rmally great.

In a similar way, the theory furnishes an explanation of the abnormal
lowering of the freezing point of electrolytes.

~The theory of electrolytic dissociation and electrolysis.~ The changes
taking place during electrolysis harmonize very completely with the
theory of dissociation. This will become clear from a study of the
following examples.

[Illustration: Fig. 32]

1. _Electrolysis of sodium chloride._ Fig. 32 represents a vessel in
which the electrolyte is a solution of sodium chloride (NaCl). According
to the dissociation theory the molecules of sodium chloride dissociate
into the ions Na^{+} and Cl^{-}. The Na^{+} ions are attracted to the
cathode owing to its large negative charge. On coming into contact with
the cathode, the Na^{+} ions give up their positive charge and are then
ordinary sodium atoms. They immediately decompose the water according to
the equation

Na + H_{2}O = NaOH + H,

and hydrogen is evolved about the cathode.

The chlorine ions on being discharged at the anode in similar manner may
either be given off as chlorine gas, or may attack the water, as
represented in the equation

2Cl + H_{2}O = 2HCl + O.

2. _Electrolysis of water._ The reason for the addition of sulphuric
acid to water in the preparation of oxygen and hydrogen by electrolysis
will now be clear. Water itself is not an electrolyte to an appreciable
extent; that is, it does not form enough ions to carry a current.
Sulphuric acid dissolved in water is an electrolyte, and dissociates
into the ions 2 H^{+} and SO_{4}^{--}. In the process of electrolysis of
the solution, the hydrogen ions travel to the cathode, and on being
discharged escape as hydrogen gas. The SO_{4} ions, when discharged at
the anode, act upon water, setting free oxygen and once more forming
sulphuric acid:

SO_{4} + H_{2}O = H_{2}SO_{4} + O.

The sulphuric acid can again dissociate and the process repeat itself as
long as any water is left. Hence the hydrogen and oxygen set free in the
electrolysis of water really come directly from the acid but indirectly
from the water.

3. _Electrolysis of sodium sulphate._ In a similar way, sodium sulphate
(Na_{2}SO_{4}), when in solution, gives the ions 2 Na^{+} and
SO_{4}^{--}. On being discharged, the sodium atoms decompose water about
the cathode, as in the case of sodium chloride, while the SO_{4} ions
when discharged at the anode decompose the water, as represented in the
equat