How to read phase diagrams?

[discowisco]

http://postimage.org/image/d96pirjcr/

How would you go about deciphering this? For example like asking what is the critical point for CO2 in this graph?

I just scanned TBR and couldnt find a graph like this in it, didnt start reading the phase diagram chapter yet.

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[Jepstein30]

http://postimage.org/image/d96pirjcr/

How would you go about deciphering this? For example like asking what is the critical point for CO2 in this graph?

I just scanned TBR and couldnt find a graph like this in it, didnt start reading the phase diagram chapter yet.

That's not a phase diagram, I think. I remember seeing a graph just like this in the same section but a phase diagram is http://serc.carleton.edu/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg . Could be wrong though.

Anyways, the critical point (in this case, the liquid-vapor critical point) would be at all points along the dotted line. Critical point refers to a set of conditions (pressure/temp) where multiple phases can co-exist. There are multiple different critical points, most known is the triple critical point which is where gas, liquid and solid co-exist. That's where all three lines on the phase diagram I linked above meet. On the graph you posted, you wouldn't be able to tell where the triple critical point is.

For something like this, you're more likely to just be asked questions about reading the graph correctly (gas or vapor at temperature X and pressure Y).

 

[discowisco]

Alright so how would you read the type of graph i posted, for example how would you know what the critical point of CO2 is?

 

[Jepstein30]

Alright so how would you read the type of graph i posted, for example how would you know what the critical point of CO2 is?

Anyways, the critical point (in this case, the liquid-vapor critical point) would be at all points along the dotted line. Critical point refers to a set of conditions (pressure/temp) where multiple phases can co-exist. There are multiple different critical points, most known is the triple critical point which is where gas, liquid and solid co-exist. That's where all three lines on the phase diagram I linked above meet. On the graph you posted, you wouldn't be able to tell where the triple critical point is.

critical point = where 2+ phases co-exist = all points within the dotted section on the graph that is labeled as having both gas and vapor existing

 

[powerfetish]

Adding to this you should know that a liquid, not a gas, can reach critical temperature when it is heated to the extent that it cannot be heated further as a liquid. And if you apply pressure past this critical point, instead of the gas liquidifying, as usually occurs when a gas is pressurized, the liquid under critical temperature and critical pressure turns into a supercritical fluid having both fluid- and gas-like properties. Critical pressure is pressure applied when the fluid is past its critical temperature. From what I understand only fluids can reach critical temperatures and pressures to form a hybrid fluid. As such only the vqpor-pressure curve can end in a critical point. Also, the triple point is where all three phases are in equilibrium like liquid and gas are in equilibrium at the critical point. This does not mean that they are in a hybrid state but that at that point they are converting phases at a constant rate. Only liquids and gases can form a hybrid fluid and that only occurs past the critical point where the substance is past its critical temperature and pressure. Also the curves represent equilibrium between two states except for when they converge at the triple point where all three phases are in equilibrium, not just two.

Posted from my Nexus 10.

 

[caduc3us]

Anyways, the critical point (in this case, the liquid-vapor critical point) would be at all points along the dotted line. Critical point refers to a set of conditions (pressure/temp) where multiple phases can co-exist. There are multiple different critical points, most known is the triple critical point which is where gas, liquid and solid co-exist. That's where all three lines on the phase diagram I linked above meet. On the graph you posted, you wouldn't be able to tell where the triple critical point is.

I'm also having trouble reading this graph. You said that "the critical point... would be all the points along the dotted line." This exact graph appears in the AAMC self assessment. Question 67 asks "Which of the points (A-D) is the critical point for CO2?

The answer is D with the explanation "The liquid and vapor phases at point D..., where the densities of liquid and gaseous CO2 are equal. Thus, answer choice D is the best answer."

I'm assuming the critical point can't be all the points along the dotted line if the question only gives one answer. Any further explanation for this graph? FYI,they called it an "Isothermal curves for CO2 near the critical point" in the AAMC.

 

[akim6890]

Ok looking at the graph, it needs to be made clear the the solid lines represent the vapor phase and the dotted line represents liquid phase. What do these lines represent? They represent the change in pressure as a result of change in volume. If you look at the slope of these lines what is that? Pressure/volume = (mass x acceleration) / (unit area x volume). Since acceleration and unit area are both constant, they can be eliminated which leaves us with mass/volume = density. Does this make sense? So when the slope of one of the vapor phase lines is equal to the slope of the liquid phase line, the densities are equal and a critical point has been reached.