Osmosis: spontaneous flow of solvent through a semipermeable membrane from dilute to concentrated solution. Osmotic pressure (pi): minimum external pressure on the concentrated side to prevent osmosis. pi = iMRT (van't Hoff equation). M = molarity, R = 0.0821 L.$\frac{atm}{mol.K}$. Osmotic pressure is the most sensitive colligative property — preferred for high-molar-mass solutes (proteins, polymers) because even small concentrations give measurable pi. Isotonic solutions: equal pi (0.9% NaCl is isotonic with blood). Hypertonic: higher pi $\frac{causes crenation}{plasmolysis}$. Hypotonic: lower pi (causes haemolysis). Reverse osmosis: applying P > pi forces solvent from concentrated to dilute side — used for desalination.