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There are two main questions I'm struggling to get my head around.

  1. What exerts osmotic pressure? Is it the molecules of the solvent (water) or solute particles (sugar) ?
  2. Where is it exerted? On the semipermeable membrane?

If we imagine the U-shaped tube scenario, does the osmotic pressure become zero when the the height of water in both tubes stop changing?

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Osmotic pressure has two definitions. First it is a physical phenomena, namely the movement of water through a semipermeable membrane by osmosis to dilute a concentrated solution. It is also a numerical value, and it is the highest level attained by water having crossed the membrane by osmosis.

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By definition osmotic pressure is the pressure that we need to exert on solution so that no osmosis occurs.

So osmotic pressure is applied by solution on piston.

Osmotic pressure is defined when both liquids have same height so it would be meaningless to define osmotic pressure when both liquids have different heights.

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    $\begingroup$ But, if both arem contain liquids of the same height and no external pressure, there is no osmotic pressure. Osmotic pressure is measured by looking at the different heiights. $\endgroup$
    – matt_black
    Commented Feb 9 at 17:51
  • $\begingroup$ @matt_black if I want to measure osmotic pressure of 5% solution and some solvent flows in solution arm then my solution would no longer be 5%. Measuring height and taking into account the dilution is very tedious. So better way is to keep solution and solvent arm at same height and measuring how much pressure I need to keep it so. $\endgroup$
    – Harjot Dhillon
    Commented Feb 10 at 3:54
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    $\begingroup$ @matt_black This is the more general answer. It accommodates the fact that solution composition changes with the dilution occurring due to the pressure difference if flow is allowed, and can therefore consider nonideal conditions. $\endgroup$
    – Buck Thorn ♦
    Commented Feb 10 at 16:45
  • $\begingroup$ @HarjotDhillon Yes, that might be more convenient, but your last sentence claims "Osmotic pressure is defined when both liquids have same height" which is not correct. You are talking about a convenient way to measure the pressure not its definition. $\endgroup$
    – matt_black
    Commented Feb 10 at 21:37

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