What is wroong with this scenario?

[Watters]

lol diagram did not work at all

[elpenmaster]

please listen to what i hgave been saying. water starts out in container 1. it goes to container two through osmosis. substance x cannot get into container #1 because of the semipermeable barrier. after going into container 2, the water rises to the top because it is less dense than substance x. at the top is another semipermeable barrier. the water gets through it, but substance x cannot. because the water went to container 2, the water level is now lower in container one. so the water goes through the semipermeable barrier, falls over a waterwheel in container one, makes energy, and then gets to the water level. then it does this all over again. please refer to my diagram on the last page if you havwe any questions.

[Rkyeun]

Water wants to be tainted. This is a simplification of osmotic pressure.
In your example, the water starts out in tube 1. It wants to get to the taint of x in tube 2. Fortunately, the barrier allows this. The water flows in to fill all available space. However at the top of tube 2, you have another barrier. Again at this barrier, water wants to get to the taint. This time that means STAYING IN, instead of going out. At this point, pushing more water through the membrane would be pointless, because it would just come out the top and the osmotic pressure would not change.

Water can still evaporate out through the barrier at the top of tube 2, and as it does more water will come in from tube 1 to take its place. Eventually you'll run out of water and have a tube 2 full of x.

[elpenmaster]

but once the water gets to the top of tube two, it has three choices: stay, go back down to mix with substance x, or go through the semipermeable membrane. it cant go down, since substtance x is denser than it, so we have pure water sitting on top of substance x, with a semipermeable membrane that it could go through. since the semipermeable membrane would take it downhill, and decrease potential energy, this is what the water goes tthrough, and it ends up in container 1 :wink:

[Rkyeun]

The water doesn't have any choice but to stay where it is.
Osmotic pressure is actively keeping the water from rising above that semi-permeable membrane at the top of tube 2.
To do otherwise requires an energy source to power the "pump" that will overcome osmotic pressure and pull the water up.
The water isn't going to escape through the semi-permeable membrane, the water is going to try to inflate it like a balloon, and when the pressure on that membrane is equal to the osmotic pressure of the water, the flow will stop.

It takes energy to power the motion.
The semi-permeable membrane at the bottom is pushing from tube 1 to tube 2.
The semi-permeable membrane at the top is pushing down into tube 2.

When tube 2 gets full, membranes 1 and 2 are pushing against each other. Nothing escapes, because the only thing the water wants to do is get to the taint.

[wendy]

Say i take two tubes, and put them next to each other. at the bottom between them, i put a semipermeable barrier, that water can pass throught butt that substance x cannot. now, i put water halfway up one andsubstance x inthe other tube. by osmosis, the water goes from its tube to the tube of substance x. and since substance x is denser than water, the water rises to the top. then i put a semipermeable barrier at the top. so only water can get through. then the water goes away from substance x and back into its own tube. then the process repeats. would not this be a perpetual motion machine? :?

If the water and the other substance don't mix, then the water can not make its way through the surface-tension layer the second substance creates: that is, one would get a very thin layer of water on the porus tube, which is held by surface tension.

If the water and the other substance do mix, then the water will mix to equal the osmotic pressure. If there is any slip between the two containers, then the water will carry substance x with it.

[Rkyeun]

If water and substance X do not mix, osmotic pressure will not apply.
The water in tube 1 will fall through the membrane into tube 2 until the water in tubes 1 and 2 are at the same height.

[elpenmaster]

If water and substance X do not mix, osmotic pressure will not apply.
The water in tube 1 will fall through the membrane into tube 2 until the water in tubes 1 and 2 are at the same height.

okay, im not getting somethging here:
the water will pass throught the bottom semipermeable membrane, right? into container 2 it wil go. once water gets to container 2, it is in something with which it can mix, but which is denser than itself. like when you put water on top of oil, the oil will go to the top and the water will go to the bottom. well, in this case, when water gets in under substance x, it will want to go to the top, will it not? and once it gets to the top, we have it set up so that the original level of substance x is just below the semipermeable membrane. then, when the water gets to the top, it will overflow and go through this second membrane. the water level in container one is below this second membrane, so the traveling water will fall into this water, and be back where it started, with the exception that it moved without being powered.

what that i said here is incorrect? which step of the process would not occur? :?

[Rkyeun]

If the substance X is water-soluble, then it is not like oil and does NOT settle to top or bottom.
There will be no top or bottom, substance X will -vanish- into the water (well, not literally) and form a single solution with one density, not two seperate substances.

Oil and water will seperate top to bottom.
The sugar isn't going to fall to the bottom of your Dr. Pepper.

The molecules of water will go between the molecules of substance X, increasing density while remaining in the same amount of space.

The part of the process which will not occur is the water rising from the top of the second semi-permeable membrane. It has absolutely no reason to do so unless it can take X with it when it leaves, because becoming LESS tainted is not one of water's natural behaviors. The water WANTS to be with X, not spilling over the top. And gravity certainly isn't going to help it fly magically out the top of the tube.

[elpenmaster]

then let us pretend that substance x is oil. will the water arriving at the bottom of container 2 rise to the top of container 2?

if it does, then it will fall through the semipermeable barrier on top, will it not? imagine that you have a container full of oil. if you pump water into the bottom of the container, it will rise to the top, right? and then once it gets to the top, it will overflow!

[chickendude]

if the oil was filled to the brim, when water enters, the oil would overflow instead of the water leading to the scenario below

if it wasnt filled to the brim, there would have to constantly be a certain volume of water in the oil container to keep this overflowing pattern. The pressure in the wateronly container needs to be enough constantly to maintain this level of pressure. because the oil is more dense, theres more pressure on the oil side because there is more overall weight in the same volume of space.

the tube with oil and water has more weight, more pressure than the one with just the water

this would lead to the water not passing through the bottom semipermeable barrier

i think >_>

[elpenmaster]

to stop the oil from overflowing, one would put another semipermeable barrier at the top, so the oil couldnt get out, but so that once the water got to the top, the water could overflow!

[Rkyeun]

No it could not.

If the oil floats, and does not dissolve, then there will be two portions of water, A and B. Portion A will flow through the barrier into tube 2 until it is at the same height as portion A in tube 1.
Portion B will have the same mass as the oil, and will remain on top of Portion A in tube 1 to counter the weight of the oil pushing down on Portion A in tube 2.

Water will not under ANY circumstances go through the membrane at the top of tube 2 unless you plug the device into a wall outlet or otherwise provide an external power source.

Both semi-permeable membranes serve as a pump. And they both pump in the same direction, towards the substance X. Water is not going to come out the top, because the membrane at the top of tube 2 is pushing DOWN. And it's pushing down EXACTLY as hard as the membrane at the bottom of tube 1 is pushing up. In the absence of other power sources, we still have gravity, which says you aren't going out the top of the tube.

[elpenmaster]

but the water in container 1 is not up to the level of the top semipermeable membrane.

and the substance x will not float on water, the water will rise to the top, where will it go throught the top membrane, falling through midair until it reaches the water level in container 1?

[RQ]

Hmm you've got a point, but one thing you don't explain is how the water gets to substance X again. Eventually due to friction, water will lose energy, and it won't have enough momentum to penetrate Substance X anymore.

This would either simply work for a very long time, or won't work at all. When the water collides with the surrounding it loses energy thus, unless you supply energy it will eventually stop.

[Rkyeun]

Why are we still talking about water coming out the top of tube 2? Have I failed to make it clear that it will not occur? What part is being misunderstood? I don't know how to explain it anymore because after every instance of describing why it won't come out the top, the reply is
"Well, won't it go through the top?"
"No, because the barrier at the top opposes this."
"Well, won't it go through the top though?"

No.
There are three situations.

A) Substance X does not mix with water, and is denser.
We might as well imagine that Substance X is a brick, or part of the shape of tube 2, since all it does is pointlessly fill space. If it will not mix with water, it cannot contribute to osmotic pressure or power your semi-permeable membrane pump. The water will fall out of tube 1 through the membrane until the surface of the water in both tubes are the same height. This is perhaps the most boring set of conditions, because there is only one force involved, gravity. Once the water is at the same height in both tubes, it just sits there.

B) Substance X does not mix with water, and is less dense.
This case is much like the example in A, with a minor change. Now instead of being a brick at the bottom of the tube, it's a brick floating on the water. This means it displaces its mass in water. That amount of water will remain in tube 1 instead of flowing through, making tube 1 have more water in it. Once that's balanced again, gravity stops the water from magically floating out the top. And even if you added more water, all it would do is push Substance X up against the membrane it can't go through. There is no osmotic pressure in this situation either, because Substance X does not mix.

C) Substance X is water-soluble, and its density is irrelevant.
This one is more interesting, because now we can use osmotic pressure.
At the start of our experiment, tube1 is full of water and tube 2 is less than full of Substance X. The water in tube 1 wants to satisfy gravity by flowing into tube 2 to equalize its height (into the extra space at the top of tube 2). The barrier at the bottom of tube 1 wants to satisfy osmotic pressure, by pushing water towards the greatest density of solute, which is in tube 2.
Water starts to flow into tube 2. Assuming the water in tube 1 is pure, osmotic pressure will be strong enough to overcome gravity and completely fill tube 2. When it hits the semi-permeable membrane at the top of tube 2, your perpetual motion machine stops working.
The barrier at the top of tube 2 wants to satisfy osmotic pressure by pumping water towards the greatest concentration of solute. Into, not out of, tube 2. The barrier at the top of tube 2 opposes the barrier at the bottom of tube 1 with the same amount of osmotic pressure, and both are cancelled. This leaves only gravity, which will prevent the water from magically floating out the top of tube 2.

You cannot convince water to fly just by dropping salt in it.

[elpenmaster]

The barrier at the top of tube 2 opposes the barrier at the bottom of tube 1 with the same amount of osmotic pressure, and both are cancelled. This leaves only gravity, which will prevent the water from magically floating out the top of tube 2.

why will bravity prevent this? this second semipermeable barrier is not on the top of tube two, but on the side. the water/substance x level will be higher than the semipermeable barrier. to water, the barrier is not really a barrier at all, so why cant it go through it, fall down into the water remaining in container 1, go over a paddlewheel on the way, and create enrgy? substance x would just sit in container 2, because it cannot get out through the semipermeable barrier

when all this is happening the water level in tube one has gone down from the top, to below semipermeable barrier number two, so the water in container two is falling into air, which has has less water than the water/substance x mixture, so the water will go into it

[Rkyeun]

===why will bravity prevent this? this second semipermeable barrier is not on the top of tube two, but on the side.
Ah. That seems to be our misunderstanding. If this opening is above tube1, as it must be to gain energy from it, then osmotic pressure will still hold the water inside tube2 instead of letting it fall out. At a certain point determined by the solubility of Substance X and its weight, it will cease to rise above the lower edge of the upper barrier.

===the water/substance x level will be higher than the semipermeable barrier.
It will raise slightly above the lower edge of the barrier. The osmotic pressure at the upper barrier will push it back into the tube, and gravity will pull it back down because now there's less in tube1 than in tube2.

===water, the barrier is not really a barrier at all, so why cant it go through it, fall down into the water remaining in container 1, go over a paddlewheel on the way, and create enrgy?
A semi-permeable membrane that is permeable by water but not by Substance X isn't a barrier to water, but it is a barrier to the solution formed by water and Substance X. The Substance X is not allowed through, and the osmotic pressure keeps the water with Substance X.

===substance x would just sit in container 2, because it cannot get out through the semipermeable barrier
Correct, and the water would stay with it.

===when all this is happening the water level in tube one has gone down from the top, to below semipermeable barrier number two
This is quite possible. Osmotic pressure can cause water to rise into tube2 until it hits the lower edge of the semi-permeable membrane.

===so the water in container two is falling into air, which has has less water than the water/substance x mixture, so the water will go into it
Err, no. The water in tube2 isn't coming out the membrane at the top. It isn't even rising much above it. Even if there was water in tube1 at that height, it would also be pumping water into tube2 where Substance X is.

[elpenmaster]

A semi-permeable membrane that is permeable by water but not by Substance X isn't a barrier to water, but it is a barrier to the solution formed by water and Substance X. The Substance X is not allowed through, and the osmotic pressure keeps the water with Substance X.

so is the solution prevents water from going throught the semipermeable barrier, because it wants to stay w/ substance x (but why?) then try substance x is insoluble with water. then, since x is denser than water, the water that gets into tube 2 will rise to the top, and be pure, clean water, with substance x just sitting on the bottom, and causing there to be a continuous draw of water into tube 2 (is this right?) then the clean water can overflow the second semipermeable barrier and go back into tube one----will water go through a semipermeable barrier into air? if not, why not?

[Rkyeun]

===so is the solution prevents water from going throught the semipermeable barrier, because it wants to stay w/ substance x (but why?)

Yes. Entrophy requires that substances become more mixed. Water wants to become more tainted for the same reason that eggs do not spontaneously unbreak and jump back onto the table. The laws of thermodynamics favor things being mixed, tainted, and broken, because it is easier to let things mix than to try to keep them seperated.

===then try substance x is insoluble with water. then, since x is denser than water, the water that gets into tube 2 will rise to the top, and be pure, clean water, with substance x just sitting on the bottom, and causing there to be a continuous draw of water

Stop. False. An insoluble substance cannot taint water, so the water is not drawn to it. If substance x is not soluble, then it does not mix with water, and there is no pressure to rise. Water simply falls into the chamber to equalize its own weight, and then stops without coming out the top or going up.

===into tube 2 (is this right?) then the clean water can overflow the second semipermeable barrier and go back into tube one----will water go through a semipermeable barrier into air? if not, why not?

Clean water only leaves the semi-permeable membrane if it is not in solution with something that can't go through the barrier. Since the blocked oil does not mix, the clean water can go through. If the substance was salt, then the water couldn't go through because it had salt in it. It's like tying a balloon to the ground with an anchor. The balloon can fly, but the anchor can't, so the balloon stays on the ground.