ABSTRACT
This chapter examines experimental data on all aspects of the phase diagram: the slopes of the phase boundaries and their points of intersection. The temperature of the transition from the solid to the liquid phase is dependent on the pressure to which the liquid/solid is subjected. In general, higher pressures tend to increase the temperature at which solid-liquid transitions take place. Since the liquid and gas phases of a substance coexist at all temperatures at which the liquid is stable, a sensible choice is made as to the pressure of vapour which defines the transition to the gaseous phase. The enthalpy of vaporisation for arsenic is anomalously low in comparison to its enthalpy of fusion. This indicates that evaporation is relatively easy in comparison with melting. Many organic substances may exist in a form intermediate between the liquid and solid states known as a mesophase or liquid crystal state.
