Page 217 - Encyclopedia of Chemical Compounds 3 Vols

CALCIUM SULFATE

Calcium sulfate. Red atoms
are oxygen; turquoise atom is
calcium; and yellow atom is
sulfur. Gray sticks show double
bonds. PUB L I S H E RS RE SO URCE
GRO U P

converted to the anhydrous form upon heating as, for exam-
ple: CaSO 4 2H 2 O ! CaSO 4 +2H 2 O.
Anhydrous calcium sulfate is essentially insoluble in
water. As their names suggest, the soluble form of the com-
pound (soluble anhydrite) is somewhat more soluble in water
than is the insoluble form (insoluble anhydrite). The dihy-
drate and hemihydrate are only slightly soluble in water.
When water is added to the hemihydrate, a reaction occurs
that results in the formation of a hard, solid mass (plaster of
Paris) used in making casts, such as those used to hold
broken bones in place. Neither the anhydrous form of cal-
cium sulfate or the dihydrate reacts with water in this way.
Both the anhydrous and dihydrate forms of calcium sul-
fate occur naturally in the form of the minerals anhydrite,
angelite, muriacite, and karstenite (CaSO 4 ); and gypsum
(CaSO 4 2H 2 O). These minerals have been known to humans
and used by them for thousands of years. The method for
converting natural gypsum to the hemihydrate (plaster of

166 CHEMICAL COMPOUNDS