100 COMPLEX COMPOUNDS.
the ordinary oxygen-acids are formed by the union of water and acid anhy-
dride, and the salts of the oxygen-acids by the union of metal oxide and
non-metal oxide. Here also there are produced, quite in accord with the
interpretation of complex salts, new ions, the separate constituents of which
are incapable of entering into independent chemical reactions:
H 2 O + SO 3 = H 2 [SOJ,
H 2 [SO 4 ] = 2H+ + [SO 4 ]—.
Thus the sulphate radical does not show the reactions of sulphur any more than
the ferrocyanide radical exhibits those of iron.
Since manifestly every compound containing more than two elements can
under certain conditions behave in such a manner as to indicate the grouping
of two of its constituents into a complex, it appears more rational to include
under complex compounds all substances which are produced from simple com-
pounds by the addition of one or several elements; in most cases the complex
radical behaves, like the hydroxyl group, as a unit, and can thus be treated as
a substituent of an atom.
The most essential difference between the complex and the simple com-
pounds lies in the great variety of ways in which the former can react or dis-
sociate, and this is more evident in proportion to the number of constituents.
There is but one way in which electrolytic and non-electrolytic dissociation can
take place in simple compounds. With complex compounds, on the other
hand, the point of division is usually not the same by the electrolytic as by the
non-electrolytic dissociation:
CaCO 3 = Ca + + CO 3 (electrolytic dissociation).^1
CaCO 3 = CaO + CO 2 (non-electrolytic dissociation).^1
Non-electrolytic dissociation may be of very different types:
KC10 3
NaNO 3
Na 2 S 2 O 3
KPbI 3
NH 4 C1= KC1 + 3 0
= NaNO 2 + 0- Na 2 SO 3 + S
= KI + Pbl 2
= NH 3 + HC1
(Nos
(No.
(No.
(No.
(No.. 74 and 75)
79).
90).
105).
4 3 + (No. 10).120).
Ni(NH 3 ) 6 Br 2 = NiBr 2 + 6 NH 3 (No. 128).
CLASSIFICATION OF COMPLEX COMPOUNDS. Of the various ways in which
complex compounds can dissociate, the electrolytic dissociation exceeds all others
as regards frequency of occurrence. We may classify the complex compounds,
therefore, as: 1. Those with a complex radical which yields an ion of negative
charge (Chapter IV); 2. Those with a complex radical which yields an ion of
positive charge (Chapter V); 3. Those characterized by very little or no
capacity for electrolytic dissociation (Chapter VI).
(^1) According to whether the one or the other possibility of dissociation is to
be brought especially to notice, two methods of writing the name and symbol
have been devised: CO 2 • CaO, " carbonate of lime "; CaCO 3 , " calcium car-
bonate "; but it would be just as biased to defend the older name as being the
only satisfactory one as it would be to reject it as " unscientific."