The critical point is a condition in thermodynamics where the distinction between gas and liquid phases disappears, creating a supercritical fluid. This point is displayed on a phase diagram and has many applications in science and industry, such as in the production of biodiesel fuels.
Critical point is a term used in thermodynamics to describe a pressure and temperature condition beyond which distinctions between phases, particularly between gas and liquid, cease to exist. Beyond that point, a substance is neither completely liquid nor completely gaseous; it exhibits properties of both the liquid phase and the gas phase and is referred to as a supercritical fluid. There is also a liquid-liquid critical point. This point denotes temperature, pressure and composition conditions beyond which a mixture will separate into two or more different liquid phases.
The critical point of a specific substance is often displayed on a phase diagram, a graph showing the behavior of the substance under different conditions of temperature and pressure. This graph shows, for example, a curve representing the equilibrium between the liquid and gaseous phases. On one side of the line, at a higher temperature and lower pressure, the substance is in the gaseous phase. On the other side of the line, at a lower temperature and higher pressure, the substance is in the liquid phase. At high enough temperature and pressure, this line terminates at a single point beyond which the liquid-gas phase distinction disappears, the critical point.
Supercritical fluids, which exist beyond a substance’s critical point, have many different properties that resemble the properties of both liquids and gases. Like liquids, supercritical fluids can act as solvents; they can dissolve other substances. Like gases, however, they have no surface tension.
Supercritical fluids that exist beyond the critical point are relatively rare in nature, but they do exist. Deep-sea volcanoes, for example, exist deep in the ocean and release extremely hot substances under the conditions of extremely high pressure on the ocean floor. This heats the water around the volcano, pushing it past its critical point and turning it into a supercritical fluid. Some planets, especially gas giants, contain substances at very high temperatures and pressures in their cores. The conditions of high temperature and pressure are well beyond the critical point for the substances involved, so they exist as supercritical fluids.
There are many different applications of supercritical fluids in science, industry and other fields. Their solubility properties make them useful to chemists for dissolving certain substances. They can also be used in the production of biodiesel fuels. Supercritical methanol, for example, undergoes a series of reactions that result in a usable amount of fuel.
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