crystallizes from a water solution.
Although hydrated crystals are solids, the water molecules can be removed through the crystal lattice by heating. Heating occasionally causes hydrated crystals to shatter in the process of removing the water molecules. If the crystal remains intact, the heating process forms an anhydrous crystal, which means "without water." Hydrated crystals generally change color once they become anhydrous.
Hydrated copper sulfate is a common type of hydrated crystal and is blue in color.