I was just reviewing some phase diagrams and how the slope of the H20 diagram shows how water is more dense in its liquid state etc...

My question to you all is how can one use the pressure vs temp diagram to gain insight on the volume of the gas?

Thanks in advance!

My question to you all is how can one use the pressure vs temp diagram to gain insight on the volume of the gas?


PV = nRT for ideal gases, to specifically answer your question.

The line ends there, since liquids and solids are incompressible.

I like to use the equation (PV)/T = (PV)/T. It is real useful for determining the volume when both the pressure and temperature changes. But it would be impossible to gain any exact insight about the volume without more information besides the graph. You should have a conceptual understanding of the volume from the graph though. Higher temperature = higher volume, higher pressure = lower volume.

Higher temperature = higher volume,

You can only say this if the pressure is constant, and the moles of gas isn't changing.


higher pressure = lower volume.
Again, only with constant temperature and moles.

Let me re-emphasize: this is for gases only.

Sorry, should have been more specific. It all boils down to Boyle's Law PV = PV, when holding temp constant and Charle's Law V/T = V/T, when holding pressure constant (all for ideal gases and holding the number of mols constant). I don't know how this has anything to do with a phase diagram though. The phase diagram is used to note the relationship between pressure, temp., and the phase changes.

so basically you would have to have one out of the three constant, and then ask what happens when altering the 2nd thing will alter the last thing? Some questions I see asks what would happen to two varibles when one is changed.

Speaking of phase diagrams, how are people doing on the isothermal phase diagrams that give the pressure and volume on the axis? I'm having a little problem trying to figure those out.

Speaking of phase diagrams, how are people doing on the isothermal phase diagrams that give the pressure and volume on the axis? I'm having a little problem trying to figure those out.

What exactly are you having trouble with?

What exactly are you having trouble with?

The questions where they ask where the critical point is on those. Usually they have the dotted parabolic line drawn on there too with different points marked, and ask to identify the critical point. I don't remember any other questions that are asked, but I'm just having a hard time trying to visualize what they represent.

The questions where they ask where the critical point is on those. Usually they have the dotted parabolic line drawn on there too with different points marked, and ask to identify the critical point. I don't remember any other questions that are asked, but I'm just having a hard time trying to visualize what they represent.

Wrong graph. Critical point is not on the "isothermal phase diagrams that give the pressure and volume on the axis"

Wrong graph. Critical point is not on the "isothermal phase diagrams that give the pressure and volume on the axis"


I have seen a question on it. There is a dotted line that's an upside down parabola that they put on there to indicate different phases. I'm not sure where I saw that, but I'll look for it.

Ok i misinterpreted your question stem. Either way I'll try to explain it if you find it :)

EK Chem 1001 has questions about that phase diagram for questions 682-689. It just threw me since I've never seen these before.