192 ALKALI AND ALKALINE EARTH METALScipitated sodium chloride on a suction filter. Test the filtrate for
sulphate by adding a little barium chloride solution to a small
sample of it diluted with water. A positive test will probably be
obtained. Now wash the crystals with successive portions of
10 cc. of 6N HC1, until the washings show no further test for
sulphates. (See Note 5 (a), page 9.) Then transfer the crystals
to a porcelain dish and heat gently, while stirring, until all decrepi-
tation ceases. Put up the product in a 2-ounce cork-stoppered
bottle.
QUESTIONS- Why must the hydrochloric acid gas be passed through a
washing bottle? Why is the safety tube necessary? - Why, in the light of the law of molecular concentration,
should one expect the solubility of sodium chloride to be lessened
by the presence of hydrochloric acid? It may be stated that
another effect known as the " salting-out effect " also comes into
play here and likewise tends to lessen the solubility of sodium
chloride. The great amount of heat liberated when hydrogen
chloride dissolves in water indicates a chemical action, and it is
very probable that the water and hydrogen chloride unite to form
an unstable compound. In the saturated solution then nearly all
the water is chemically combined and very little is left to hold
sodium chloride in solution. - Mention two possible causes for the very considerable
amount of heat produced when the hydrochloric acid gas is ab-
sorbed by the solution in the beaker. - Why does not the solution in the washing bottle also grow
hot?
PREPARATION 19
AMMONIUM BROMIDE, NH 4 Br
Ammonium bromide could be prepared by the neutralization
of ammonium hydroxide with hydrobromic acid,
NH 4 0H + HBr = NI^Br + H 2 O
Since, however, hydrobromic acid is a more expensive material
than uncombined bromine, the latter would have the preference
as a source of bromine, provided it yielded as satisfactory a prod-
uct. Bromine reacts upon a cold solution of sodium hydroxide