The Pressure – Temperature of a pure substance is commonly plotted as follows:
When you take a look at such a graph, there are two points on it that are particularly fascinating. (rest are all boundaries) : The Triple and Critical point.
Most people might be familiar with a triple point. This is where the solid liquid
and gaseous phase are in equilibrium.
In the above animation, Gas phase is at the Top half of the container, Liquid at the bottom and Solid phase is sandwiched between the two phases.
If one keeps moving past the triple point and along the liquid vapor boundary line on the P-T diagram, one would notice that this boundary line terminates at a distinct temperature and pressure !!!
This point is known as a Critical point.
Beyond this point distinct liquid and gas phases do not exist and the substance is known as a super-critical fluid.
In order to understand what that really means, watch the following didactic demonstration of a super-critical fluid transition: (It is absolutely beautiful)
One of the reasons why the critical point is crucial is that close of the critical point, small changes in pressure or temperature result in large changes in density. And industries love to take advantage of this to fine tune their processing methods.**
Have a good one!
* We have discussed the phase diagrams for only pure substances here. But phase diagrams for mixtures are interesting too. Check this out.