66 ACIDS, BASES, AND SALTS.This double possibility of dissociation enables substances to react in different
ways. According to the first of the above equations, hydrogen iodide is a
reducing agent; according to the second, it is an acid.
Electrolytic dissociation can be detected and measured by physical methods,
either by estimating the apparent molecular weight of the dissolved substance
by one of the osmotic methods, or by determining the conductivity of the solu-
tion (Arrhenius); cf. No. 34. As has already been stated in the discussion of
the preparation of lithium by the electrolysis of fused lithium chloride (No. 14),
the unit of electricity, the electron, can be represented by the symbol e and
the process of electrolytic dissociation, or ionization, of a binary compound
AB can be represented as follows:
AB <=> A+ + B-which means that neutral molecules of AB have dissociated, or ionized, into
positively-charged cations A+ and negatively-charged anions, B~. The
element A has lost one of its original electrons and thereby obtained a positive
charge, the element B has gained an electron and thereby acquired a unit
negative.A" - e = A+; B° + e = B~.The same reasoning applies when A or B is a combination of two or more
elements. In the original compound, the transfer of an electron from A to B
has probably taken place already and a crystal of a salt consists of a rigid
lattice of the charged particles, the electrostatic attraction of the opposite
charges holding each particle in its definite place so that there can be no move-
ment of the charges and hence no conductivity. Dissolving the salt separates
the charged components so that they are no longer held to definite places.
Each then possesses its own independent motions and shows the osmotic
properties of a complete molecule even although the motion is still restricted
in that particles of opposite charge must always be approximately evenly
spaced throughout the solution. The application of an outside potential will
now draw particles of opposite charge in opposite directions, — in other words,
the solution conducts electricity.
LAW OF DILUTION. The application of the law of mass-action to the
dissociation of a binary electrolyte gives the following expression:
[cation] [anion]
= k.
[undissociated compound]Thus, in a given case, if the fraction of a gram molecule of a binary electrolyte
which has undergone dissociation is denoted by a (the degree of dissociation),
then 1 — a is the undissociated fraction of the gram molecule which is in
undissociated compound is and that of each ion is -. Then according