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In a large number of cases the molecular weights of dissolved substances are found to be the same as those deduced from the vapour densities. When the substance is not volatile, it often corresponds with the simplest molecular formula- e.g., cane-sugar, C12H22O11. Solutions of organic substances in water, alcohol, and ether usually show normal molecular weights. Raoult, however, observed that many substances dissolved in benzene, nitrobenzene, and ethylene dibromide gave depressions of freezing point, or lowerings of vapour pressure, only half the normal, and he explained this by the association of the solute to form double molecules. Many such substances in fact (e.g., acetic acid) gave abnormally high vapour densities.
An abnormally small depression is also produced when the dissolved substance crystallises out with the solvent to form a homogeneous solid solution; the freezing point may thus even be elevated.
But when aqueous solutions of acids, bases, and salts, (i.e., electrolytes) were found to give molecular depressions considerably in excess of the normal, which increased with dilution until they approached double or treble the normal depression, or an even higher multiple in others, the interpretation was not so clear. It might be supposed that all the so-called normal depressions produced by organic solutes were really due to double molecules, and that acids, bases, and salts are normal, but the higher depressions for some salts, the identity of the values of the gas constant R from measurements of gaseous density and osmotic pressure, and other experimental evidence, tell against this hypothesis. The only other explanation possible, if we regard the laws of solution as valid in all cases, is to suppose that the salts are dissociated in solution. The molecules then break up into sub-molecules, and at high dilution the dissociation must be practically complete. This, however, is exactly the state of affairs postulated by Arrhenius in 1887 in his theory of electrolytic dissociation. The sub-molecules are the electrically charged ions: KCl <=> Kยท + Cl', and the increase in the number of molecules of solute so produced would account for the abnormally large depression of freezing point. The ions produce depression of freezing point like neutral molecules. The electrolytic dissociation theory, therefore, not only gave a clear explanation of the facts of electrolysis as discovered by Faraday, but cleared away the perplexing difficulties which had surrounded the properties of solutions of electrolytes as investigated by Raoult.
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