# Combined Phase Diagram

#### SaintJude

##### Full Member
The following is a combined phase diagram diagram for materials A and B.

1.) Which of the following would have its volume reduced if the pressure is increased?

A. Material A at its triple point
B. Material B at its triple point
C. Material A at 1000 K and 1 atm
D. Material B at 273 K and 0.006 atm

#### MedPR

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Can rule out A, B, and D and be left with C.

Can't tell you much more than that though :/

#### chiddler

##### Full Member
10+ Year Member
how is A ruled out? triple point features gases, in addition to the other phases, which are particularly voluminous.

so converting it to solid should reduce volume, shouldn't it?

B and D are obvious because they are identical.

#### MedPR

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how is A ruled out? triple point features gases, in addition to the other phases, which are particularly voluminous.

so converting it to solid should reduce volume, shouldn't it?

B and D are obvious because they are identical.

That's true, I didn't think about A like that. Looking at it again, at 1000K and 1atm unknown A is a gas. If you increase the pressure it becomes a liquid. Gas to liquid results in definite volume loss.

I don't know why A is wrong though :/

#### SaintJude

##### Full Member
I don't see how B& D are identical, one is a the critical point, the latter is when B is a gas. And when material B is at 273 K and 0.006 atm and the pressure is increased, wouldn't it move from gas--> solid ?

But I do see how C is correct but that's quite a pressure jump! And the fact that's it's at 1000K value that's off the chart, really makes me want to understand why D is wrong.

#### pfaction

##### Full Member
10+ Year Member
A is valid because it's going from a multiple to a solid, mostly.
B is valid too, unless it turns liquid.
C is clearly a gaseous molecule through and through, and seems to be the "most correct".
D is going to be a solid, St Jude. And it'll stay a solid when it's increased, but MAY turn into a liquid at higher pressures.

I think the trick here is the negative slope. Or something. Gay.

#### SaintJude

##### Full Member
Oh, yeah I misread the graph when it comes to D. About A & B , I have this notion, that you can't really predict a increase/decrease in volume b/c at the triple point all three states exist, so how can you accurately predict a change. It just seems like it would take too much of an assumption to make without additional information.

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#### MrNeuro

##### Full Member
7+ Year Member
C: b/c its most probably a super critical liquid (aka in liquid and gaseous phase). Increasing its pressure by any value would turn more supercritical gas into supercritical liquid (less voluminous)

also w/ a negative slope
Volume of g > s > l
w/ a postivie slope
volume of g > l > s

w/ the triple points you have all three phases so your overall volume change would probably not be as great as the one w/ the super critical fluid

#### SaintJude

##### Full Member
Pfaction, you're really awesome and thank you sooo much for your input.

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#### folktale

##### Full Member
C: b/c its most probably a super critical liquid (aka in liquid and gaseous phase). Increasing its pressure by any value would turn more supercritical gas into supercritical liquid (less voluminous)

also w/ a negative slope
Volume of g > s > l
w/ a postivie slope
volume of g > l > s

w/ the triple points you have all three phases so your overall volume change would probably not be as great as the one w/ the super critical fluid

I thought that once you pass the critical temperature, there's no way of converting the supercritical fluid into its liquid state? Only lowering the temperature would cause a conversion to a liquid.

#### MrNeuro

##### Full Member
7+ Year Member
I thought that once you pass the critical temperature, there's no way of converting the supercritical fluid into its liquid state? Only lowering the temperature would cause a conversion to a liquid.

supercritical fluids have multiple densities b/c they exist in both a supercritical gas and supercritical liquid state

wiki
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be "fine-tuned"

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