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 such a point, a substance is neither completely liquid nor completely gaseous; it displays properties of both the liquid phase and the gas phase and is referred to as a supercritical fluid. There is also such a thing as a liquid-liquid critical point. Such a point denotes conditions of temperature, pressure, and composition beyond which a mixture will separate into two or more different liquid phases.
The critical point of a specific substance is often viewed on a phase diagram, a chart that shows the behavior of the substance at different temperature and pressure conditions. Such a chart shows, for example, a curve representing the equilibrium between the liquid and gaseous phases. On one one side of the line, at higher temperature and lower pressure, the substance is in the gaseous phase. On the other side of the line, at lower temperature and higher pressure, the substance is in the liquid phase. At sufficiently high temperature and pressure, this line terminates in 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 are reminiscent of the properties of both liquids and gases. Like liquids, supercritical fluids can serve as solvents; they can dissolve other substances. Like gases, however, they have no surface tension.
The supercritical fluids that exist beyond the critical point are relatively rare in nature, but they do exist. Deep sea volcanoes, for instance, exist deep in the ocean and release extremely hot substances into the extremely high-pressure conditions at the ocean floor. This heats the water around the volcano, pushing it beyond its critical point and turning it into a supercritical fluid. Some planets, particularly gas giants, contain substances at very high temperature and pressure conditions at their cores. The high temperature and pressure conditions are far 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 makes them useful to chemists for dissolving certain substances. They can also be used in the production of biodiesel fuels. Supercritical methanol, for example, is put through a series of reactions that result in a quantity of usable fuel.