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

Answer Check Make sure that you understand the

critical temperature correctly; liquefaction can occur only below the critical temperature if sufficient pressure has been applied to the gas.

Exercise 11.4 Describe how you could liquefy the following gases: a. methyl chloride, CH3Cl (critical point, 144°C, 66 atm); b. oxygen, O2 (critical point,

-119°C, 50 atm).

■ See Problems 11.57 and 11.58

Removing Caffeine from Coffee

Legend has it that a goat herder named Kaldi, in the ancient country of Abyssinia

(now Ethiopia, in northeast Africa), discovered the pleasant effect of coffee beans when he noted that his goats danced wildly about after eating the shiny leaves and red berries of a small tree. Later he tried eating the whole berries himself, and he soon found that he was dancing with his goats. Whether the legend is true or not, it does effectively describe the stimulant effect of this berry from the coffee tree.As we now know, coffee, which today is obtained by roasting the berries’ green pits, or beans as they are called, contains the stimulant caffeine, a bitter-tasting white substance with the formula C8H10N4O2. (Figure 11.14 shows the structural formula of caffeine.) For those who like the taste of roasted coffee but don’t want the caffeine, decaffeinated coffee is available. A German chemist, Ludwig Roselius, first made “decaf” coffee about 1900 by extracting the caffeine from green coffee beans with the solvent chloroform, CHCl3. Later, commercial processes replaced chloroform with the safer organic solvent methylene chloride, CH2Cl2.Today, though, most of the commercial decaffeinated coffee produced (Figure 11.15) uses supercritical carbon dioxide as the extracting fluid. In a tank of carbon dioxide under pressure (for example, in a CO2 fire extinguisher), the substance normally exists as the liquid in equilibrium with its gas phase. But we know from the previous text discussion that above 31°C (88°F), the two phases, gas and liquid, are replaced by a single fluid phase. So, on a hot summer day (above 31°C, or 88°F), the carbon dioxide in such a tank is above its critical temperature and pressure and exists as the supercritical fluid. Supercritical carbon dioxide is a near-ideal solvent Under normal conditions, carbon dioxide is not a very good solvent for organic substances, but supercritical carbon dioxide readily dissolves many of these substances, including caffeine. It is nontoxic and nonflammable. It also has no effect on the stratospheric ozone layer, whereas methylene chloride does. (See the essay on stratospheric ozone at the end of Chapter 10.) Carbon dioxide does contribute to the greenhouse effect (discussed in the essay at the end of Chapter 5), but the gas once used can be recirculated for

solvent use and not vented to the atmosphere. Supercritical fluids have gained much attention recently because of the possibility of replacing toxic and environmentally less desirable solvents. For example, at the moment, the usual solvent used to dry-clean clothes is perchloroethylene, CCl2CCl2. Although nonflammable and less toxic than carbon tetrachloride, which was the solvent previously used, perchloroethylene is regulated as an air pollutant under the Clean Air Act. Some scientists have shown that you can dry-clean with supercritical carbon dioxide if you use a special detergent. Substances other than carbon dioxide have also shown intriguing solvent properties. For example, whereas water under normal conditions dissolves ionic and polar substances, above its critical point (374_C, 217 atm) it becomes an excellent solvent for nonpolar substances. Supercritical water and carbon dioxide promise to replace many toxic or environmentally unfriendly organic solvents.