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In our last publication about phases and phase diagrams, we left off the critical point. As one may recall, phase diagrams show the phases of substances at specific temperatures and pressures. Most often, we look at the lines separating the phases. These represent temperatures at which either phase can be found in equilibrium. We also discussed the triple point, a point at which three phases can be found in equilibrium. Lastly, it is important to note once more that the fourth phase of matter has just recently been discovered and lacks research and understanding. For the purpose of this discussion, we will once more set aside the fourth phase of matter.
So what is this strange point that ends the boundary between the liquid phase and the solid phase -- also known as the critical point? It is the temperature and pressure at which we can no longer make the distinction between the liquid and gaseous phase of a substance (check out a video demonstration of this below). At that temperature and pressure, the densities of both phases are equal. This will cause them to combine, no longer revealing an evident boundary between them. The resulting body is known as a supercritical fluid. They were discovered in 1822 by a man named Baron Charles Cagniard de la Tour. Supercritical fluids have the viscosity of a gas, but the density of a liquid. Since they have the density of a liquid, they cannot be compressed into the liquid phase.
Looking at the molecular level of the substance helps explain why such strange fluids form. At very high temperatures and pressures, the particles and molecules have so much energy stored in them that the intermolecular forces are no longer strong enough to hold them together. Without the intermolecular forces, the liquid loses its surface tension and the two phases can combine (there is a video explaining surface tension below). This is also because the pressure and temperature are so elevated that their densities are equal and no phase is attracted to the Earth more than the other.
One may recall the phase diagram that was used in a previous publication. Here is a new phase diagram that helps explains the critical point.
From the graph, one can notice a green shaded area. That area is labeled SCF which stands for supercritical fluid. Scientists have not been able to find definite barriers between the SCF, liquid, and gas areas of the graph which is why the surfaces seem to overlap. It also helps explain why some graphs, like the one used in a previous publication, do not have anything drawn past the critical point. As of today, the critical point, the triple point, and the barrier lines are the only significant points labeled on the phase diagram. Who knows, with a new phase of matter and future discoveries, this graph may appear incomplete in the future.
One last interesting thing to note is that without the surface tension, the wetting behavior becomes more complex. I must admit, this is not a subject I have completely mastered yet, so I would not be able to explain it without taking several sources word for word. That being said, there is a unique wetting behavior that happens at the critical point. I have included some links to start your research. Feel free to comment down below if you know more about the subject or could direct others toward a resource.
Sources & More Information
The Triple Point || Solid, Liquid, & Gas Here is a direct link to the STEMx article that was referred to several times in this article.
Time Crystals, Mastodons, and a Trend-Defying Black Hole || Also In STEM News This is a direct link to the STEMx article that first introduced the idea that there is a fourth phase of matter: time crystals.
Important Points. This is a very short page that summarizes the important points of phase diagrams. It was made by MIT, also known as Massachusetts Institute of Technology, a university in the United States.
Critical Point - Chemistry LibreTexts “At the critical point, the particles in a closed container are thought to be vaporizing at such a rapid rate that the density of liquid and vapor are equal and thus form a supercritical fluid. As a result of the high rates of change, the surface tension of the liquid eventually disappears…”
Supercritical Fluids - Chemistry LibreTexts “Matter can be pushed to temperatures and pressures beyond those of its critical point. This stage is characterized by the inability to distinguish whether the matter is a liquid or a gas, as a result, Supercritical fluids (SCF) do not have a definite phase. In 1822 Baron Charles Cagniard de la Tour discovered supercritical fluids while conducting experiments involving the discontinuities of the sound of a flint ball in a sealed cannon barrel filled with various fluids at various temperatures…”
Contact Angle and Wetting Properties If you are interested in learning more about the strange wetting properties that can be noticed at the critical point, here is a video about the critical point at normal temperatures. This video helps explain many of the things we see on an everyday basis.
Surface Tension - What is it, how does it form, what properties does it impart “How does surface tension affect the surface properties of a liquid? Looking at surface tension from a particle perspective and a macro perspective, this video shows what causes surface tension, how surface tension is manifested in our everyday lives, and how intermolecular forces are involved in surface tension…”
Thermodynamics - Explaining the Critical Point “This experiment demonstrates the behavior of carbon dioxide around the critical point. It shows the transition of a liquid-vapour-mixture to the supercritical state and back. Watch how the phase boundary disappears and reappears. There is also a short description of the process in a P-T-diagram...”
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