Q) Why does phase equilibrium/inter-conversion ONLY occur at boundary lines between phases in phase diagrams?
Example: According to water phase diagram, at 200 C and 200 atm ONLY liquid water exists. So equilibrium: H2O (l) ⇌ H2O (g) doesn't exist at this point? Why?
Any clarification will be appreciated...
Water is so ubiquitous that we can hardly imagine it except as a liquid with at least a little bit of vapor, and the solid is not far from our thought. We are familiar with all three states of matter for $\ce{H2O}$: solid, liquid and gaseous. But the phase diagram contains a lot more information.
From Wikipedia[1]:
In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and plasma.
We don't often confront plasma, but you could imagine it on the surface of the sun.
A phase diagram is a very compressed data presentation, compared to our usual experience.
From Wikipedia[2]:
In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.
The area of the graph with which we ordinarily interact is a short ($\text ~\pu{-20°C}$ to $\text ~\pu{+200°C}$), thin ($\text ~\pu{0}$ to $\text ~\pu{3 atm}$) line that you can hardly see, and it is in this area that two phases can exist because equilibrium has not been completely established throughout the entire system.
The purpose of a phase diagram is to show what conditions make it possible for a given state of matter to exist or not exist. There are conditions where only one state of matter is stable. And there are exceptions. For instance, consider supercooled water at $\pu{-10°C}$. It's liquid, not in the right state, according to the phase diagram - but also not at equilibrium. When equilibrium occurs, it will conform to the phase diagram.
Consider a different material: zinc. Zinc is a solid; we essentially ignore its liquid state unless we heat it to temperatures greater than $\pu{420°C}$. We especially ignore its vapor pressure; however, at temperatures greater than $\pu{980°C}$, it can be and sometimes is, distilled as a manufacturing process.
A phase diagram for zinc on the internet doesn't even show the vapor. What would you get out of this simple phase diagram? A melting point and its variation with imposed pressure. The line is the condition of temperature and pressure where two states of matter (solid and liquid) can exist together for a long time i.e in equilibrium.
If you are some slight distance farther on one side of the line, you have only a solid; at some slight distance away on the other side, only the liquid is stable. With only one state of matter, only one phase to deal with, there is nothing stable to equilibrate to.
If you wonder about zinc atoms vaporizing, they will not affect your investigation - they are the same as not even being there. (Unless a few atoms do affect your investigation, in which case you get to submit a research paper!)
Zinc phase diagram from Wikimedia Commons[3]:
References:
(1) Wikipedia contributors. State of matter https://en.wikipedia.org/w/index.php?title=State_of_matter&oldid=1030288068 (accessed Jul 1, 2021).
(2) Wikipedia contributors. Phase (matter) https://en.wikipedia.org/w/index.php?title=Phase_(matter)&oldid=1013386008 (accessed Jul 1, 2021).
(3) File:Phase diagram of zinc (1975).png https://commons.wikimedia.org/wiki/File:Phase_diagram_of_zinc_(1975).png (accessed Jul 1, 2021).
A point in the phase diagram off a line means, for a ONE component substance, that is the only phase that exists. In the case of water at 200Cand 200 atmosphere there is no head space and the pressure is maintained by a solid piston or such device. If the pressure is reduced by lifting the piston and maintaining temperature, when the pressure reaches the line gas will form and water will continue to evaporate as head space is increased until the water is all gone; then the gas will be in the space below the line. If the pressure is caused by an inert-gas-head-space the added degree of freedom means that water will evaporate until its appropriate partial pressure is reached. This means that if the external pressure is caused by an inert-gas-head-space a new "line" is generated for every inert gas pressure. [Imagine a 3D phase diagram.,] Since the change in vapor pressure with changes in inert gas pressure is relatively small this change is usually ignored in simple treatments of phase changes.
![[3]](https://file-hippo.netlify.app/host-https-commons.wikimedia.org/wiki/File:Phase_diagram_of_zinc_(1975).png){kind=link}