Supercritical fluid is a state of matter with many of the properties of a gas as well as a liquid. The French physicist Charles Cagniard de la Tour first investigated supercritical fluids in the 1800’s. They play a role in many physical and chemical processes in the fields of chemistry, engineering, geology, physics, and applied technology.
A chemical element or compound becomes a supercritical fluid above a certain temperature and pressure, called a critical point. For example, carbon dioxide (chemical formula, CO2) is a supercritical fluid above a critical temperature of 88 °F (31 °C) and a critical pressure of 73 atmospheres. An atmosphere is a unit of air pressure roughly equal to the pressure of Earth’s atmosphere at sea level. Water (H2O) becomes a supercritical fluid above a critical temperature of 705 °F (374 °C) and a critical pressure of 218 atmospheres. Further altering the pressure and temperature of a supercritical fluid can change its density. In this way, supercritical fluids can reach densities comparable to those of liquids.
The supercritical forms of carbon dioxide and water can dissolve substances that will not dissolve in their nonsupercritical forms. For example, mothballs will dissolve in supercritical carbon dioxide. Likewise, the mineral quartz will dissolve in supercritical water.
The dissolving power of supercritical fluids has many uses in industry. Supercritical fluids, such as CO2, can be used to extract flavors from spices and oil from soybeans. Supercritical CO2 can also remove caffeine from moist coffee beans, resulting in decaffeinated coffee. Supercritical water is used to break down toxic waste into nontoxic compounds. Fossil fuel power plants have increased efficiency by replacing regular water with supercritical water in their power-generating turbines.
Supercritical CO2 and liquefied CO2 are often used as alternatives to traditional dry cleaning solvents. They can also be used as a refrigerant. Some experts consider such uses of CO2 to be more environmentally friendly than the alternatives.
The supercritical form of a substance may be indicated using the abbreviation SC before the substance’s chemical formula. For example, supercritical carbon dioxide may be written SC-CO2.
