BR Gen Chem Chapter 7 (Phase Changes)

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BestDoctorEver

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On page 98 and 99 questions 10 and 14...Can someone explain to me why D is the answer on number 10. Also on number 14, if the question states: Which of the following conclusions is INVALID for Compound A? I believe that they must be looking for a false statement... Answer A is a false statement but it is not the answer... That is kind of weird...May be my interpretation is wrong...

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For 10: At which temp is sublimation impossible? Sublimation is the direct change of solid --> gas. So, you look at the phase diagram, and put a vertical line at the temperatures offered as choices. Which line doesn't directly cross the solid/gas boundary? That's your answer, because sublimation is a direct transition. Remember, 0 degrees = 273 K. You also have to be able to decide which area is gas, solid, liquid, since they aren't labeled. But it's a typical diagram, that shouldn't be an issue.

For 14: It is looking for a false statement, you're right. Answer A is a true statement, because in Figure 1, there is not a direct solid-->gas transition at 25 degrees, or 298 K. You have to cross the liquid phase to go between solid and gas. D is the false statement, it says, "For compound A (figure 1) there exists a solid less dense than its liquid phase." If that were true, you'd be able to draw a temperature line somewhere on the diagram, and while moving from low pressure to higher pressure, you would move from solid to liquid. By adding pressure, you would COMPRESS the solid to a liquid.

Water is the common example of this, and you can quickly recognize it's phase diagram because of the negative slope separating the liquid and solid phases. That negative slope means you can do that constant temperature, increasing pressure transition from solid to liquid.
 
For 10: At which temp is sublimation impossible? Sublimation is the direct change of solid --> gas. So, you look at the phase diagram, and put a vertical line at the temperatures offered as choices. Which line doesn't directly cross the solid/gas boundary? That's your answer, because sublimation is a direct transition. Remember, 0 degrees = 273 K. You also have to be able to decide which area is gas, solid, liquid, since they aren't labeled. But it's a typical diagram, that shouldn't be an issue.

For 14: It is looking for a false statement, you're right. Answer A is a true statement, because in Figure 1, there is not a direct solid-->gas transition at 25 degrees, or 298 K. You have to cross the liquid phase to go between solid and gas. D is the false statement, it says, "For compound A (figure 1) there exists a solid less dense than its liquid phase." If that were true, you'd be able to draw a temperature line somewhere on the diagram, and while moving from low pressure to higher pressure, you would move from solid to liquid. By adding pressure, you would COMPRESS the solid to a liquid.

Water is the common example of this, and you can quickly recognize it's phase diagram because of the negative slope separating the liquid and solid phases. That negative slope means you can do that constant temperature, increasing pressure transition from solid to liquid.
I understand your explanation...However, my problem was that they did not say anything about the pressure should be constant. I overanalyze the questions thinking that you can manipulate (increase or decrease) the pressure.
 
I understand your explanation...However, my problem was that they did not say anything about the pressure should be constant. I overanalyze the questions thinking that you can manipulate (increase or decrease) the pressure.

Eek, maybe I confused you worse. Unless you meant "temperature should be constant." I don't have my book on me at work but I think I remember how it goes.

For 10, its essentially asking that you hold temperature constant (at each of the optional temperatures), and move from high to low pressure. It wants to know at which temperature can you do that and NOT experience sublimation.

For 14, where we're looking for which statement it false, the A statement is correct because as it says, at 25celsius you in fact CAN'T do that constant T/changing P trick and see sublimation. You just see melting and then vaporization. Figuring out that D is the false one also involves a constant T/changing P approach.

Nowhere in these two problems do you keep pressure constant.
 
Eek, maybe I confused you worse. Unless you meant "temperature should be constant." I don't have my book on me at work but I think I remember how it goes.

For 10, its essentially asking that you hold temperature constant (at each of the optional temperatures), and move from high to low pressure. It wants to know at which temperature can you do that and NOT experience sublimation.

For 14, where we're looking for which statement it false, the A statement is correct because as it says, at 25celsius you in fact CAN'T do that constant T/changing P trick and see sublimation. You just see melting and then vaporization. Figuring out that D is the false one also involves a constant T/changing P approach.

Nowhere in these two problems do you keep pressure constant.
You are right...It did not cross my mind that coompound B exist as solid and compound A exist as liquid. These 2 questions were so easy then. Thanks for replying...
 
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