تسجيل الدخول

تسجيل

Phase boundaries

المؤلف:  Peter Atkins، Julio de Paula

المصدر:  ATKINS PHYSICAL CHEMISTRY

الجزء والصفحة:  118

2025-11-10

476

Phase boundaries

The phase diagram of a substance shows the regions of pressure and temperature at which its various phases are thermodynamically stable (Fig. 4.1). The lines separating the regions, which are called phase boundaries, show the values of p and T at which two phases coexist in equilibrium. Consider a liquid sample of a pure substance in a closed vessel. The pressure of a vapour in equilibrium with the liquid is called the vapour pressure of the substance (Fig. 4.2). Therefore, the liquid–vapour phase boundary in a phase diagram shows how the vapour pressure of the liquid varies with temperature. Similarly, the solid vapour phase boundary shows the temperature variation of the sublimation vapour pressure, the vapour pressure of the solid phase. The vapour pressure of a substance increases with temperature because at higher temperatures more molecules have sufficient energy to escape from their neighbours.

Fig. 4.1 The general regions of pressure and temperature where solid, liquid, or gas is stable (that is, has minimum molar Gibbs energy) are shown on this phase diagram. For example, the solid phase is the most stable phase at low temperatures and high pressures. In the following paragraphs we locate the precise boundaries between the regions.

Fig. 4.2 The vapour pressure of a liquid or solid is the pressure exerted by the vapour in equilibrium with the condensed phase.