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A Source Book in Chemistry, 1400-1900
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General SummaryThe fact that dissolved substances lower the freezing points of their solvents had been observed as early as 1788 by Blagden, and the similar effect on vapor pressure was discovered by yon Babo in 1847, but the early investigators used solutions of ionizing substances, and the unsuspected effect of the dissociation prevented the discovery of any regular laws governing these effects. Then in 1882, F.-M. Raoult published his results on the effects of nondissociating organic solutes, from which he deduced a general law controlling the lowering of freezing points. Four years later he extended this work to show the effect of solutes on vapor pressure. Having established the effect of nondissociating compounds, he was in a position to show that salts produced an effect which, though anomalous, could nevertheless be explained by the supposition that a dissolved molecule broke up into other molecules. This work was of great value in supplying a new method for determining molecular weights, since the depression of freezing point and vapor pressure (as well as the related rise in boiling point later discovered) are proportional to the molecular concentrations of the solutions; it was of equal value in supporting the ideas of van’t Hoff on osmotic pressure (pages 435–458). With the announcement of the dissociation theory of Arrhenius, the anomalies were explained and the full significance of the generalizations of Raoult was recognized. Raoult published the formulations of his laws in the Comptes rendus for 1882 and 1887. The selections which follow are taken from these two papers.
François-Marie Raoult
GENERAL LAW OF THE VAPOR PRESSURE OF SOLVENTS.2
The molecular reduction of the vapor pressure K of a solution, that is to say, the relative reduction of pressure produced by 1 molecule of a substance held in 100 grams of a volatile liquid, can be calculated by means of the following formula:
in which f is the vapor pressure of the pure solvent, f’ the vapor pressure of the solution, M the molecular weight of the dissolved substance, P the weight of this substance in solution in 100 grams of the solvent; if it is admitted that the relative reduction of pressure
is proportional to the concentration. As this proportionality is rarely rigorous, even when the solutions are very dilute, I have been obliged in these comparative studies to use solutions which always have nearly the same molecular concentrations and contain four to five molecules of substance held per 100 molecules of volatile solvent. A greater dilution would not allow sufficiently exact measurements. All the experiments were performed by the barometric method and conducted like those I have run in ether solutions. The tubes were plunged in a water bath limited by parallel glasses, constantly agitated, and heated at will.
In each ease the temperature was so chosen that the vapor pressure of the pure solvent was about 400 millimeters of mercury. The measurements were made from fifteen to forty-five minutes after agitation of the contents of each tube, the temperature being constant.
I have used 12 different volatile liquids as solvents, namely, water, phosphorous chloride, the sulfide of carbon, the bichloride of carbon (
), chloroform, amylene, benzene, methyl iodide, ethyl bromide, ordinary ether, acetone, methyl alcohol.
In water I have dissolved the following organic materials: cane sugar, glucose, tartaric acid, citric acid, urea. All these substances have produced sensibly the same molecular reduction in vapor pressure:
I have, for the present, left the mineral substances to one side; actually, the effect of these substances has been determined by enough conclusive experiments performed by Wüllner, by myself, and recently by M. Tammann.
In solvents other than water, I have dissolved materials as little volatile as possible, chosen among the following: oil of turpentine, naphthalene, anthracene, sesquichloride of carbon
methyl salicylate, ethyl benzoate, antimonous chloride, mercury ethyl, benzoic, valeric, and trichloroacetic acids, thymol, nitrobenzene, and aniline. The error due to the vapor pressure of these compounds can often be rendered negligible. The vapor pressure of the dissolved substances is, in fact, considerably reduced by their mixture with a great excess of the solvent; and in order that it should not exercise a sensible influence on the results, it is enough that it should not surpass 5 or 6 millimeters at the experimental temperature.
The molecular reductions in vapor pressure caused by these different bodies in the same solvent are constantly grouped around two values, of which one, which I call normal, is double the other. The normal reduction is always produced by simple and chlorinated hydrocarbons and by ethers; the anomalous reduction is almost always produced by acids. There are found, however, solvents in which all the dissolved bodies produce the same molecular reduction of pressure; such are, for example, ether and acetone.
Among the volatile solvents examined there are two, water and benzene, in which I have studied carefully the lowering of freezing point. The comparison of the results obtained shows that for all solutions made in the same solvent there is a nearly constant relation between the molecular lowering of freezing point and the molecular reduction of vapor pressure. In water this ratio is equal to 100; in benzene it is equal to 60, nearly 1/20.
If we divide the molecular reduction of vapor pressure K produced in a determined volatile liquid by the molecular weight M’ of the liquid, the quotient obtained K/M’ represents the relative reduction of pressure which will be produced by 1 molecule of substance held in 100 molecules of volatile solvent. In making this calculation for the normal values of K produced in the various solvents by the organic materials and nonsaline metallic compounds, I have obtained these results:
The values of K and M’ assigned in the table vary in the ratio from 1 to 9; in spite of this, the values of K/M’ vary very little and remain always near the mean 0.0105. We can then say,
1 molecule of nonsaline substance (held in the solvent) dissolved in 100 molecules of any volatile liquid decreases the vapor pressure of this liquid by a nearly constant fraction, nearly 0.0105.
This law is entirely analogous to that which I announced in 1882 relating to the lowering of the freezing point of solvents. The anomalies which it presents are explained for the most part by admitting that, in certain liquids, the dissolved molecules can be formed from two chemical molecules.
1 Compt. rend., 95: 1030–1033 (1882).
2 Compt. rend., 104: 1430–1433 (1887).
Contents:
Chicago: François-Marie Raoult, "General Law of the Vapor Pressure of Solvents.2," A Source Book in Chemistry, 1400-1900 in A Source Book in Chemistry, 1400-1900, ed. Henry M. Leicester and Herbert S. Klickstein (New York: McGraw-Hill Book Company, Inc., 1952), 473–475. Original Sources, accessed May 11, 2025, http://www.originalsources.com/Document.aspx?DocID=CPS767B3E54D86P.
MLA: Raoult, François-Marie. "General Law of the Vapor Pressure of Solvents.2." A Source Book in Chemistry, 1400-1900, Vol. 104, in A Source Book in Chemistry, 1400-1900, edited by Henry M. Leicester and Herbert S. Klickstein, New York, McGraw-Hill Book Company, Inc., 1952, pp. 473–475. Original Sources. 11 May. 2025. http://www.originalsources.com/Document.aspx?DocID=CPS767B3E54D86P.
Harvard: Raoult, F, 'General Law of the Vapor Pressure of Solvents.2' in A Source Book in Chemistry, 1400-1900. cited in 1952, A Source Book in Chemistry, 1400-1900, ed. , McGraw-Hill Book Company, Inc., New York, pp.473–475. Original Sources, retrieved 11 May 2025, from http://www.originalsources.com/Document.aspx?DocID=CPS767B3E54D86P.
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