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In Chemistry, what is a Critical Point?

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  • Written By: Daniel Liden
  • Edited By: Jenn Walker
  • Last Modified Date: 16 November 2016
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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.

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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.

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PelesTears
Post 5

Is the critical point in chemistry the same as the critical point in calculus? I am taking a calculus class this fall, and I am trying to brush up on some concepts. Can anyone explain what a critical point in calculus is?

FrameMaker
Post 4

@GiraffeEars- When examining the phases of matter, there is an important distinction between plasma and a supercritical fluid. Supercritical fluids are stable as long as the temperature and pressure remain constant.

The factors that determine when something is in its plasma state, on the other hand, depend on temperature. Plasma is matter that has been heated to the point where the molecules become unstable. The best example of plasma I can give you is a spark. Plasmas are mixtures of neutral atoms, free electrons, molecules and polyatomic particles, not stable substances.

GiraffeEars
Post 3

Is an element in the plasma phase at critical pressure and temperature? Basically, I want to know if a supercritical fluid is considered to be in the plasma phase? If this is not the case, can someone describe to me what a plasma is and how it is different from a supercritical fluid? Thanks!

parmnparsley
Post 2

@Highlighter- Oh the joys of thermodynamics...I was never a big fan of chemistry, but I always thought that thermodynamics was interesting, especially when elements or combinations thereof are being discussed. The critical point only refers to the phase change between liquids and gases or two liquids. A supercritical liquid is at equilibrium as a partial liquid and partial solid.

An example of a temperature and pressure where a substance is at its critical point would be that of would be gold at 5,000 atmospheres, and about 7,000 degrees Celsius. At this pressure and temperature, gold is a supercritical fluid.

The latter half of your inquiry actually describes a substance at its triple point. This is the point where a substance

is exists (or coexists) at equilibrium as a liquid, solid, and gas. Like the critical point, the factors that influence this phase are temperature and pressure.

Unlike the earlier example, gold does not have a triple point, but the triple critical point of mercury occurs at approximately negative 39 degrees Celsius and two atmospheres. Another fun example is the triple critical point of water. At one one-hundredth of a degree Celsius and one six thousandth of an atmosphere, water is a liquid, solid, and gas at the same time.

highlighter
Post 1

So if I understand this article right, a supercritical fluid is something that has reached its critical temperature to transition to a gaseous state, but the pressure is preventing the substance from acting completely like a gas? I am trying to study for a science CLEP test and there are questions about supercritical fluids in the practice exams.

Is there such thing as a supercritical solid? If a gas was under enough pressure to create a solid but it was at the temperature required to be a gas, would this be a supercritical solid? If the pressure on a supercritical substance were to be spontaneously reduced, would the substance instantly turn to a gas or liquid? Some of these questions are simply out of curiosity, but I would appreciate anyone who would attempt to explain phase changes a little more.

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