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Here's the question. I have no idea how to approach this question.
Thanks
EK Chemistry Chapter 5 In lecture Question 118
- Thread starter StretchDoe
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Hi there, good question! Remember the definition of critical point - with regard to liquid/vapor phase changes, it's the temperature above which liquid and vapor can't be distinguished from each other. From this fact, we know that "liquid" and "vapor" must have the same density above this point.
Looking at the graph, do you see how the "vapor" line starts at a very low density on the left, then gradually increases in density as temperature goes up? In contrast, the "liquid" line begins at its highest point (1 g/cm^3, which is the typical density of water with which we are familiar) and decreases as temperature rises. At a certain point (the critical point), the two lines are going to meet where the liquid density has gotten low enough to match the rising vapor density. This is the point to the farthest left along the curved line, which corresponds to some value between 350 and 400 degrees C.
You can also answer this question using process of elimination. Choice A can't be the correct answer - this is the melting point of water, so it's a boundary between the solid and liquid phases. This temperature is far too low to be the critical point.
Looking at choice B, if you examine the curve at 135 degrees C, you'll see a very different density for liquid and vapor. Again, this contradicts the definition of critical point. Finally, 506 degrees C is far too high - this will put us way to the right of the curved line, meaning that liquid and vapor are indistinguishable at temperatures well below this level.
Good luck with your studying!
Looking at the graph, do you see how the "vapor" line starts at a very low density on the left, then gradually increases in density as temperature goes up? In contrast, the "liquid" line begins at its highest point (1 g/cm^3, which is the typical density of water with which we are familiar) and decreases as temperature rises. At a certain point (the critical point), the two lines are going to meet where the liquid density has gotten low enough to match the rising vapor density. This is the point to the farthest left along the curved line, which corresponds to some value between 350 and 400 degrees C.
You can also answer this question using process of elimination. Choice A can't be the correct answer - this is the melting point of water, so it's a boundary between the solid and liquid phases. This temperature is far too low to be the critical point.
Looking at choice B, if you examine the curve at 135 degrees C, you'll see a very different density for liquid and vapor. Again, this contradicts the definition of critical point. Finally, 506 degrees C is far too high - this will put us way to the right of the curved line, meaning that liquid and vapor are indistinguishable at temperatures well below this level.
Good luck with your studying!
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Hi there, good question! Remember the definition of critical point - with regard to liquid/vapor phase changes, it's the temperature above which liquid and vapor can't be distinguished from each other. From this fact, we know that "liquid" and "vapor" must have the same density above this point.
Looking at the graph, do you see how the "vapor" line starts at a very low density on the left, then gradually increases in density as temperature goes up? In contrast, the "liquid" line begins at its highest point (1 g/cm^3, which is the typical density of water with which we are familiar) and decreases as temperature rises. At a certain point (the critical point), the two lines are going to meet where the liquid density has gotten low enough to match the rising vapor density. This is the point to the farthest left along the curved line, which corresponds to some value between 350 and 400 degrees C.
You can also answer this question using process of elimination. Choice A can't be the correct answer - this is the melting point of water, so it's a boundary between the solid and liquid phases. This temperature is far too low to be the critical point.
Looking at choice B, if you examine the curve at 135 degrees C, you'll see a very different density for liquid and vapor. Again, this contradicts the definition of critical point. Finally, 506 degrees C is far too high - this will put us way to the right of the curved line, meaning that liquid and vapor are indistinguishable at temperatures well below this level.
Good luck with your studying!
Hey nextstep. I actually had trouble with that question, but figured it out. I was referring to the question below that, question 118. Sorry for the possible confusion.
Also in terms of difficulty compared to the AAMC questions how would you rank that question you just explained. I found it to be hard initially because I had trouble interpreting a phase diagram in that manner.
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Whoops, thanks for clearing that up! I guess I was overeager to explain #117, haha. Glad you were able to figure it out on your own. In terms of difficulty, I'd say it's on par with what you could actually see on the MCAT, for one particular reason - it presents a familiar topic in a new way, in graph form. The AAMC loves to take concepts that you know very well and see if you recognize them when they're presented slightly differently (especially in a biological context).
#118 is impossible to answer because it is an error. If you look at old editions of the same book, they have the same two diagrams paired with a non-calculation-based question about the lines on the phase diagram. #118 is supposed to have a different diagram instead, where the values 3.5 and 14.4 kJ are clearly marked along the plateau of a heating diagram.
#118 is impossible to answer because it is an error. If you look at old editions of the same book, they have the same two diagrams paired with a non-calculation-based question about the lines on the phase diagram. #118 is supposed to have a different diagram instead, where the values 3.5 and 14.4 kJ are clearly marked along the plateau of a heating diagram.
