Reverse Osmosis And Semipermeable Membranes

Reverse osmosis (RO) is the process of forcing solvent (water) through a semipermeable membrane from the concentrated solution side (sea water) to the dilute side (pure water) by applying external pressure exceeding the osmotic pressure. Normal osmosis occurs when water flows spontaneously from pure water to sea water (low to high concentration). To reverse this flow, the applied pressure must exceed the osmotic pressure of the sea water. Since π = 27 atm, the minimum external pressure needed is > 27 atm. Under this applied pressure, water molecules in sea water are forced through the semipermeable membrane toward the pure water side — this is option A, which is correct. Option B incorrectly states the direction: in reverse osmosis, the solvent moves from the concentrated (sea water) side to the dilute (pure water) side, not the other way. Option C is incorrect because applying only 13.5 atm (less than the osmotic pressure) would not overcome the natural osmotic tendency and would cause normal osmosis. Option D is wrong because membranes are selective and do not pass water equally in both directions without a driving force. This concept is covered in NCERT Chapter 2 and is a critical application of osmosis in industrial water desalination. Plausibility check: desalination plants typically operate at 55–80 atm, well above the 27 atm osmotic pressure of sea water, consistent with needing pressure above π.