products of the reaction
is removed as fast as it is formed._

~Equilibrium in solution.~ When reactions take place in solution in water
the same general principles hold good. The matter is not so simple,
however, as in the case just described, owing to the fact that many of
the reactions in solution are due to the presence of ions. The
substances most commonly employed in solution are acids, bases, or
salts, and all of these undergo dissociation. Any equilibrium which may
be reached in solutions of these substances must take place between the
various ions formed, on the one hand, and the undissociated molecules,
on the other. Thus, when nitric acid is dissolved in water, equilibrium
is reached in accordance with the equation

H^{+} + NO_{3}^{-} <--> HNO_{3}.

~Conditions under which reversible reactions in solution are complete.~
The equilibrium between substances in solution may be disturbed and the
reaction caused to go on in one direction to completion in either of
three ways.

1. _A gas may be formed which escapes from the solution._ When sodium
nitrate and sulphuric acid are brought together in solution all four
ions, Na^{+}, NO_{3}^{-}, H^{+}, SO_{4}^{--}, are formed. These ions are
free to rearrange themselves in various combinations. For example, the
H^{+} and the NO_{3}^{-} ions will reach the equilibrium

H^{+} + NO_{3}^{-} <--> HNO_{3}.

If the experiment is performed with very little water present, as is the
case in the preparation of nitric acid, the equilibrium will be reached
when most of the H^{+} and the NO_{3}^{-} ions have combined to form
undissociated HNO_{3}.

Finally, if the mixture is now heated above the boiling point of nitric
acid, the acid distills away as fast as it is formed. More and more
H^{+} and NO_{3}^{-} ions will then combine, and the process will
continue until one or the other of them has all been removed from the
solution. The substance remaining is sodium acid sulphate (NaHSO_{4}),
and the reaction can therefore be expressed by the equation

NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.

2. _An insoluble solid may be formed._ When hydrochloric acid (HCl) and
silver nitrate (AgNO_{3}) are brought together in solution the
following ions will be present: H^{+}, Cl^{-}, Ag^{+}, NO_{3}^{-}. The
ions Ag^{+} and Cl^{-} will then set up the equilibrium

Ag^{+} + Cl^{-} <--> AgCl.

But silver chloride (AgCl) is almost completely insoluble in water, and
as s