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Solid, Liquid, and Gas

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Caffeine-Free, Now with Less Cancer
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Caffeine-Free, Now with Less Cancer

Credit: Waferboard
Source: http://www.flickr.com/photos/60944931@N00/7417277818
License: CC BY-NC 3.0

Coffee and tea are two of the world’s most popular beverages. However, not everyone wants a jolt of caffeine every time they have a cup. The first commercial decaffeination process involved extracting coffee beans with the organic solvent benzene, which effectively removed most of the caffeine while leaving various other natural flavors and aromas behind. Unfortunately, some of the extracting solvent is bound to be absorbed by the beans, and benzene is carcinogenic (cancer-causing) even in very small amounts.

Eventually, benzene was replaced by other organic solvents, such as methylene chloride or ethyl acetate. However, these substances are also slightly toxic, even if they are not as dangerous as benzene. More recently, an even safer decaffeination process has been developed in which the caffeine is removed by pressurized carbon dioxide, which is nontoxic and abundant. The process makes use of a peculiar state of matter: the supercritical fluid.

News You Can Use

  • In general, if a gas is compressed with a high enough pressure, it will transform into the liquid or solid state. However, all substances have something called a critical temperature. When the gas is heated above this temperature, it will not liquefy (or solidify), no matter how much pressure is applied.
  • Although a pressurized gas above its critical temperature will never be a true liquid, it can be compressed until it is almost as dense as a liquid. At this point, it is referred to as a fluid, and its properties are intermediate between those of a liquid and a gas.
  • Watch liquid CO2 be heated to form a supercritical fluid, then re-condense into a liquid upon cooling: http://www.youtube.com/watch?v=N_Mp2Cb62yM

Explore More

With the links below, learn more about supercritical fluids and phase diagrams. Then answer the following questions.

  1. The video above starts out with a sample of liquid CO2. What is the minimum possible pressure inside the container?
  2. Are there substances that can be compressed into supercritical fluids at room temperature (25°C)? If so, name three. If not, why not?
  3. What properties of a supercritical fluid are more similar to those of a liquid than a gas?
  4. What properties of a supercritical fluid are more similar to those of a gas than a liquid?

Image Attributions

  1. [1]^ Credit: Waferboard; Source: http://www.flickr.com/photos/60944931@N00/7417277818; License: CC BY-NC 3.0

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