Page:Outlines of Physical Chemistry - 1899.djvu/235

 water is neutral and not an electrolyte, it may, in presence of an acidic or basic molecule, itself become electropositive (basic) or electronegative (acidic).

Now, a molecule of water thus ionised is different from ordinary water, and becomes active from the point of view of the osmotic pressure, the depression of the freezing point, &c. ; and that is why caustic soda and hydrochloric acid have the same value for i as sodium chloride has.

From the thermo-chemical point of view this theory meets with no difficulty. The neutralisation of a solution of hydrochloric acid by a solution of caustic soda gives rise to the following reaction :

Solution of Solution of Solution of Neutral

caustic soda hydrochloric sodium water

acid chloride

For the neutralisation of the affinities of the base and of the acid, and for the formation of two molecules of neutral water, there is an evolution of 18*7 Calories.

The law of ihermoneutrality, analytical reactions, &c., can be equally well explained by the two hypotheses. The explanations already given (pages 218-214) remain as they stand. It is only necessary to replace the free ions by the new products of dissociation and consider that they (the ionised molecules) have a real and independent existence in the aqueous solution.

For the electmolytic phenomena we come back to a

somewhat modified form of Grotthus's hypothesis. The

+ -

system Na OH + H CI + wH 2 does not form a stable

+ — but only a stationary equilibrium. The molecule Na CI

may be momentarily formed, but only to be broken up

again. The result of this is that the sodium and the

chlorine ions possess great mobility. The atom Na, for

example, is not constantly united to the same OH,

nor is the atom CI always joined to the same H, but these

atoms are tossed about from molecule to molecule, and

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