Pressure of a Pure Liquid

Liquid-Vapor Coexistence Curve and the Critical Point 

When a pure liquid is placed in an evacuated bulb, molecules will leave the liquid phase and enter the gas phase until the pressure of the vapor in the bulb reaches a definite value, which is determined by the nature of the liquid and its temperature. This pressure is called the vapor pressure of the liquid at a given temperature. The equilibrium vapor pressure is independent of the quantity of liquid and vapor present, as long as both phases exist in equilibrium with each other at the specified temperature. As the temperature is increased, the vapor pressure also increases up to the critical point, at which the two-phase system becomes a homogeneous, one-phase fluid. 

If the pressure above the liquid is maintained at a fixed value (say by having the bulb containing the liquid open to the atmosphere), then the liquid may be heated up to a temperature at which the vapor pressure is equal to the external pressure. At this point vaporization will occur by the formation of bubbles in the interior of the liquid as well as at the surface this is the boiling point of the liquid at the specified external pressure. Clearly the temperature of the boiling point is a function of the external pressure in fact, about a given T, p point, the variation of the boiling point with external pressure is the inverse of the variation of the vapor pressure with temperature. 

In this experiment the variation of vapor pressure with temperature will be measured and used to determine the molar heat of vaporization. 

We are concerned here with the eqnilibrinm between a pnre liquid and its vapor  

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The vapour-pressure of a pure liquid depends on, and increases with, the temperature. 

Why is the vapor pressure of a solution less than the vapor pressure of a pure liquid How does this impact boiling point What is the quantitative relationship between solute concentration and vapor pressure ... 

Increasing temperature will decrease the solubility of a gas in a liquid because kinetic energy opposes intermolecular attractions and permits more molecules to escape from the liquid phase. The vapor pressure of a pure liquid increases with temperature for the same reason. Greater kinetic energy will favor material in the gas phase. 

The lowering of the vapour pressure of a pure liquid A by the addition of B (as above) is one example of a colligative property (defined in Frame 51) where such properties are further discussed and include ... 

Other Thermometric Devices. The vapor pressure of a pure liquid or solid is a physical property sensitive to temperature and thus suitable for use as a thermometer. The use of a liquid-nitrogen vapor-pressure thermometer is suggested for the range 64 to 78 K in Exp. 47. At very low temperatures (1 to 4.2 K), the vapor pressure of liquid helium can be used. 

The vapor pressure of a pure liquid is determined solely by the temperature. (Section 12.6 showed that the vapor pressure of water is determined by its temperature alone.) If the surface area for evaporation is larger, so also is the surface area for condensation. If there is more volume for the gas to occupy, more molecules will be able to evaporate before the rate of condensation equals the rate of evaporation, but it will take that many more molecules to build up to the same vapor pressure. The volume and shape occupied by the liquid also have no effect on the vapor pressure of a pure liquid. 

The partial pressure of a pure liquid at the svuface varies as a function of the surface temperature, Tg, according to the Clausius Clapeyron equation [16] ... 

Let and 2 be values of the saturation vapor pressure of a pure liquid at absolute... 

We still have a formula that relates mole fraction to pressure. Note, however, that with the exception of the atmospheric pressure (P t n) the other pressures are those of pure liquids ( and Pg). Now, how does the vapor pressure of a pure liquid vary with temperature Smite your forehead and say that you could have had a V-8. The Clausius-Clapeyron equation ... 

An indication of whether or not the above condition for ideality is met is obtained from the vapor pressure of the solution. At a given temperature, the vapor pressure of a pure liquid is a measure of the ability of molecules to escape from the liquid to the gas phase. By studying the vapor pressure of a solution as a function of its composition at constant temperature one may assess the solution s ideality or its degree of departure from ideality. For an ideal solution, the tendency of molecule A to escape is proportional to its mole fraction, that is, to its concentration expressed in terms of the fraction of molecules which are of type A. The proportionality constant must be the vapor pressure of pure component A because this vapor pressure is reached when the mole fraction is unity. This result is Raoult s law, which is expressed mathematically as... 

Vapour pressure of a pure liquid is 100 torr at 25°C while its vapour pressure from 95 mole per cent solution at the same temperature is only 90 torr. The activity of the liquid in the solution. The activity coefficient for it... 

The dependence of vapor pressure of a pure liquid on the liquid s temperature is ... 

To understand the principles of distillation, a review of the effect of impurities on the vapor pressure of a pure liquid is necessary. The discussion starts with consideration of the consequences of having nonvolatile impurities present and then turns to the more common case of contamination of the liquid with other volatile substances. 

Equation 8.4.15 shows that the curve of a plot of n(p/p°) versus /T (where p is the vapor pressure of a pure liquid or solid) has a slope at each temperature equal, usually to a high degree of accuracy, to — Ayap/f/i or — at that temperature. This kind of... 

Psat Saturation vapor pressure (of a pure liquid) Pa... 

We have already addressed vapor pressure depression, in the form of Raoult s law. The vapor pressure of a pure liquid is lowered when a solute is added, and the vapor pressure is proportional to the mole fraction of the solvent ... 

In Section 8.4.2 we recognized the vapor pressure of a pure liquid - at a specified temperature - as the pressure read by a manometer when this liquid is in equilibriiun with the vapor above it. Thus, if in the vapor-liquid equilibrium experiment of Section 13.5.1 we introduced a pure liquid, the pressure read by the manometer represents the vapor pressure of this liquid at the bath temperature. 

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