Water vaporisation curve

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. 

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... 

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... 

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. 

Figure 3.25 TGA-DTG-MS curves of the thermal decomposition of calcium oxalate monohydrate measured at 30 K/min in a 70-p.L alumina pan. Purge gas argon, 50 mL/min. The diagram shows that calcium oxalate monohydrate decomposes in three distinct steps. The MS fragment ion curves for water (rrVz 18), CO (rrVz 28) and CO2 Mz44) display peaks that correspond closely to the individual steps in the TGA curve. The first mass loss step relates to the elimination and vaporisation of water of crystallisation (1) the second step to the decomposition of anhydrous calcium oxalate with formation of CO (2) and the third step to the decomposition of calcium carbonate to calcium oxide and CO2 (3). The m/z44 ion curve shows that CO2 is also formed in the second step at 550 C (besides CO). This is a result of the disproportion reaction of CO to CO2 and carbon.

Figure 3 The vapour pressure as a reciprocal function of temperature for water. The slopes of the tangents to the curve are the enthalpies of vaporisation.

Figure 4 shows a plot of the enthalpy of vaporisation versus the temperature (C). The arrow denotes the value of the enthalpy of vaporisation at 100 C (-9.73 kcal/mole). This is an interesting curve considering that the value for the enthalpy of vaporisation decreases at an accelerated rate until it reaches a value of 0.00 kcal/mole at 374°C. This happens to be the critical temperature of water and can help to explain the nature of the critical temperature. Recall that the enthalpy of vaporisation is the energy (or heat) required to convert one mole of liquid from the liquid state to the vapour state. If one finds that no energy is required to transfer between the physical states, it impHes that only one phase now exists. Figures 2, 3 and 4 represent experimental values for water however, the same behaviour with different values of pressure, temperature, and enthalpy exists for all piue compounds. 

In the reference case, the gas has no influence on liquid kinetics and saturation time. But we have to check this result, if the gas can escape from the chamber. In Pollock (1986), the heating of a partially saturated alluvium induces a heat-pipe effect. Liquid water vaporises near the heating source. The vapour is carried away in colder areas by means of gas darcean flux. There, vapour is liquefied. The reference case does not show any significant vaporization near the heating source, still the model we use and Pollock s one are quite similar. The non-existence of this phenomenon can not be imputed to the model. Two other explanations are then possible. It can be due to clay and alluvium behaviour differences, or to a bad evaluation of hydraulics parameters. Hence, tests have been performed to see if gas conductivity or retention curve parameters uncertainties can activate a heat-pipe phenomenon. 

Coming again to the diagram, we know that the vaporisation (change of liquid to vapour) is accompanied by absorption of heat. Hence, if heat is given to the system (as along OA), then according to Le Chatelier s principle, the equilibrium will shift in that direction in which heat is absorbed, i.e., more liquid will pass into vapour. Thereby, the pressure of the system wiU increase. The curve OA extends to the point A, which is critical temperature (374°C) of liquid water, beyond which only one phase, i.e., vapom will exist. 

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