Relationship between vapor pressure and liquid volume--so confused

[drlovett]

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Hey Everybody,

So I was doing a TPR passage the other night and it basically was a passage comparing two cylinders-- one called a "compressed-shock absorber", and the other a "conventional spring shock absorber".

Briefly, the compressed-shock absorber consisted of a cylinder--- one end of it having a movable piston, and the other end was exposed to a compression force. Both ends were airtight. The test runs looked at the displacement of the piston while a force was being applied to one end.

Basically, as the force reached a certain magnitude, the piston stopped moving, but the air-tight seals were not breached, and the shock regained all of its normal characteristics after the force was removed.

The question I am confused about was asking the reason for the above. I chose "the vapor pressure of the gas decreased as the applied pressure increased". The correct answer was "the gas liquefied under the high pressure".

Now my question is, aren't those two answers essentially the same? If vapor pressure is decreased, then there are less gas molecules above the surface of the liquid, and therefore, more molecules within the liquid itself... so isn't that the same as "it liquefied"? Maybe it's semantics...

Been trying to understand this for a couple of days. Any thoughts would be appreciated. Thanks!

 

[sc4s2cg]

I think it has more to do with how specific the answer was.

the vapor pressure of the gas decreased as the applied pressure increased

The vapor pressure can decrease due to multiple reasons - it can decrease because the molecules came closer together and became liquid, like you said. But it can also decrease if any escaped through the other end (despite the passage saying that it was intact), or through the piston, and so on. In these cases the piston would not go back to the original position after the force is taken off.

the gas liquefied under the high pressure

This answer is a lot more specific. It says that the gas molecules specifically liquified under the force, and nothing else happened. This takes into account the passage details on how the seals weren't breached, and how the piston went back to the original position once the force was taken off. I think they wanted this answer specifically to make sure you understood that gas can turn into liquid under high pressure and airtight conditions.

That's my take on it anyways. I don't have TPR so take as you will!

 

[drlovett]

Ok, ya I can see that, thank you. I also realized later that my choice didn't make sense because there has to be liquid present in the first place for there to be vapor pressure.