[gear2d]

I was wondering how can I tell the difference if fluid is moving due to osmotic pressure, or hydrostatic pressure?

If I have a U-tube that separates right and left with a semipermeable membrane, and I poor water in on the left side, that hydrostatic will force water to the right side then (high pressure -> low pressure)?

Now if I add a salt to the left side, then osmotic pressure will force some of the water on the right side to go to the left side?

[nj_bartel]

Hydrostatic pressure only occurs due to weight and the pressure due to that weight.  For example, you have a U-tube, both sides equally filled with an semipermeable membrane in between.  You pour water into one side, and it'll be very temporarilly higher, but due to hydrostatic pressure (gravity basically) the two sides will even out.  Now if that same semipermeable membrane isn't permeable to Na⁺ or Cl^- ions, and you add NaCl to one side of the U-tube, water will shift to that side.

[gear2d]

Hydrostatic pressure only occurs due to weight and the pressure due to that weight.  For example, you have a U-tube, both sides equally filled with an semipermeable membrane in between.  You pour water into one side, and it'll be very temporarilly higher, but due to hydrostatic pressure (gravity basically) the two sides will even out.  Now if that same semipermeable membrane isn't permeable to Na⁺ or Cl^- ions, and you add NaCl to one side of the U-tube, water will shift to that side.

What your are saying is that after hydrostatic has made the level of water on both sides equal, that adding a salt will make osmotic pressure take over the flow of water in the direction where more of the solute is. Then the system will come to a dynamic equilibrium when osmotic pressure = hydrostatic pressure.

[nj_bartel]

Yes