Water vapour, phase diagram

It is unnecessary to go to the lengths of calculating the oxygen or sulphur potentials of gas phases in order to use these diagrams in certain simple cases. Consider the oxidation of a metal by a hydrogen/water-vapour atmosphere. The reaction involved here is... 

Fig. 12. Schematic representation of variations in dehydration rates (ft) with prevailing water vapour pressure (Ph2o) These examples include Smith—Topley behaviour and indicate correlations with phase stability diagrams. (After Bertrand et al. [596], reproduced with permission, from Journal of Inorganic and Nuclear Clemistry.)...

Remember that the phase diagram s y-axis is the applied pressure. At room temperature and pressure, liquid water evaporates as a consequence of entropy (e.g. see p. 134). For this reason, both liquid and vapour are apparent even at s.t.p. The pressure of the vapour is known as the saturated vapour pressure (s.v.p.), and can be quite high. 

We now look at the phase diagram of water in Figure 5.6, which will help us follow the modem method of removing the water from coffee to yield anhydrous granules. A low temperature is desirable to avoid charring the coffee. Water vapour can be removed from the coffee solution at any temperature, because liquids are always surrounded by their respective vapour. The pressure of the vapour is the saturated vapour pressure, s.v.p. The water is removed faster when the applied pressure decreases. Again, a higher temperature increases the rate at which the vapour is removed. The fastest possible rate occurs when the solution boils at a temperature we call T oii). 

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. 

It is convenient to use phase diagrams [46] to represent the thermodynamic properties that determine the stability and equilibrium composition of water-containing aerosols. The properties of interest are the temperature, the vapour pressure and composition of the various components in the condensed phases. This is particularly important with respect to the composition and stability of the various hydrates formed at low temperature in the nitric acid-water [47] and sulfuric acid-water binary systems [48], and the ternary systems HjSO/HNOj/HjO and HjSO/HCl/HjO [49],... 

The phase diagram for water, shown close to the triple point H, at which ice is in equilibrium with liquid water and water vapour (steam) is sketched in Figure 24.4. 

Systems consisting of two different hydrates and vapour have a definite vapour pressure at every temperature. The vapour pressure of the solution, which is a measure of the tendency of the hydrate to give up water, is not completely defined unless the compound (containing less water) which results on the dissociation of the hydrate is also specified. The corresponding pressure temperature diagrams, i.e. the vapour pressure curves, also intersect at quadruple points, namely, those at which three sohd phases are in equihbrium with water vapour, e.g. ice, hexa-hydrate, and tetrahydrate /5, or hexahydrate, tetrahydrate /3 and dihydrate. 

The information from such diagrams, if available at a series of pressures, may be represented in phase diagrams such as Fig. 4.3. Line AB is the vapour pressure curve for water. AD is the melting curve. AC gives the vapour pressure of ice. Point A, at which all three lines meet, is called the triple point (for water it is at 273.16 K and 4.58 mmHg pressure, i.e. 0.61 kPa). 

The curve TK is not an infinite curve, it is terminated by the point K corresponding to the so-called critical state. The temperature and pressure corresponding to this state are called the critical temperature t and critical pressure p. In the critical states the densities of both states are equal and any differences between both phases — liquid and vapour — disappear, and they form one state only (the fluid zone ). At very high pressures the phase diagram of water becomes more complicated (Fig. 3.6). At such... 

In a one-component, or unary, system, only one chemical component is required to describe the phase relationships, for example, iron (Fe), water (H2O) or methane (CH4). There are many one-component systems, including all of the pure elements and compounds. The phases that can exist in a one-component system are limited to vapour, liquid and solid. Phase diagrams for one-component systems are specified in terms of two variables, temperature, normally specified in degrees centigrade,... 

Fig. 1.1 A part of the phase diagram of water, showing the domains of the solid (icel, S), the liquid (L), and the vapour (V). The square denoted the triple point and the triangle the critical point. (From data in (Dorsey 1940))...

Fig. 1. Liquid-vapour phase boundary in the temperature-entropy diagram for a substance of low and high specific heat. Water cV=3.5, PPl (C6F14) Cv=39.3.

Extrapolation to low temperatures of the CO2 hydrate dissociation pressure curve measured above 152K by Miller and Smythe [12] suggested to those workers that it should intersect the vapour pressure curve of solid CO2 at about 121K. Below this temperature the hydrate should be unstable relative to solid CO2 and ice. The phase diagram of the carbon dioxide-water system is shown in Figure 1. 

Fig. 6 T-G (temperature-field) phase diagram for a system that exhibits first-order transitions (solid line) that are terminated at a single critical point of the water-vapour type. The supercritical evolution emanates further from the critical point along a Widom line (dashed line). The heat-capacity Cp and order-parameter S temperature profiles corresptmding to different G values are shown in file graph. The dotted lines represent the discrmtinuity in the Cp(T) and S(J) profiles, corresponding to the phase-coexistoice regirai in which the latent heat is released...

Santos, C., Pazo, A., and Guitian, F. (1995) Water vapour pressure influence on Ca0-P20s system phase diagram, in Materials in Clinical Applications (ed. P. Vincenzini), Techna, Faenza, Italy,... 

I dP <0. In instances such as the phase diagram of water, where the melting curve does not display an initial maximum, it is predicted that this occurs in the metastable tensile-strained regime where the equilibrium state is the vapour. 

A suitable method to measure the temperature during the drying process is to determine the saturation vapour pressure. If the flow of water vapour between the chamber and the ice condenser is interrupted for a short period of time, a saturation vapour pressure in accordance with the water vapour partial pressure diagram will settle in the chamber. The corresponding ice temperature can be taken from the phase diagram (Figure 14.2). This type of measurement is called a... 

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