PROPERTIES ASSOCIATED WITH WATER HYDRATES: Solids that contain water molecules as part of their crystalline structure. The water in the hydrate is known as the water of hydration or the water of crystallization. HYGROSCOPIC: A substance is hygroscopic if it readily absorbs water from the atmosphere and forms a hydrate. DELIQUESCENT: A substance is deliquescent if it absorbs water from the air until it forms a solution. DESICCANTS: Compounds that absorb water and are used as drying agents. EFFLORESCENCE: The process by which crystalline materials spontaneously lose water when exposed to air.

Example: BaCl2•2H20 is called barium chloride dihydrate. HYDRATES Hydrates formulas: are expressed by writing the anhydrous formula (without water) for the compound and then adding a dot followed by the number of water molecules present. Example: BaCl2•2H2O this formula tells that each formula unit contains one barium ion, two chloride ions and two water molecules. Naming Hydrates: first name the compound exclusive of the water, then add the term hydrate, with proper prefix, representing the number of water molecules in the formula. Example: BaCl2•2H20 is called barium chloride dihydrate.

BaCl2•2H2O(s)  BaCl2(s) + 2H2O(g) HYDRATES Water molecules in hydrates are bonded by electrostatic forces between polar water molecule and the positive or negative ions of the compound. These forces are not as strong a covalent or ionic chemical bonds. As a result water of crystallization can be removed by moderate heating of the compound. 100ºC BaCl2•2H2O(s)  BaCl2(s) + 2H2O(g) When a solution of copper (II) sulfate (CuSO4) is allowed to evaporate, beautiful blue crystals containing 5 moles water per 1 mole CuSO4 are formed. This formula for this hydrate is CuSO4•5H20 called copper (II) sulfate pentahydrate. When CuSO4•5H2O is heated, water is lost,a pale green-white powder , anhydrous CuSO4 is formed. 250ºC CuSO4•5H2O(s)  CuSO4(s) + 5H2O(g) When water is added to anhydrous copper (II) sulfate, the reaction is reversed and the compound turns blue again. The formation of the hydrate is noticeably exothermic.

PRACTICE PROBLEMS See empirical Formula Magnesium carbonate, MgCO3, forms a hydrate containing 39.1 % water of hydration. Calculate & name the formula of this hydrate. 2. A 25.0 g sample of a hydrate of FePO4 was heated until all the water was driven off. The mass of anhydrous sample is 16.9 g. What is the formula & name of the hydrate? MgCO3 . 3 H2O Magnesium carbonate trihydrate FePO4 . 4 H2O Iron(III) phosphate tetrahydrate