is exploded, it is entirely converted into water. This is also the case
if two substances are brought together in solution, by the action of
which upon each other a third body is formed which is insoluble in the
solvent employed, and which also does not tend to react upon any of the
substances present; for instance, when a solution of a chloride is added
to a solution of a silver salt, insoluble silver chloride is
precipitated, and almost the whole of the silver is removed from
solution, even if the amount of the chloride employed be not in excess
of that theoretically required.
But if there be no tendency to form an insoluble compound, Or one which
is not liable to react upon any of the other substances present, this is
no longer the case. For example, when a solution of a ferric salt is
added to a solution of potassium thiocyanate, a deep red coloration is
produced, owing to the formation of ferric thiocyanate. Theoretically
the reaction takes place in the case of ferric nitrate in the manner
represented by the equation
Fe(NO3)3 + 3KCNS = Fe(CNS)3
Ferric nitrate. Potassium thiocyanate. Ferric thiocyanate.
+ 3KNO3;
Potassium nitrate.
but it is found that even when more than sixty times the amount of
potassium thiocyanate required by this equation is added, a portion of
the ferric nitrate still remains unconverted, doubtless owing to the
occurrence of the reverse change--
Fe(CNS)3 + 3KNO3 = Fe(NO3)3 + 3KCNS.
In this, as in most other cases in which substances act upon one another
under such circumstances that the resulting compounds are free to react,
the extent to which the different kinds of action which may occur take
place is dependent upon the mass of the substances present in the
mixture. As another instance of this kind, the decomposition of bismuth
chloride by water may be cited. If a very large quantity of water be
added, the chloride is entirely decomposed in the manner represented by
the equation--
BiCl3 + OH2 = BiOCl + 2HCl,
Bismuth chloride. Bismuth oxychloride.
the oxychloride being precipitated; but if smaller quantities of water
be added the decomposition is incomplete, and it is found that the
extent to which decomposition takes place is proportional to the
quantity of water employed, the decomposition being incomplete, except
in presence of large quantities of water, because of the occurrence of
the reverse action--
BiOCl + 2HCl = BiCl3 + OH2.
Chemical change which merely involves simple decomposition is thus seen
to be influenced by the masses of the reacting substances and the
presence of the products of decomposition; in other words the system of
reacting substances and resultants form a mixture in which chemical
action has apparently ceased, or the system is in equilibrium. Such
reactions are termed reversible (see CHEMICAL ACTION).