oon as a very little of it has formed the solution becomes
supersaturated, and the excess of the salt precipitates. More silver and
chlorine ions then unite, and this continues until practically all of
the silver or the chlorine ions have been removed from the solution. We
then say that the following reaction is complete:

AgNO_{3} + HCl = AgCl + HNO_{3}.

3. _Two different ions may form undissociated molecules._ In the
neutralization of sodium hydroxide by hydrochloric acid the ions H^{+}
and OH^{-} come to the equilibrium

H^{+} + OH^{-} <--> H_{2}O.

But since water is almost entirely undissociated, equilibrium can only
be reached when there are very few hydroxyl or hydrogen ions present.
Consequently the two ions keep uniting until one or the other of them is
practically removed from the solution. When this occurs the
neutralization expressed in the following equation is complete:

NaOH + HCl = H_{2}O + NaCl.

~Preparation of acids.~ The principle of reversible reactions finds
practical application in the preparation of most of the common acids. An
acid is usually prepared by treating the most common of its salts with
some other acid of high boiling point. The mixture is then heated until
the lower boiling acid desired distills out. Owing to its high boiling
point (338 deg.), sulphuric acid is usually employed for this purpose, most
other acids boiling below that temperature.

EXERCISES

1. What would take place when solutions of silver nitrate and sodium
chloride are brought together? What other chlorides would act in the
same way?

2. Is the reaction expressed by the equation NH_{3} + H_{2}O = NH_{4}OH
reversible? If so, state the conditions under which it will go in each
direction.

3. Is the reaction expressed by the equation 2H + O = H_{2}O reversible?
If so, state the conditions under which it will go in each direction.

4. Suggest a method for the preparation of hydrochloric acid.

CHAPTER XIV

SULPHUR AND ITS COMPOUNDS

~Occurrence.~ The element sulphur has been known from the earliest times,
since it is widely distributed in nature and occurs in large quantities
in the uncombined form, especially in the neighborhood of volcanoes.
Sicily has long been famous for its sulphur mines, and smaller deposits
are found in Italy, Iceland, Mexico, and especially in Louisiana, where
it is mined extensively. In combination, sulphur occurs abundantly in
the form of sulphides and su