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Phases of matter: Identifying the triple point and critical point in a phase diagram

The first step in identifying the circumstances under which thermodynamically different phases arise and coexist at equilibrium is to draw a phase diagram.


These diagrams are useful tools for scientists to investigate what state the majority of matter present in a system is at a given temperature and pressure.


Among the most important points to note on these graphs are the critical point and the triple point.



What is the critical point?


A critical point is the “ending” between notable differences in density between phases of matter.


Here, the densities of two different phases of matter, such as the gas liquid phase, are identical. Therefore, there is no longer value in differentiating between the two different phases.


This is primarily due to the fact that phases and states of matter are primarily measured by their density.


The density of phases of identical molecules will always result in the same trend. Solids have the least energy and thus the most structure, followed by liquids and then gasses. The trend follows higher energy and less structure as phases change from solid to gas.


The critical point however is essential to our understanding of thermodynamics.

It was first found by Charles Cagnaird, who noticed that carbon dioxide could be liquified at 31 degrees centigrade.


However, if moving to a higher temperature, increasing pressure did not manage to liquefy carbon dioxide even at levels as high as 3,000 times our current air pressure on earth.




What is the triple point?


The triple point is where matter exists as liquid, solid and gas.


This is importantly used as the baseline for the temperature scale of Kelvin, where the triple point of water (273.16) was the baseline reference point for temperature.


In modern times, however, this has been altered by the 2019 redefinition of SI units, where it is slightly different.


The reference point was changed from a set definition and value to that of a measured result. This means that while the value is not wildly inaccurate, it is better to physically measure your triple point personally than to trust a sourced value if you have adequate equipment and instrumentation to investigate these values.



Different examples of phase diagrams


To go above and beyond the standard three-phase phase diagram, there are many advanced examples of phase diagrams.


A commonly-observed one is how temperature and pressure affects the crystal structure of ice.


In this instance, there are subtle differences in the alignment of water molecules when frozen which are significantly different and stable enough to be considered different from one another.