Vaporisation curve

Figure 3.7) [241], Some consider the SCF state to be more extended and comprising the area of the phase diagram above Tc independent of p0 [242], Critical temperature and pressure are usually defined as the maximum temperature at which a gas can be converted to a liquid by an increase in pressure, and the maximum pressure at which a liquid can be converted to a gas by an increase in temperature, respectively. In a PT diagram the vaporisation curve ends at the critical point. At a temperature above the critical point, the vapour and liquid have the same density. The critical parameters for some common fluids in analytical studies are listed in Table 3.11, but others may be found elsewhere [243], in particular, rc = 31.3 °C and pc = 7.38MPa for the most common SCF (C02). Supercritical C02 (scC02) is widely used because of its convenient critical parameters, low cost, and safety aspects (low toxicity, nonexplosive). 

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. 

A sublimation process is controlled primarily by the conditions under which phase equilibria occur in a single-component system, and the phase diagram of a simple one-component system is shown in Figure 15.30 where the sublimation curve is dependent on the vapour pressure of the solid, the vaporisation curve on the vapour pressure of the liquid, and the fusion curve on the effect of pressure on the melting point. The slopes of these three curves can be expressed quantitatively by the Clapeyron equation ... 

Combinations of P and r falling on the boundary O-C allows coexistence of liquid and vapour phases and the boundary is called vaporisation curve. 

The vaporisation curve (O-C) does not extend indefinitely, but ends abruptly at point C which is called the critical point. Let s examine the physical significance of the critical point. 

Metastable Equilibria The change of SR to SM occurs very slowly. If enough time for the change is not allowed and SR is heated rapidly, it is possible to pass well above the transition point (B) without obtaining SM. In that case, the curve AB extends to O. The curve AO is known as metastable vaporisation curve of SR. The phases SR and Sv will be in metastable equilibrium along this curve. It is a univariant system. 

On super-cooling along DC, the curve CO is obtained. It is, infact, the back prolongation of DC. The curve CO, known as vaporisation curve of supercooled SL, represents the metastable equilibrium between supercooled SL and Sv. It is also univariant. 

Equilibrium between Liquid and Vapour. Vaporisation Curve. 

Upper Limit of Vaporisation Curve.—On continuing to add heat to a liquid contained in a closed vessel, the pressure of the vapour will continuously increase. Since with increase of pressure the density of the vapour must increase, and since with rise of temperature the density of the liquid must decrease, a point will be reached at which the densities of liquid and vapour become identical the system cea es -to49nJieterogeneous, and-passes into one homogeneous phase. The temperature at which this occurs is called the critical temperature. To this temperature there will, of course, correspond a certain definite pressure, called the critical pressure. The curve representing the equilibrium between liquid and vapour must, therefore, end abruptly at the critical point- At temperatures above this point no pressure, however great, can cause the formation of the liquid phase at temperatures above the critical point the vapour becomes a gas. In the case of water, the critical temperature is 374 , and the critical pressure 217-5 atm. at the point representing these conditions the vapour-pressure curve of water must end. The lower determined by the range of tiie m gfa f-A... 

Although, as we have seen, the vaporisation curve ends at the critical point, the fusion curve may be followed continuously up to temperatures much above the critical temperature for liquid—vapour. Thus the fusion curve of phosphonium chloride (critical point, 49°-50°)... 

Although these z ules admit of a considerable variety of possible arrangements of curves around the triple point, only two of these have been experimentally obtained in the case of the triple point solid— liquid— vapour. At present, therefore, we shall consider only these two cases. In Figs. 3 and 4 the curve AO is the sublimation curve, OB the vaporisation curve, and OC the fusion curve. 

It has been found possible to follow the vaporisation curve of (supercooled) liquid phosphorus downwards to more than 80° below the triple point. 

The relations which are found here will be best understood with the help of Fig. 72 In this figure, OB represents the sublimation curve of ice, and BC the vaporisation curve of water the curve for the solution must lie below this, and must cut the sublimation curve of ice at some temperature below the melting-point. The point of intersection A is the cryohydric point. If the solubility increases with rise of temperature, the increase of the vapour pressure due to the latter will be partially annulled. Since at first the effect of increase of temperature more than counteracts the depressing action of increase of concentration, the vapour pressure will increase on raising the temperature above the cryohydric point. If the elevation of temperature is continued, however, to the melting-point of the salt, the effect of increasing concentration makes itself more and more felt, so that the vapour-pressure curve of the solution falls more and more below that of the pure liquid, and the pressure will ultimately become equal to that of the pure salt that is to say, practically equal to zero. The curve will therefore be of the general form AMF shown in Fig. 72. If the solubility should diminish with rise of temperature, the two factors, temperature and concentration, will act in the same direction, and the vapour-pressure curve will rise relatively more rapidly than that of the pure liquid since, however, the pure salt is ultimately obtained, the vapour-pressure curve must in this case also finally approach the value zero.2... 

Corresponding to the point Q/the melting-point of pure iodine, there is the point C, which represents the vapour pressure of iodine at its melting-point. At this point three curves cut i, the sublimation curve of iodine 2, the vaporisation curve of fused iodine 3, CiB, the vapour-pressure curve of the saturated solutions in equilibrium with solid iodine. Starting, therefore, with the system solid iodine— liquid iodine, addition of chlorine will cause the temperature of equilibrium to fall continuously, while the vapour pressure will first increase, pass through a maximum and then fall continuously until the eutectic point, B (Bjl), is reached. At this point the system is invariant, and the pressure will therefore remain constant until all the iodine has disappeared. As the concentration of the chlorine increases in the manner represented by the curve B/H, the pressure of the vapour also increases as represented by the curve Bj/iHi. At the eutectic point for iodine monochloride and iodine trichloride, the pressure again remains constant until all the monochloridc has disappeared. As the concentration of the solution passes along the curve HF, the pressure... 

Univariant Systems.—Equilibrium between liquid and vapour. Vaporisation curve. Upper limit of vaporisation curve. Theorems of van t Hoff and of Le Chatelier. The Clausius-Clapeyron equation. Presence of complex molecules. Equilibrium between solid and vapour. Sublimation curve. Equilibrium between solid and liquid. Curve of fusion. Equilibrium between solid, liquid, and vapour. The triple point. Complexity of the solid state. Theory of allotropy. Bivariant systems. Changes at the triple point. Polymorphism. Triple point Sj—Sg— V. Transition point. Transition curve. Enantiotropy and monotropy. Enantiotropy combined with monotropy. Suspended transformation. Metastable equilibria. Pressure-temperature relations between stable and metastable forms. Velocity of transformation of metastable systems. Metastability in metals produced by mechanical stress. Law of successive reactions. 

The curve OA is known as vapour pressure curve or vaporisation curve of liquid water, because it gives the vapour pressure of liquid water at different temperatures. 

Curve CO Metastable vaporisation curve of Sl Sl Sy estable equili- ... 

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