116 DIY Science: Illustrated Guide to Home Chemistry Experiments
LABoRAToRy 6.6:
dETERmINE THE foRmULA of A HydRATE
Many ionic compounds exist in two or
more forms. The anhydrous form of the
compound contains only molecules of the
compound itself. The hydrated form or forms
of the compound contains molecules of the
compound and one or more molecules of
water loosely bound to each molecule of
the compound. These water molecules are
referred to as water of hydration or water
of crystallization, and are incorporated
into the crystalline lattice as the compound
crystallizes from an aqueous solution.
RIREEqU d EqUIpmENT ANd SUppLIES£ goggles, gloves, and protective clothing£ balance and weighing papers£ crucible with cover and tongs£ gas burner£ ring stand, ring, and clay triangle£ copper sulfate pentahydrate (~5 g)SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute a Pyrex custard cup and saucer or
similar heat-resistant items for the crucible and cover,
which also eliminates the need for the ring stand, ring,
and clay triangle. Make sure that the saucer allows
venting of the water vapor driven off during heating. You
can place a small nail or similar heat-resistant object
between the lip of the custard cup and the saucer to
provide a vent. A Pyrex vessel with a small spout (like a
liquid measuring cup) will vent nicely as well. If you use
a Pyrex container with a gas burner, use ceramic wire
gauze between the flame and the container. - You may substitute a test tube for the crucible and use
a correspondingly smaller amount of copper sulfate
pentahydrate. When you heat the test tube, hold it
nearly horizontal to keep the hydrate spread out in as
thin a layer as possible. With a test tube, you’ll find that
the water vapor that’s driven off tends to condense
in the upper, cooler part of the test tube. You can
eliminate this problem by also heating the upper part
of the test tube to vaporize the water. - If you use a Pyrex custard cup or similar substitute for the
crucible, you can also substitute a hotplate for the gas
burner. If you do that, don’t place the custard cup directly
on the burner coil. Use a large tin can lid or burner cover
to isolate the custard cup from the burner element. - You may substitute any other available hydrate for
the copper sulfate pentahydrate. Magnesium sulfate
heptahydrate (MgSO 4 · 7H 2 O) is inexpensive and
widely available in drugstores as Epsom salts.
Because these water molecules assume defined positions
within the crystalline lattice, the proportion of water molecules
to compound molecules is fixed and specific. For example,
copper sulfate exists as an anhydrous compound (CuSO 4 ) and
in hydrated form as the pentahydrate (CuSO 4 · 5H 2 O). Copper
sulfate does not exist in the tetrahydrate (CuSO 4 · 4H 2 O) or
hexahydrate (CuSO 4 · 6H 2 O) forms, because the physical
geometry of the crystalline lattice does not permit four or six
water molecules to associate with one copper sulfate molecule.
The number of molecules of water in a hydrate is usually an
integer, but not always. For example, some hydrates exist in
the form X 2 · 5H 2 O, where each molecule of the compound X is
associated with a fractional number (in this, case 2.5) molecules
of water.
Some compounds, including copper sulfate, have only one
stable hydrated form. (Monohydrate and trihydrate forms of
copper sulfate are known, but are difficult to prepare and tend
to spontaneously convert to the more stable anhydrous or
pentahydrate forms by absorbing or giving up water molecules.)
Other compounds have two or more common hydrated forms.
For example, sodium carbonate exists in anhydrous form
(Na 2 CO 3 ), monohydrate form (Na 2 CO 3 · 1H 2 O), heptahydrate
form (Na 2 CO 3 · 7H 2 O), and decahydrate form (Na 2 CO 3 · 10H 2 O).
Many anhydrous compounds are hygroscopic, which means they
absorb water vapor from the air and are gradually converted to
a hydrated form. Such compounds, such as calcium chloride
(CaCl 2 ), are often used as drying agents. (Some of these
compounds absorb so much water vapor from the air that
they actually dissolve in the absorbed water, a property called