58 WATER AND SOLUTION
filtrate and washings in the flask of " mother liquors " (G). Again
test for presence of chloride. A trace will probably be shown.
Continue the washing process, using 15-cc. portions of ice water
each time until no test is shown for chloride. Transfer the crystals
to white paper towels. Fold the towels over the crystals to make
a compact package, and leave the package over night to dry at
room temperature. Transfer the dry crystal meal to a dry 4-ounce
bottle and label the preparation neatly.
If a sufficient quantity of pure product is not obtained, all the
mother liquor should be boiled down in the 750-cc. casserole and
used as a starting point in repeating the foregoing procedure.
Fifty grams may be regarded as a satisfactory yield.
The sequence of operations in this preparation can be readily
followed on the flow sheet, page 55.
QUESTIONS- In the recrystallization of the potassium nitrate why is it
preferable first to dissolve completely the batch of crystals in water
by heating and then cool to obtain the crystals again, instead of
merely washing the crystals with the same amount of cold water? - Define metathesis.
- If we had barium nitrate and potassium sulphate from which
to prepare potassium nitrate, make a list of the solubilities of the
four salts concerned and arrange a flow sheet of a method by which
pure potassium nitrate could be obtained. Which of the opera-
tions would offer the most difficulty?
PREPARATION 2
PREPARATION OF A HYDRATE
CRYSTALLIZED SODIUM CARBONATE, Na 2 CO 3 -10H2O, FROM
ANHYDROUS SODIUM CARBONATE
Many substances, as acids, bases, salts, and even elements,
when they separate from solution, or crystallize, carry water with
them. Familiar examples are (H 3 AsO4) 2 -H 2 O, Ba(OH) 2 -8H 2 O,
CuSO 4 -5H 2 O, and C1 2 -8H 2 O. Such substances, when dried, show
no evidence of fluidity, that is, of the property one must natu-
rally think of as belonging to water, even sometimes when more
than hah" the weight of the substance is water. Such substances
are called hydrates, and the same substance without the water is
known as the anhydrous substance. Substances containing water