Illustrative Phase Diagrams for Pure Substances

In accordance with the Clapeyron equation and Le Chatelier s principle, the more highly ordered (low-entropy) phases tend to lie further to the left (at lower 7), whereas the higher-density phases tend to lie further upward (at higher 7). The mnemonic (7.32) allows us to anticipate the relative densities of adjacent phases. From the slope, for example, of the ice II-ice III coexistence line (which tilts forward to cover ice III), we can expect that ice II is denser than ice III (pn pm). Similarly, from the forward slopes of the liquid coexistence lines with the high-pressure ices II, V, and VI, we can expect that cubes of ice II, ice V, and ice VI would all sink in a glass of water, whereas ice I floats (in accord with the backward tilt of its phase boundary). Many such inferences can be drawn from the slopes of the various phase boundaries in Fig. 7.3, all consistent with the measured phase densities Pphase (in gL 1), namely, 

Six new triple points are seen to appear in Fig. 7.3, and still others are found at higher P and lower 7. However, no quadruple points (or coexistence features of higher order) are found, as predicted by the Gibbs phase rule. 

Surprising behavior of liquid and ice phases is found if we follow various 7, P paths in this extended phase diagram. Sidebar 7.4 illustrates how to determine the expected phase transitions and properties of H20 for various temperatures and pressures far outside the realm of ordinary experience. It is remarkable that such multiplicity of forms and properties can result from a pure substance composed of only a single type of molecule. 

Problem At what temperature will water freeze to ice if the pressure is (a) 2 kbar, (b) 10 kbar What phase of ice is formed in each case  

Problem An ice cube, initially of ordinary ice I, is subjected to increasing pressures at various fixed temperatures  

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