Synthetic Inorganic Chemistry

CRYSTALLIZED SODIUM CARBONATE 59

90°

80

of this nature are crystalline, and the water is also known as
water of crystallization. This fact should not lead one to think
that water is necessary for the formation of a crystal, because
many crystals do form that do not contain water.
Soda ash is commercial anhydrous sodium carbonate. When a
solution saturated at boiling temperature is allowed to cool to
below 33°, a transparent crystalline mass separates of the formula
Na 2 CO 3 -10H 2 O, known commercially as soda crystals or sal soda.
If water is evaporated
from the solution, satu-
rated at 100°, a solid of the
composition Na 2 CO 3 -H 2 O
separates. Besides the
dekahydrate and mono-
hydrate there is at least
one other well-recognized
hydrate, the heptahydrate,
Na 2 CO 3 -7H 2 O. This com-
pound separates from so-
lution only over a narrow
range of temperature and
concentration.
The curves in Fig. 15
show the different hydrated
forms which are in equilib-
rium with the saturated
solution at different tem-
peratures. It will be seen
that the dekahydrate is the
stable hydrate only below
33°. In the following prep-
aration, if barely enough
water to form the dekahydrate were added to the anhydrous soda
ash and the mixture heated to 100°, a complete solution would not
be obtained because some monohydrate would separate. Enough
water, therefore, is taken to hold all the monohydrate in solution
at the boiling point. When this solution cools to below 33° it
may become highly supersaturated with the dekahydrate unless a
few crystal fragments of the dekahydrate are added. It occasion-
ally happens that if no seed crystals are added the heptahydrate

Na 2 C

/

o 3 -i<ra 2 <

/

.Ni

N

2 eo 8 .*H 2

l2CO 3 -7H

1

•y

/

\

0 10 2 0 30 40 50

Solubility (Grams Anhydrous Na 2 CO 3 in 100 Grama Water)

FIG. 15