7.phase eq

7.phase eq - T Y Tan 7 PHASE EQUILIBRIA AND PHASE TRANSFORMATION An essentially chemically pure and homogeneous substance which may be either

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T. Y. Tan 1 7. PHASE EQUILIBRIA AND PHASE TRANSFORMATION An essentially chemically pure and homogeneous substance, which may be either elemental or compound in nature, constitutes a phase if it exists in one of the three defined states of gas, liquid, or solid. In the case of solids, a phase possesses a definite structure . Adopting the struc- ture criterion for defining a solid phase, a substance may have many different phases in the solid state, e.g., Fe. Some solid materials may not possess a long range ordered structure, and they are called the amorphous phase. Insofar as there is no change incurred to the state and/or structure of the phase, the existence of a certain amount of impurities dissolved in the matrix material as at- oms, and a deviation from a strict compositional stoichiometry in the case of compounds, are permitted for a phase. By casual observation, a phase always has a definite temperature range over which it is sta- ble, i.e., it will not change into other phases. A familiar example is that, for P=1 atm, pure H 2 O is a solid phase material for T 0 o C (ice), a liquid phase material for 0 o C T 100 o C (water), and a gas phase material for T 100 o C (water vapor or steam). Ice and water coexist at 0 o C, and water and steam coexist at 100 o C. Thus, when T is changed, phase changes will occur. Since this H 2 O experiment (so are almost all other laboratory experiments) is carried out under the constant pressure condition at 1 atm, from Chapter 3 we see that the proper energetic quantity under con- stant temperature and pressure conditions is Gibbs free energy. Thus, we expect that the exis- tence of a temperature range for each of the three stable H 2 O phases is due to the simple reason that the Gibbs free energy of the particular phase in the appropriate temperature range is lower than that of the other two phases. More careful observations would show, however, that there is a {P,V,T, n } range within which the stable phase has a dominating stability, and when the {P,V,T, n } condition changes, another phase of the material will become dominant. The word dominating means that this stable phase does not preclude the existence of other phases to a minute amount, but instead coexists with them. Consider again the example of pure H 2 O. More careful observations have shown that the vapor phase H 2 O always coexists with either water or ice below 100 o C. On the other hand, in steady state water and ice do not coexist over a temperature range larger than 0 o C, and neither will coexist with water vapor above 100 o C. Thus, the situation is more complicated than can be explained by the simple criterion that the Gibbs free energy of the phase is the lowest among all possible phases. Considering the situation more carefully, we realize that it is the Gibbs free en-
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T. Y. Tan 2 ergy of the total system, which may consist of all three different phases, that is minimized. This means, the fact that the Gibbs free energy of one of the three phases is the lowest predicts only
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This note was uploaded on 07/13/2011 for the course ME 218 taught by Professor Dr.tan during the Fall '11 term at Duke.

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7.phase eq - T Y Tan 7 PHASE EQUILIBRIA AND PHASE TRANSFORMATION An essentially chemically pure and homogeneous substance which may be either

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