CALCIUM SULPHATE HEMIHYDRATE. 195
24% and about 33%, it rises from - 8.4° to above 70°. If the
eutectic point itself is not actually obtained in one of the experi-
ments, it is easily found as the intersection point of the two curves.- Calcium Sulphate Hemihydrate.
Gypsum, as it occurs in nature, consists of calcium sulphate dihydrate.
On heating gypsum, its aqueous tension increases until at 101.5° it is equal to
the atmospheric pressure, and at 107° it is 970 mm. or the same as that of
liquid water at the same temperature; in a closed tube, therefore, water and
the dihydrate co-exist up to a temperature of 107°. At higher temperatures
the dihydrate breaks down into the hemihydrate, CaSO 4 .iH 2 O, and water,
but these recombine to again form the dihydrate when the tube is allowed to
Cool. In analogy with the transformation of allotropic forms into one another,
the temperature at which the above reaction is reversible is known as the
transition point.
Gypsum is dehydrated technically by heating the powdered mineral in
iron kettles to about 130° while stirring. The product is the ordinary plaster
of Paris, which consists of calcium sulphate hemihydrate, and which when
mixed with water at a temperature below 107° takes up some of the latter to
again form the dihydrate. This solidifies to a solid mass of interlocking
crystal needles. The production of plaster casts is accomplished in this
manner. When heated to 160°, gypsum becomes "dead burnt," or com-
pletely dehydrated, in which form it does not readily unite with water again.
The conversion of the dihydrate to hemihydrate can be effected below
107° if the aqueous tension is lowered by the presence of other substances.
Thus under a saturated solution of common salt the hemihydrate is formed
at 77°, and under a saturated solution of magnesium chloride at about 11°.
The dehydration is effected very rapidly by heating gypsum in concentrated
nitric acid (sp. gr. 1.4) on the water bath.
Prepare calcium sulphate dihydrate by adding 120 c.c. of
2-normal sulphuric acid to an aqueous solution of 50 g. of calcium
chloride;, wash the precipitate by decantation with water, and
then with a little alcohol, and dry it in the hot closet. The pro-
duct consists of small needles (Microscope). Mix 20 g. of the
preparation with 50 c.c. of concentrated nitric acid (sp. gr. 1.4)so as to form a thick paste, and heat the mixture on the
water-bath with occasional stirring. Watch the course of the
transformation by removing a few drops of the mixture from
time to time and examining it under the microscope; after about
10 minutes the finely pointed needles disappear, and compact
prisms with right-angled outlines appear in their place. After
half an hour cool the contents of the dish, allow the solid to settle.