HYDRATES 63Experiments
HYDEATES- Water of Hydration. Heat a crystal of blue vitriol
CuSO4-5H 2 O rather cautiously in a dry test tube holding the
tube in a nearly horizontal position. Observe that the
crystal gradually loses its blue color and becomes white and
powdery, also that drops of water condense on the cooler
part of the tube. Let the tube cool to room temperature and
add a few drops of water to the white copper sulphate.
Observe that the material regains at once its original blue
color and that it grows so hot that the hand cannot be held
on that end of the tube.
Water combined as water of hydration in crystals is in a true
state of chemical combination, for the hydrated crystal shows
the following characteristics of a chemical compound.
First: Its crystalline form is different from that of the anhy-
drous compound. In some cases (as in this instance) there is a
difference in color.
Second: It follows the law of definite proportions, for example,
every sample of blue vitriol which has been crystallized from water
at room temperature has exactly the composition expressed by the
formula CuSO4-5H 2 O. (The next experiment is a quantitative
one designed to prove that a crystal hydrate has a definite com-
position.)
Third: There is a marked heat effect produced by the combina-
tion of the anhydrous salt and water.
- Composition of a Crystal Hydrate. The mineral gyp-
sum contains, besides calcium sulphate, also a certain pro-
portion of water. The latter may be completely driven off
by heat, leaving anhydrous calcium sulphate.
Weigh accurately a clean, dry, 15-cc. porcelain crucible.
Place approximately 2 grams of gypsum in it and weigh again.
Cover the crucible, support it on a nichrome triangle, and
heat (to avoid breaking the cover warm this first uniformly by
playing the flame over it carefully from above) to redness
for 20 minutes in the Bunsen flame. Rest the triangle and