A salt with associated water of crystallization is known as a hydrate.  The structure of hydrates can be quite elaborate, because of the existence of hydrogen bonds that define polymeric structures.
Historically, the structures of many hydrates were unknown, and the dot in the formula of a hydrate was employed to specify the composition without indicating how the water is bound. Per IUPAC's recommendations, the middle dot is not surrounded by spaces when indicating a chemical adduct. Examples:
* – copper(II) sulfate pentahydrate
* – cobalt(II) chloride hexahydrate
* – tin(II) (or stannous) chloride dihydrate
For many salts, the exact bonding of the water is unimportant because the water molecules are made labile upon dissolution.  For example, an aqueous solution prepared from  and anhydrous  behave identically.  Therefore, knowledge of the degree of hydration is important only for determining the equivalent weight: one mole of  weighs more than one mole of .  In some cases, the degree of hydration can be critical to the resulting chemical properties.  For example, anhydrous  is not soluble in water and is relatively useless in organometallic chemistry whereas  is versatile.  Similarly, hydrated  is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions.  Samples of  must therefore be protected from atmospheric moisture to preclude the formation of hydrates.
Crystals of hydrated copper(II) sulfate consist of  centers linked to  ions.  Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water.  A fifth water resides elsewhere in the framework but does not bind directly to copper.  The cobalt chloride mentioned above occurs as  and .  In tin chloride, each Sn(II) center is pyramidal (mean  angle is 83°) being bound to two chloride ions and one water.  The second water in the formula unit is hydrogen-bonded to the chloride and to the coordinated water molecule.  Water of crystallization is stabilized by electrostatic attractions, consequently hydrates are common for salts that contain +2 and +3 cations as well as −2 anions.  In some cases, the majority of the weight of a compound arises from water.  Glauber's salt, , is a white crystalline solid with  greater than 50% water by weight.
Consider the case of nickel(II) chloride hexahydrate.  This species has the formula .  Crystallographic analysis reveals that the solid consists of  subunits that are hydrogen bonded to each other as well as two additional molecules of .  Thus one third of the water molecules in the crystal are not directly bonded to , and these might be termed "water of crystallization".