Hydrated Ionic Compounds
Many ionic (metal + non-metal) compounds crystallize
with one or more water molecules loosely bonded to
each formula unit. We call these compounds
“HYDRATES”.
Think about the word “hydrated”. If you keep hydrated,
you drink water. So a HYDRATE has water in it.
A hydrate has a SPECIFIC number of water molecules
bonded to it.
For example:
Epsom salts that you might put in a bath to relax tense
muscles is actually MgSO4●7H2O
This is called magnesium sulphate heptahydrate,
meaning 7 water molecules bonded to each
magnesium sulphate unit.
*Remember these Prefixes*
Mono
1
Di
2
Tri
3
Tetra
4
Penta
5
Hexa
6
Hepta
7
Octa
8
When showing the chemical formula of a hydrate, we put
a dot (●) between the ionic compound and the water.
The dot tells us that the water molecule(s) is/are bonded
loosely to the ionic compound.
Compounds that have no water molecules attached are
called ANHYDROUS.
For example:
Alabaster (or gypsum) is a white stone used for
decoration (statues, jewellery, lamps etc.). It is actually
CaSO4●2H2O or calcium sulphate dihydrate
Sometimes, it exists as CaSO4, with no waters
bonded to it, so we call it anhydrous calcium
sulphate.
When naming a hydrated ionic compound:
1) name the ionic compound first, as normal (some
may be simple ionic compounds and some may
be “polyatomics”)
2) indicate the number of water molecules (shown in
the formula as the number after the dot and and
before the H2O) by using the correct prefix (see
table above)
3) write the word “hydrate”
Name the following Hydrates:
1. Zn(NO3)2 • 5H2O
2. MgCl2 • 2H2O
3. ZnCl2 • 6H2O
4. Ba(OH)2 • 8H2O
5. CoCl2 • 6H2O
6. CuCl2 • 2H2O
7. Ba(OH)2 • 8H2O
Go to this website for some more practice:
http://www.chem.purdue.edu/gchelp/nomenclature/h
ydrates_2009.htm