Vapour pressure of liquid

If the total pressure of the vapour at constant temperature is represented as a function of the compositions of the two phases, the p-liquid and p-vapour curves are obtained. The p-liquid curves—that is, the curves representing the total vapour pressures of liquid binary mixtures as functions of the composition of the liquid phase—are most important they are usually referred to simply as the vapour-pressure curves of the mixture. Each curve is an isotherm. 

Fig. 2.6. Vapour pressure of liquid 3He and liquid 4He. ( ) indicate the lowest temperature that can be practically obtained reducing the vapour pressure on these liquids.

Temperature and pressure are the two variables that affect phase equilibria in a one-component system. The phase diagram in Figure 15.1 shows the equilibria between the solid, liquid, and vapour states of water where all three phases are in equilibrium at the triple point, 0.06 N/m2 and 273.3 K. The sublimation curve indicates the vapour pressure of ice, the vaporisation curve the vapour pressure of liquid water, and the fusion curve the effect of pressure on the melting point of ice. The fusion curve for ice is unusual in that, in most one component systems, increased pressure increases the melting point, whilst the opposite occurs here. 

The vapour pressure of liquid Hg at 433 K is 4.19 mm Hg. The free energy change accompanying the expansion of one... 

The existence of two polymorphs Was questioned by Weber,3 who maintained that the j8-form was a hydrate of sulphur trioxide, and this also was the conclusion arrived at by Berthoud,4 after determinations of the melting-points of the two forms, and of the vapour pressures of liquid sulphur trioxide. The latter investigator, however, described it as a hydrate unique in its remarkably small water content—estimated at less than 1 molecule per million molecules of trioxide. 

Few liquid mixtures are actually ideal over their entire composition range. Figure 33.2 illustrates two cases where the vapour pressure of liquid mixtures (solutions) deviate from Raoult s Law (Frame 32 and this frame, equations (33.3) and (33.4)) (positively A/B or negatively C/D) over the composition range but shows (see caption to figure) the end composition members (both representing cases of dilute solutions) do follow Raoult s Law for a limited, small, composition range. 

As the area is diminished below some thousands of sq. A., where the molecules cover only a small fraction of the surface, the surface pressure rapidly becomes much smaller than that of a perfect gas, and in the four acids with the longest chains becomes constant over a considerable region. The curves are indeed a very faithful reproduction of Andrews s curves for the relation between pressure and volume, for carbon dioxide, at temperatures near the critical. The horizontal regions in the curves correspond to the vapour pressure of liquids, and indicate the presence of an equilibrium between two surface phases, the vapour film, and islands of liquid, coherent film. 

Vapour Pressures.—The vapour pressures of liquid phosphorus are tabulated overpage in three sections, namely—... 

THE VAPOUR PRESSURES OF LIQUID PHOSPHORUS. (a) Liquid White Phosphorus. 

Thus at 279° C. the vapour pressure of liquid phosphorus is 753 mm., while at 289° C. that of red phosphorus is 23 mm.s The pressures of red phosphorus are not in equilibrium, but fall slowly as the solid changes into the more stable pyromorphic (violet) form. 

Tellurium—Kelley 59 quotes some data on the vapour pressure of liquid tellurium, but states that the composition of the vapour is uncertain. It has since been shown by Niwa i that tellurium vapour at a few hundred degrees is composed of diatomic molecules. The N.B.S. 529 derive from this Ai (Tc2 g) =41 kcal,... 

A differential manometer of the Rayleigh type ( 14.VII A) was used in accurate measurements of the vapour pressure of water, and a simple apparatus for measuring the difference in vapour pressures of two liquids was described by Lord Kelvin. Stock s vapour pressure thermometer ( 19.V1 Q can be used to measure the vapour pressures of liquids, including those at low temperatures. Special techniques are necessary for very small vapour pressures,... 

The expression shows that the vapour pressures of liquid phases act independently of each other within the miscibility gap. This relationship is the basis for steam distillation. Here components with high boiling points can be distilled close to the boiling point of water with a vapour composition according to their vapour pressure ... 

The vapour pressure of liquid COF j from the triple point to the normal boiling temperature is given by Equation (13.12) [1580] the experimental data from which this... 

Fig. 13.3 The vapour pressure of liquid COFj as a function of temperature, according...

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