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.
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.
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
1. Magnesium carbonate, MgCO3, forms a
hydrate containing 39.1 % water of
hydration. Calculate & name the formula
of this hydrate.
MgCO3 . 3 H2O
Magnesium carbonate trihydrate
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
FePO4 . 4 H2O
hydrate?
Iron(III) phosphate tetrahydrate