Practice question

Osmotic pressure is a colligative property describing tendency of water to move across semipermeable membranes that allow solvent but restrict solute permeation. When compartment containing dilute solution is separated from concentrated solution, water chemical potential is higher where solute activity is lower. Spontaneous diffusion drives water toward higher solute until chemical potentials equalize, causing volume increase and hydrostatic pressure rise on concentrated side. The additional hydrostatic pressure required to stop net influx and hold volumes constant at equilibrium defines osmotic pressure. Thermodynamically it can be approximated by van't Hoff equation pi equals i times M times R times T, where i is van't Hoff factor for dissociation, M molarity, R gas constant, T absolute temperature. It depends only on total number of solute particles, not identity. This concept explains cellular swelling, water absorption in roots, and clinical use of isotonic solutions. Reverse osmosis applies pressure exceeding pi to invert flow. It is not force exerted by water passing through membrane nor pressure generated by ion channels, but prevention pressure balancing osmotic tendency.