Phase Changes (Critical Points)

[DeathandTaxes]

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The question asks, which letter (A-D) in Figure 2 represents the critical point for CO2? I honestly had no idea how to interpret this graph and somehow guessed it correctly (D). The explanation says that the liquid and vapor phases coalesce there where the densities of liquid and gaseous CO2 are equal, but how am I exactly supposed to get that from this graph?

 

[NextStepTutor_1]

So, to answer this question, you must look at the second figure which shows the isothermal curves for CO2 near the critical point. Then, as the question asks, we must look at the point where the density of the liquid is equal to the vapor. In the graph, we can see that everything that is under the dotted line is a liquid-vapor phase. At that point there is a transition that occurs to go from one phase to another. However, as we move up the isothermal lines, we can see that there is a point where the dotted lines and the isothermal lines just touch each other. This point is where there is no transition period between liquid and gases--and hence the densities are the same.

 

[premed1001]

So, to answer this question, you must look at the second figure which shows the isothermal curves for CO2 near the critical point. Then, as the question asks, we must look at the point where the density of the liquid is equal to the vapor. In the graph, we can see that everything that is under the dotted line is a liquid-vapor phase. At that point there is a transition that occurs to go from one phase to another. However, as we move up the isothermal lines, we can see that there is a point where the dotted lines and the isothermal lines just touch each other. This point is where there is no transition period between liquid and gases--and hence the densities are the same.

how can you tell densities of the phases from the isothermal curve?

 

[DrknoSDN]

how can you tell densities of the phases from the isothermal curve?

Density is mass/volume

Lower the volume (and assume constant moles) and your density will increase.

 

[premed1001]

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[DrknoSDN]

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Sorry if that was too vague. Basically you can look at Figure 2 of the image and the bottom axis shows volume in Liters. You can qualitatively know that if you decrease the volume from point B around 55L to point D of 45 Liters, the Pressure and Density will increase.

No matter how much CO2 you had at point B, it's mass would remain the same but the volume would decrease when moving from B to D. This would translate to an increase in density. (same mass / smaller volume)

Opposite is true for C to D. You are increasing volume along the X axis and no matter how much you start with. Mass/(increased volume) = lower density.

At point D, the density of vapor and liquid is the same. This is the critical point. and the answer to OPs question.

The explanation says that the liquid and vapor phases coalesce there where the densities of liquid and gaseous CO2 are equal, but how am I exactly supposed to get that from this graph?

 

[DeathandTaxes]

Sorry if that was too vague. Basically you can look at Figure 2 of the image and the bottom axis shows volume in Liters. You can qualitatively know that if you decrease the volume from point B around 55L to point D of 45 Liters, the Pressure and Density will increase.

No matter how much CO2 you had at point B, it's mass would remain the same but the volume would decrease when moving from B to D. This would translate to an increase in density. (same mass / smaller volume)

Opposite is true for C to D. You are increasing volume along the X axis and no matter how much you start with. Mass/(increased volume) = lower density.

At point D, the density of vapor and liquid is the same. This is the critical point. and the answer to OPs question.

Does this mean that at C, CO2 is in liquid form, and in point B, it's in vapor form? Isn't the straight line supposed to represent a transition period in which you have both liquid and vapor?

 

[DrknoSDN]

Yes, C and B represent the start and end of a phase change. The area inside the dome would be part of the phase change where there is liquid and vapor are present.
The dotted line represents the limits of where the phase change will begin and end depending on what pressure you are at. The start and end points vary with changing pressure and volume.

D shows where the transition is basically seamless. The liquid and vapor phases are not distinguishable from one another. So you don't have liquid "and" vapor. It's just properties of both at the same time.

Using an analogy i would say it's like a sunrise. It's not day or night, it's just its own thing called dawn. Here it's called the critical point. Awful analogy but best I have atm. hah