I. Dry ice is placed into an acrylic plastic cylinder. A valve is closed and pressure in the cylinder increases. When the pressure reaches 350 kPa (50 psi) it stops increasing and liquid CO2 appears. The liquid begins to boil and when all solid CO2 is gone, the pressure increases further. The valve is opened and the pressure drops again, holding constant for a while at 350 kPa. Eventually solid CO2 reforms, the liquid disappears, and the pressure drops completely.
II. The critical temperature for carbon dioxide is 304K (87.8°F [31°C]). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 304K (87.8°F [31°C]) will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 304K (87.8°F [31°C]) is 72.9 atmospheres (~73000 kPa). In other words, the application of a pressure of 72.9 atmospheres of pressure on a sample of carbon dioxide gas at 304K (87.8°F [31°C]) will cause the gas to liquefy.
III. Two important properties of gases are important in developing methods for their liquefaction: critical temperature and critical pressure. The critical temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the critical temperature is called the critical pressure.
For example, the critical temperature for carbon dioxide is 304K (87.8°F [31°C]). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 304K (87.8°F [31°C]) will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 304K (87.8°F [31°C]) is 72.9 atmospheres. In other words, the application of a pressure of 72.9 atmospheres of pressure on a sample of carbon dioxide gas at 304K (87.8°F [31°C]) will cause the gas to liquefy.
Differences in critical temperatures among gases means that some gases are easier to liquify than are others. The critical temperature of carbon dioxide is high enough so that it can be liquified relatively easily at or near room temperature. By comparison, the critical temperature ofnitrogen gas is 126K (-232.6°F [-147°C]) and that of helium is 5.3K (-449.9°F [-267.7°C]). Liquefying gases such as nitrogen and helium obviously present much greater difficulties than does the liquefaction of carbon dioxide.
