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What Is a Supercritical Fluid?

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  • Written By: M.R. Anglin
  • Edited By: Kristen Osborne
  • Last Modified Date: 23 October 2014
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A supercritical fluid is a substance that has been heated above its critical point. The critical point of a substance is the point where the critical pressure and the critical temperature coexist, allowing the substance to exist in its liquid and gaseous forms at equilibrium. The result of taking a substance beyond this point is a fluid has the dissolving properties of a liquid, but the diffusing properties of a gas. That means that it can dissolve substances like a liquid while expanding to fill a container like a gas. These unique properties allow it to be used in different industries.

When a substance is taken above its critical point, it becomes a supercritical fluid. In order for a fluid to become supercritical, a person must know the critical temperature and the critical pressure. Normally, a gas at high temperature can be turned into a liquid with the addition of pressure, and a liquid at high pressure can turn into a gas with the addition of heat. The critical pressure is the pressure above which a substance cannot exist as a gas despite how high the temperature is. Similarly, the critical temperature is the point above which a substance cannot be a liquid despite how high the pressure is.

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When a substance is heated above the critical temperature and is put under a pressure above that of the critical pressure, an interesting phenomenon occurs. The substance cannot be either a liquid or a gas. Rather, it has the properties of both. The lines between the phases of matter virtually disappear, and the fluid changes properties. The new supercritical fluid has properties of both a liquid and a gas.

One supercritical fluid that is often used is carbon dioxide. It is a good substance to turn into a supercritical fluid because its critical temperature is 87.8°F (31°C) and its critical pressure is 73 atmospheres (about 55,480 mm Hg). Once it is a supercritical fluid, the properties of carbon dioxide can be altered with changes in temperature and pressure. For example, manipulating the pressure can change which substances will dissolve in the fluid. Since carbon dioxide is a non-polar molecule, modifiers can be added to increase its ability to dissolve polar molecules.

Some supercritical fluids can be used to extract a greater amount of a desired substance in a shorter period of time. These and other properties allow them to be used in a vast array of industries. The food and pharmaceutical industry, for example, can use the fluid to extract certain compounds from food. Using a supercritical fluid, such as carbon dioxide, a scientist can extract fatty acids, oils, and antioxidants without leaving a chemical residue on the extract. Supercritical carbon dioxide also has a relatively low temperature so it can be used in instances where a chemical cannot be exposed to high heat.

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