When saturated aqueous solutions of some salts are cooled the soluble salts present in them crystallize out. Every salt has a definite crystalline shape. During the crystallization, a water molecule becomes the part of their crystal lattice and remains there even after separation of crystals from their solutions. Such molecules are called water of crystallization. And the compounds are known as hydrates or hydrated compounds. This process is called hydration. Examples of hydrates are copper sulphate, potassium aluminium sulphate, ferrous sulphate, barium hydroxide.The formulas of hydrates are often written with dot between the water molecules and the compounds that has been hydrated. The hydrates contain a fix number of water molecules and have a definite composition. They may loose their water of crystallization on heating and may be reformed by the crystallization from water. These crystalline compounds after losing their water of crystallization are called anhydrous compounds.
Water is remarkably versatile solvent. It is an excellent medium for transporting ions molecules in the environment and in living things. Ionic compounds are easily soluble in water. For example; caustic soda, sodium chloride and ammonium chloride. In these compounds the attractive forces between water molecules and ions are stronger than those of the ions themselves. The majority of non polar compounds do not dissolve in water because water molecules are more strongly attracted to each other than to non polar molecules. E.g. camphor and naphthalene.