Practice question

Natural osmosis describes spontaneous water flow from region of high water chemical potential, meaning low solute concentration, to low water potential, high solute, across semipermeable membrane until equilibrium. Reverse osmosis inverts this natural tendency by imposing mechanical energy. In purification devices impure feed solution with high total dissolved salts and industrial effluent contacts one side of thin film composite polyamide membrane with pores smaller than 0.1 nanometer that reject ions by charge repulsion and size exclusion. A high pressure pump applies hydrostatic pressure typically 15 to 70 bar exceeding inherent osmotic pressure defined by pi equals iCRT. When external pressure surpasses osmotic pressure, chemical potential gradient reverses and water forced opposite to spontaneous direction. Water leaves concentrated feed moving toward dilute permeate compartment producing fresh water while salts concentrated as brine rejected. This process relies on applied hydrostatic pressure overcoming colligative driving force. Therefore under reverse conditions water moves from high solute side to low solute side driven by external work input.