Colligative Properties II

This setup illustrates the measurement of osmotic pressure by identifying the exact external force required to counteract natural solvent migration.

• The Barrier: A semipermeable membrane separates pure solvent from a solution, allowing only small solvent molecules to pass while blocking larger solute particles.
• The Process: Solvent naturally moves toward the solution side to equalize concentration, but an external pressure ($P_i$) is applied via a piston to resist this flow.
• The Equilibrium: When $P_i$ is exactly enough to stop the flow (net movement = 0), that value is defined as the osmotic pressure ($\Pi$) of the solution.
• The Threshold: This state is the "tipping point"; if $P_i$ increases further, it forces solvent back through the membrane, initiating reverse osmosis.