Page:Text-book of Electrochemistry.djvu/215

 200 EQUILIBRIUM BETWEEN ELECTROLYTES, chap.xii.

It is remarkable that the ions in the solution occupy a smaller volume than the compound molecules. In this branch of our subject, however, many peculiarities are to be found, as, for instance, the fact that certain substances (sodium hydroxide, etc.) cause such a contraction of the water that the solution has a smaller volume than the solvent used.

It can easily be seen that all strong acids and bases must have almost the same neutralisation volume; that this is actually the case may be gathered from the following numbers : —

��Acid.

�A. on neutralifting with KOH NaOH HCl

HBr HI

��For ammonia the volume change on neutralising with a strong acid amounts to 26 c.c.

The opinion of Nemst and Drude (19) is that the ions influence the volume of the water on account of the strong electric field which they give rise to. All liquids, whose dielectric constant is increased by pressure — and this is probably always the case (J^O) — suffer a contraction in a strong electric field. This phenomenon is known as dcctrostriction. The ions in water, alcohol, or other solvent cause such an electrostriction.

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