Synthetic Inorganic Chemistry

MERCURIC SULPHIDE 151

ing material, but plaster of Paris, CaSO^JEbO, which is made in
enormous quantities by gently heating gypsum, is so available
that we shall choose it and let it become fully dehydrated in the
furnace.

Materials: plaster of Paris, CaSO 4 4H 2 O, 145 grams = 1 F.W.

powdered charcoal, C, 48 grams.

Apparatus: 30-gram clay crucible and cover,

gas furnace.

Procedure: Unless the charcoal is already very finely powdered,

grind it thoroughly in a large porcelain mortar. Add the plaster

of Paris, mix the two materials, and pack the mixture in the clay

crucible. Heat the crucible in a gas furnace to between a bright

red and a yellow heat for 1£ hours. At the end of this time remove

the crucible from the furnace. When cold, inspect the contents

of the crucible, particularly the inner portions to which the heat

would have penetrated least; there should be no unburned char-

coal left; a small sample should dissolve with effervescence (hood)

in hydrochloric acid and leave no residue more than a slight

turbidity.

QUESTIONS

1. Treat a gram of calcium sulphide with 20 cc. of water.
   Is there any visible change? Filter. Add hydrochloric acid to
   the nitrate. What is dissolved in the nitrate? What is the
   substance on the filter? Write equations.


2. Evidently calcium sulphide hydrolyzes extensively. Ex-
   plain how this is possible without the evolution of hydrogen sul-
   phide. Write equation. Compare this case of hydrolysis with
   that of aluminum sulphide.

PREPARATION 12

MERCURIC SULPHIDE, HgS

Mercuric sulphide is conveniently made by direct synthesis from

the elements. Two modifications of this compound are known,

one black, which is formed first in this preparation and also by

precipitation of mercuric and sulphide ions; the other a bril-

liant red (vermilion), which is more stable and into which the

black form tends to change.