Page:The New International Encyclopædia 1st ed. v. 06.djvu/354

* DISTILLATION. 304 DISTILLATION. ticallj' pure water may be isolated from wine and otlifr liquid mi.xtures by distillation. The al- oliiMiii-t- olitiiini'd the niiiu'ial auids and other valuable --ubslanees liy proeesses of distillation, and at the present dajr distillation is one of the nio-t potent proeesses in the hands of the scien- tific and industrial chemist. Flo. 3. MODEBS- DISTILLING API'ABATrS. What usually takes place during the distilla- tion of a mixture is very simple: The given liquid becomes divided into portions of unequal volatility, every portion that distils over being more volatile than the jjorlion that still remains behind. And as less volatile liquids boil at higher lemperatiires than more volatile ones, it is obvious that the boiling-point of a distilling mi.xture must continually rise. If the mi.xture were allowed to evaporate at constant tempera- ture its vapor-tension must continually decrease. A liquid whose vapor-tension, during evapora- tion, might increase, is an impossibility for al- most obvious reasons. Such a liquid and its vapor would constitite a system whose volume would increase, aning ])ower was brought to act upon it ; which is absurd. Further, by means of such a liipiid, it would be easy to obtain a form of what physicists call "perpetual motion of the second kind,' which the science of thermodynamics recognizes as no less impossible than the crea- tion of mechanical work out of nothing. (See TiiERMODYXA>rics. ) But while the vapor-ten- sion of an evaporating mixture cannot possibly increase, it may, and in certain cases does, re- main constant. Thus a mixture of three part.i of water and 07 jiarls of absolute alcohol will evaporate without change of va|)or-tcnsion. The question therefore remains, what tnixtures are subject to decrease of vapor-tension, and what mixtures evaporate without much change. The answer is: If the ingredients are present in such proportions that the vapor-tension of the mix- ture is either the highest or the lowest that can be obtained with the given substances, then the vapor-tension will remain constant during evapo- lation; in every other case it will decrease. If for a certain proportion of the ingredients the vapor-tension of the mixture is lower than for any other proportion, the composition of the boiling liquid will approach nearer and nearer to that proportion as a limit: once that limit reached, the vapor-tension cannot decrease any further, and thenceforward the composition of the distilling liquid must remain unelmnged: the vapor passing off and the liquid remaining Ik-- Iiind will have precisely the same relative com- position. Again, if at a itrtain tcm|)erature a given mixture has the highest vaportcnsiun that can possibly Ix' obtained with its ingredients, then a separation into mure and less volatile por- tions is evidently impossible, and hence the dis- tillation can produce no change of composition. A mixture of 3 parts of water and 97 parts of absolute alcohol has, at its boiling-temperature, a higher vapor-tension than any other mixture of water and alcohol at the same temperature; and this is whj- that mixture distils over with- out change of composition. From the above it may be seen that the pos- sible changes of the total vapor-tension of a mixture determine in a general niaiimr the di- rection in which the changes of comjiosition will proceed in course of a distillation. To recapitu- late, distillation can effect a change in the composition of a given mi.xture only if both of the following conditions are fuKilled: (1) If there exist mixtures of the same substances, whose vapor-tensions are less than that of the given mixture; (21 if there also exist mixtures of the same substances, whose vapor-tensions are greater than that of the given mixture. Distilla- tion will then divide the given mixture into, say, two portions, one of which will be more, the other less, volatile than the given mixture. But the exact course of a distillation is far from being determined by these laws. For example, no answer is thus furnished to the question: licing given a mixture of a pound of water and a pound of alcdlud. and supposing that half the mixture has distilled over, what is the composition of the distillate? The total vapor-tension of a mixture, it must be remembered, is the sum of the 'partial' vapor-tensions of its ingredients. The relative amounts of the ingredients passing over at any moment are proportional to their partial vapor- tensions at that moment. The partial vapor- tensions in a mixture arc known to be lower than the vapor-tensions of the ingredients in an isolated state. Rut what they are exactly is but seldom known. To determine them experi- mentally is a matter of considerable difficulty; and no theory is known which woiild permit of calculating them in all ordinary cases. Hence our knowledge of the process of distillation is as yet extremely meagre. An empirical rule capable of yielding some practical results has been pro- posed liy Brown, and has been somewhat de- veloped by Thomas. Barrell. and Young. But it is by no means sulliciently established to require explanation here. The modern theory of solutions, too. lias been brought to l>car on the problem of distillation: but that theory permits of calculating the partial vapor-tensions only in mixtures containing a very large excess of one of the ingredients ('dilute solutions'), and hence cannot furnish a sufficiently general solution of the problem of distillation. It remains to notice briefly the processes of fractional distillation and distillation with a reflex condenser kept at constant temperature. In fractional distillation, which is very fre- quently employed by organic chemists, a mixture is first divided into a series of 'fractions' by dis- tillation: then the fractions are subdivided by further separate distillations: then some of the frartioiis are mixed togi'ther in pairs or threes, the resulting liquids again distilled separately, and so forth until the required separation has