Page:Outlines of Physical Chemistry - 1899.djvu/135

 great importance, because the osmotic pressure can be mathematically deduced from certain other properties of solutions, easy to determine and known for a large number of substances. The direct measurement of the osmotic pressure is, therefore, not an absolute necessity. Besides, Tammann, De Vries, and Pringsheim, by indirect methods, have confirmed the laws of the osmotic pressure.

The great majority of organic substances (in general, those of a not very pronounced chemical character) behave like sugar. There is, however, a very numerous class of substances which behave quite abnormally : strong acids, strong bases, and salts, or, as they are termed, electrolytes. The osmotic pressure developed by solutions of these substances is very much greater than it should be according to van't Hojfps law, and in very dilute aqueous solution may even become n times too large, n expressing the number of ions l into which the molecule splits up. For potassium chloride, for instance, the osmotic pressure is nearly double that calculated for unitary molecules of KC1. We shall have occasion to return to these irregu- larities later and also to offer an explanation ; in the meantime it is sufficient to have noticed them.

Vapoub Tension and Boiling Point of Solutions

The vapour tension of a solution is lower than that of the pure solvent.

If we denote the tensions by/ (for the solvent) and/' (for the solution), the difference /— /' is at all temperatures a constant fraction of/:

r

and this difference is proportional to the concentration of the solution.

If, in equal quantities of the same solvent, different

1 See the chapter on Electro-chemistry.

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