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��46 VAPOUR PRESSURE OF SOLUTIONS, chap, iv.

It is clear that in the case of the not too concentrated solutions of the four weak acids — succinic, citric, lactic, and boric — ^the agreement between the calculated and the experi- mental values is satisfactory, and woidd be better if a correc- tion were introduced for the increase of volume which takes place on dissolving the substance. On the other hand, the strong acids and bases .(HaS04 and KOH) and all the salts give results wldch are not at all in agreement with the values calculated. The solutions giving apparently anomalous results are all good conductors of electricity, and it may be noticed that the deviation between calculated and experi- mental value is greater the more radicles (ions) the dissolved substance contains, just as was found to be the case with the osmotic pressure.

We return later to this behaviour of strong electrolytes.

For very concentrated solutions enormous differences are found between the theoretical and the experimental results. Particularly is this the case with very hygroscopic substances such as caustic potash, calcium chloride, and sulphuric acid, and the application of these as drying agents depends on the fact that the vapour pressure of their solutions is small, and consequently water passes to them from places of higher pressure.

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