Concentrated Solutions

CONCENTRATED SOLUTIONS The Ideal Solution.—In solutions of two liquids, which are miscible in all proportions, and may both be volatile, each con tributes to the total vapour pressure of the solution. If the rela tive proportions of the two substances in the vapour are known, the proportion of the total vapour pressure due to each com ponent can be found. In this way the total vapour pressure is divided into the partial vapour pressures of the two components. Now in dilute solutions of a non-volatile solute, according to Raoult's law, the vapour pressure of the solvent is proportional to its molar fraction. A solution in which Raoult's law holds for all the components, over the whole range of compositions, is known as an ideal or perfect solution. Thus an ideal solution may be defined as one in which the partial vapour pressure of each component is proportional to its molar fraction through the whole range of composition, i.e., Raoult's relation, holds for the partial vapour pressure of each component.

The behaviour of solutions may be represented by a graph in which the partial vapour pressures of the components are plotted against the corresponding molar fractions (figs. 2 and 3). Points along the base line represent the molar fractions of the two sub stances, the extremities represent ing the two pure liquids, and the vertical distances from the base line the corresponding partial pressures. In ideal solutions the partial pressures, being propor tional to the corresponding molar fractions, are represented by straight lines, extending from the point which represents thevapour pressure of the pure liquid to zero pressure at the other extremity of the base line. Deviations from the ideal relation are indicated by deviations of the partial vapour pressures from this straight line. The partial vapour pres sures of many binary solutions were determined by Jan von Zawidski in 1900. Two substances which are closely related in constitution and properties usually form solutions which are nearly ideal. This is the case with ethylene bromide and propylene bromide (fig. 2). Substances not closely related may deviate.

Fig. 3 shows some typical examples.

Deviations from Raoult's Law.

The deviations from Ra oult's law which are exhibited in many solutions have been the subject of much experiment and speculation. On the one hand an explanation was sought in physical effects of the same nature as those which cause deviations from the law of perfect gases (J. D. van der Waals, 1890; J. J. van Laar, 1910). On the other hand a group of workers, led by Friedrich Dolezalek (1906), attempted to show that all deviations were capable of explanation by purely chemical effects, such as the formation of compounds between the components of the solution and "association" or the combi nation of two or more molecules of one component. While it has become certain that purely physical effects are capable of pro ducing deviations, it cannot be said that chemical effects never occur.

In a solution which obeys Raoult's law, the properties of a substance are the same as in its pure liquid, except in so far as they are modified by a reduction in the number of molecules pres ent. Thus in an ideal solution containing so% of molecules of a kind A, its vapour pressure is half that of pure liquid A at the same temperature. The change in vapour pressure is accounted for entirely by the change in the number of molecules present in the liquid. Thus each molecule in the solution makes the same contribution to the vapour pressure as in the pure liquid. Now the molecules of a liquid are in a condition of thermal agitation, vibrating about their mean positions, and are held together by attractive or cohesive forces acting between them. Molecules which escape into the vapour are those which acquire sufficient energy to overcome the attractive forces exerted on them in the liquid. The greater these attractive forces, the less is the chance of a molecule getting loose, and the smaller is the vapour pres sure. It is evident that in an ideal solution in which the vapour pressure of each component depends only on the number of mole cules present, the forces exerted on each molecule must be the same as in the pure liquid.