Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

Critical Temperature and Pressure

Imagine an experiment in which liquid and gaseous carbon dioxide are sealed in a

thick-walled glass vessel at 20°C. At this temperature, the liquid is in equilibrium

with its vapor at a pressure of 57 atm. You observe that the liquid and vapor are separated by a well-defined boundary, or meniscus. Now suppose the temperature is raised. The vapor pressure increases, and at 30°C it is 71 atm. Then, as the temperature approaches 31°C, a curious thing happens. The meniscus becomes fuzzy and less well defined. At 31°C, the meniscus disappears altogether. Above this temperature and above the critical pressure, there is only one fluid state, called a supercritical fluid (see Figure 11.13). The temperature above which the liquid state of a substance no longer exists regardless of the pressure is called the critical temperature. For carbon dioxide it is

FIGURE 11.12