196 HYDRATES.
decant off the liquid as completely as possible, shake the residue
with 50% alcohol, and filter at once with suction. Throw away
the filtrate, wash the precipitate, and dry it in the hot closet.
Yield, almost quantitative.
Determine the water content of the product by igniting a
weighed sample to faint redness. The hemihydrate contains
6.2% water.
The hemihydrate prepared as above consists of larger crystalsthan commercial plaster of Paris, and for this reason it " sets "
more slowly. Mix half of the preparation with water to form a
thick paste, and allow it to stand until, at the end of about 30
minutes, solidification takes place. The process may be watched
more closely by placing a few drops of the fresh mixture underthe microscope; after about 20 minutes, fine needles of the dihy-
drate are seen to appear, and after that to increase rapidly in
quantity while the compact prisms of the hemihydrate disappear.
Simultaneously with the formation of the crystal needles under
the microscope, the larger sample grows hard. This change may
be compared with the similar transformation of potassium lead
iodide (cf. No. 105). Moisten the remainder of the hemihydrate
with a mixture of alcohol and water; this time the mass solidifies
much more slowly because the vapor pressure of the water is
diminished by the presence of the alcohol.
- Hydrates of Sodium Sulphate. Supersaturated Solutions.
The solubility of ordinary, crystallized sodium sulphate, Na^jSO,. 10 H 2 O,
increases rapidly with rise in temperature, but can be followed only to 32.4°
because at this temperature the solid decahydrate, standing in contact with
the solution, is completely dehydrated. The solubility of the anhydrous
salt is peculiar in that it diminishes with rise of temperature; thus at 32.4°
the saturated solution contains the maximum amount of sodium sulphate.
Accordingly, when a saturated solution is allowed to evaporate, the anhy-
drous salt separates if the temperature is above 32.4°, whereas the decahydrate
is deposited if the solution is below this temperature. The solutions, how-
ever, can very easily become supersaturated, provided they are protected
from dust, etc., which tends to start crystallization. By allowing such
supersaturated solutions to remain in the cold, a different hydrate,
NajSO 4 .7H 2 O, crystallizes spontaneously, but since this salt is more soluble
than the decahydrate, the solution standing over it remains supersaturated
with respect to the decahydrate.
Dissolve 170 g. of crystallized sodium sulphate in 75 c.c. of