112 THE THEORY OF IONIZATIONis large, the amount of NH 3 necessary to produce equilibrium is
small. Such a substance as pure ammonium hydroxide of the
composition shown by the formula is unknown, because, if it
existed for a moment, it would at once undergo non-electrolytic
dissociation until it came to a state of equilibrium with the prod-
ucts H2O and NH 3 , the first product remaining as a part of the
NH4OH solution, and the larger part of the NH 3 escaping as gas.
The more water present, however, obviously the less NH 3 is
necessary to maintain equilibrium with the NH 4 OH.
Basic Properties of the Metal Oxides. The oxides of the metals
are characterized as a class in that they are basic. The term
" basic " implies the presence of OH" ions when these oxides are
dissolved in or suspended in water. The most strongly basic
oxides, such as Na20, K 2 O, CaO, BaO, react violently with water
and produce well-defined hydroxides NaOH, KOH, Ca(OH) 2 ,
Ba(OH) 2 which can be isolated as well-defined solid substances of
the exact composition shown by the formulas. These hydroxides
are soluble (Ca(OH) 2 sparingly so) in water, and the dissolved
hydroxides are highly ionized into simple metal ions and OH~
ions.
The oxides of the heavy metals are likewise basic, but they are
only feebly basic. These heavy metal oxides are as a class almost
insoluble, yet when they are suspended in water a certain low con-
centration of metal ions and OH" ions is built up in the solution.
Let us discuss two heavy-metal oxides, ferric oxide, Fe 2 O 3 , and
cupric oxide, CuO, as typical of this class.
Powdered black copper oxide when stirred into water gives a
black suspension and does not visibly dissolve. If the suspension
is allowed to stand the black powder settles to the bottom and the
clear liquid above contains so few ions of Cu++ and OH~ that they
cannot be detected by the reagents commonly used for detecting
these ions, NH4OH, Na 2 S. Yet we are confronted with the
fact that, when acid is added to the suspension of copper oxide,
the black powder dissolves completely and we obtain a clear blue
solution of the cupric salt. The initial and final substances in this
reaction are given in the equation
CuO + 2HC1 -* CuCl 2 + H 2 Owhich shows that 1 F.W. of CuO neutralizes 2 F.W. of HC1.
In a similar manner red powdered ferric oxide stirred into