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

Phase diagrams for carbon dioxide and sulfur (not to scale)

Top: Carbon dioxide. At normal atmospheric pressure (1 atm), the solid sublimes when warmed. Bottom: Sulfur. This substance has a complicated phase diagram with three triple points.

11 States of Matter; Liquids and Solids

FIGURE 11.13

Observing the critical phenomenon

1. 
   Carbon dioxide liquid in equilibrium with its vapor at 20°C. (B) The same two states in equilibrium at 30°C ( just below the critical point); the densities of the liquid and vapor are becoming equal. (C) Carbon dioxide at 31°C (the critical temperature); the liquid and vapor now have the same densities—in fact, the distinction between liquid and vapor has disappeared, resulting in what is called a supercritical fluid.
   

31°C. The vapor pressure at the critical temperature is called the critical pressure

(73 atm for carbon dioxide). It is the minimum pressure that must be applied to a gas at the critical temperature to liquefy it. On a phase diagram, the preceding experiment corresponds to following the vaporpressure curve where the liquid and vapor are in equilibrium. Note that this curve in Figure 11.12 top ends at a point at which the temperature and pressure have their critical values. This is the critical point. If you look at the phase diagram for water, you will see that the vapor-pressure curve for the liquid similarly ends, at point C, which is the critical point for water. In this case, the critical temperature is 374°C and the critical pressure is 218 atm. Many important gases cannot be liquefied at room temperature. Nitrogen, for example, has a critical temperature of -147°C. This means the gas cannot be liquefied until the temperature is below -147°C.

Example 11.4 Relating the Conditions for the Liquefaction of a Gas to Its Critical Temperature

The critical temperatures of ammonia and nitrogen are 132°C and -147°C, respectively. Explain why ammonia can be liquefied at room temperature by merely compressing the gas to a high enough pressure, whereas the compression of nitrogen requires a low temperature as well.