by Rkyeun » Sun Jan 23, 2005 11:23 pm
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.