Being the lucky 15 year old chum I am, I’ve been stuck with Advanced Chemistry this year. Recently, we were discussing phase diagrams - crticial points, triple points, etc. All we were shown were phase diagrams for solid, liquid, and vapors in terms of temperature and pressure. Our teacher vehemently argued that after the critical point, nothing happens. I argued that at some point, the molecules would have to change into a plasma. Am I right? If so, when does this happen? Does the phase diagram become 3D? Is there a quadruple point, where all 4 stages of matter exist simultaneously?
Maybe your chemistry teacher didn’t want to talk about what happens after the critical point because at that point you leave chemistry and start into physics
My copy of Concepts in Physical Chemistry by Peter Atkins (1995, W.H. Freeman and Company, NYC) says that:
It looks like what you get is not a plasma phase, but a supercritical fluid phase, of which Petey A. has this to say:
“Fluid”, of course, refering to any non-solid phase. So you don’t get a plasma, but something between a gas and liquid–and it sounds like that something is non-condensed like a gas, but has the density of a condensed phase, like a liquid.
Strictly speaking, you and your chemistry teacher are both correct. The critical point is defined as the temperature and pressure at which the liquid and gaseous states of matter can no longer be distinguished from each other. Matter in this state is said to be a * supercritical fluid, *but sometimes scientists just call it gas, because that’s a shorter name to use in daily conversations
Now, plasma is basically ionized gas. The word “gas” is used in the broadest sense here, which includes the traditional gas before the critical point, and the gas we just call gas after the critical point, or supercritical fluid. Some people like to call plasma the 4th state of matter, but I’m under the impression that it’s just a flashy name for popular science. Plasma is commonly generated in two ways. The first is by raising the temperature of the gas you have. When the kinetic energies of the molecules get high enough, their collisions can actually knock electrons out of their orbits, thus ionizing the gas molecules. The second is by using the discharge of a strong electric field. Lightning is an example of this, as the flash you see is just ionized air, or plasma.
So, back to your original post… your teacher was correct in the context of your question, because strictly speaking, plasma is just gas. It’s ionized all right, but ionization is usually not a criterion to define a new state of matter on. Maybe he should have mentioned supercritical fluids, but that’s another issue altogether. Then again, you were correct in a sense too, because gas will ionize when it gets hot enough. The phase diagram will not turn 3D however, unless you forcefully stick a third axis on to it. I suppose you can mark this axis with some measure of the degrees of ionization, but why mess up a perfectly good 2D diagram and make it unnecessarily confusing? As for a quadruple point, it probably doesn’t exist. First of all, plasma is not a 4th state of matter, and furthermore, its definition is ionized * gas. * Now * maybe * there’s a state of matter where solids, liquids, and gases can coexist while being ionized, but I doubt that state will be called plasma in such a scenario.
