Phase changes diagram

PHARMSEARCH Phase behavior Phase change recording Phase contrast Phase diagram Phase diagrams... 

The phase diagram for a binary alloy (Fig. A1.13) shows single-phase fields (e.g. liquid) and two-phase fields (e.g. liquid plus A). The fields are separated by phase boundaries. When a phase boundary is crossed, a phase change starts, or finishes, or both. 

From 160°C to room temperature. The lead-rich phase becomes unstable when the phase boundary at 160°C is crossed. It breaks down into two solid phases, with compositions given by the ends of the tie line through point 4. On further cooling the composition of the two solid phases changes as shown by the arrows each dissolves less of the other. A phase reaction takes place. The proportion of each phase is given by the lever rule. The compositions of each are read directly from the diagram (the ends of the tie lines). 

Because a phase change is usually accompanied by a change in volume the two-phase systems of a pure substaiice appear on a P- V (or a T- V) diagram as regions with distinct boundaries. On a P- V plot, the triple point appears as a horizontal line, and the critical point becomes a point of inflection of the critical isotherm, T = T (see Figure 2-78 and Figure 2-80). 

Use the phase diagram in Fig. 8.6 to describe the physical states and phase changes of water as the pressure on it is increased from 5 Torr to 800 Torr at 70°C. 

Self-Test 8.5A The phase diagram for carbon dioxide is shown in Fig. 8.7. Describe the physical states and phase changes of carbon dioxide as it is heated at 2 atm from — 155°C to 25°C. 

The general form of a phase diagram. Any point on the diagram corresponds to a specific temperature and pressure. Lines trace conditions under which phase changes occur, and the blue arrows show six types of phase transitions. 

Phase diagrams are constructed by measuring the temperatures and pressures at which phase changes occur. Approximate phase diagrams such as those shown in Figures 11-39 and 11-40 can be constmcted from the triple point, normal melting point, and normal boiling point of a substance. Example illustrates this procedure. 

Phase diagrams can be used to determine what phase of a substance is stable at any particular pressure and temperature. They also summarize how phase changes occur as either condition is varied. Example provides an illustration. 

The phase diagram for NHy shows the boundaiy lines for the liquid domain. These boundary lines can be used to determine the conditions under which phase changes occur. 

As diagrammed in Figure 14-5. ice melts spontaneously if a mixture of water and ice is placed on a table at 25 °C H2 O (.S 0 ° Table top, 25 C 0 ° C) Recall from Section 11- that a phase change from solid to... 

Either a negative deviation or a positive deviation is regularly observed. In any phase diagram, composition is plotted against temperature. In this way, we can see how the interactions between phases change as the temperature changes and the behavior as each solid phase then melts. Either two-phase or three phase systems can be illustrated. This is shown in the following ... 

Vary the value of A from -1.0 to +1.0 and examine the changing stability of the system as shown on a phase-plane diagram. 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

Enthalpy-concentration diagrams greatly facilitate the calculation of energy balances involving concentration and phase changes this is illustrated in Example 3.6. 

KEY TERMS allotrope phase change phase diagram... 

When a reversible transition from one monolayer phase to another can be observed in the 11/A isotherm (usually evidenced by a sharp discontinuity or plateau in the phase diagram), a two-dimensional version of the Gibbs phase rule (Gibbs, 1948) may be applied. The transition pressure for a phase change in one or both of the film components can be monitored as a function of film composition, with an ideally miscible system following the relation (12). A completely immiscible system will not follow this ideal law, but will... 

The implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

Fig. 9 Phase solubility diagram showing the changes in the apparent aqueous solubility of p-aminobenzoic acid (PABA) brought about by the addition of the complexing agent, caffeine, at 30°C. (The data are adapted from Ref. 51.)...

Figure 5.1 Schematic phase diagram showing pressures and temperatures at which two phases are at equilibrium. Phase boundary (a) represents the equilibrium between steam and ice boundary (b) represents equilibrium between water and ice and boundary (c) represents equilibrium between water and steam. The point D represents p and I on a warm, sunny day. Inset warming an ice cube from — 5 °C to the mouth at 37 °C at constant pressure causes the stable phase to convert from solid to liquid. The phase change occurs at 0 °C at...

It will be useful to concentrate on the diagram in Figure 5.2 when considering why a phase change occurs spontaneously. We recall from Chapter 4 that one of the simplest tests of whether a thermodynamic event can occur is to ascertain whether the value of AG is negative (in which case the change is indeed spontaneous) or positive (when the change is not spontaneous). 

We see this phenomenon in a different way when we look back at the phase diagram in Figure 5.6. The stable phase is liquid before applying a vacuum. After turning on the water pump, to decrease the applied pressure, the s.v.p. exceeds P(appiied)> and the solvent boils at a lower pressure. The bold arrow again indicates how a phase change occurs during a depression of the external pressure. 

Make suitable changes in the initial conditions of X and S, and plot the phase plane diagram X versus S. By making many runs at a range of initial conditions, the washout region can be identified. 

Types of intermolecular forces Properties of liquids Surface tension Viscosity Capillary action Structures of solids Phase changes and diagrams... 

In looking at phase diagrams, be careful when moving from point to point to pay attention to any phase changes which might occur. 

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