# What is happening at the various boundaries of a phase diagram? [closed]

Hoping to clarify a few questions I have that the textbook doesn't address.

Lets just say I have a piston (at equilibrium) with 100 moles of gaseous compound that looks like this: And here is the phase diagram for this compound: Now lets say I start increasing the pressure on the piston so that the gas starts to approach the gas-liquid boundary.

1.) What would start to happen as you approach the boundary? For example: Would a few moles of gas begin to condense to liquid? Is this process exponential as you approach boundary?

2.) What is happening at the boundary? For example: Are the gas and liquid in perfect equilibrium (50 moles of each)? By that logic, at the triple point, are there 33 moles of each?

3.) Now let's say we approach the critical point along the liquid-vapor boundary. I read online that its when the gas and liquid become "indistinguishable." If so, that means it has a uniform density. Does that make it another state of matter? If not, why is it special?

• Why the downvote? Let me know what I can do to clarify! – Nova Jul 24 at 22:02
• (1) No, the process is not exponential, but abrupt. (2) Yes they are in perfect equilibrium, and that's not what you think it is. The number of moles is irrelevant and can be anything. (3) Call it whatever you like. – Ivan Neretin Jul 24 at 22:04
• Thanks for reply! Can you specifically describe what would be happening inside the piston at the boundaries then? – Nova Jul 24 at 22:07
• The gas will start condensing into a liquid. As you move the piston further, more gas will condense, until there is none of it left. Then you will be just compressing the liquid. – Ivan Neretin Jul 24 at 22:10
• Why is saying that half the molecules would be in one phase (and the other half in the other) erroneous? I'm imaging a layer of liquid with a layer of gas on top. – Nova Jul 24 at 22:15