The reverse osmosis occurs when
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Pressure on solution side is larger than osmotic pressure
When a solution and the pure solvent are separated by a semipermeable membrane, solvent molecules flow through the membrane into the solution: this is osmosis. The osmotic pressure is the extra pressure that must be applied on the solution side to just stop this flow.
Reverse osmosis needs a pressure greater than this. In desalination plants the membrane is a film of cellulose acetate, which lets water through but holds back the dissolved ions.
Why the other options are incorrect
Option (2) Pressure on solution side is smaller than osmotic pressure — Incorrect. The applied pressure is not enough to stop osmosis, so solvent still flows into the solution, only more slowly.
Option (3) Pressure on solution side is equal to osmotic pressure — Incorrect. This is the very pressure that just stops the flow: there is no net movement of solvent either way.
Option (4) Pressure on solution side is equal to atmospheric pressure — Incorrect. No extra pressure is applied on the solution, so ordinary osmosis takes place.
NCERT (2026-27 edition):
Reverse Osmosis and Water Purification – “The direction of osmosis can be reversed if a pressure larger than the osmotic pressure is applied to the solution side. That is, now the pure solvent flows out of the solution through the semi permeable membrane. This phenomenon is called reverse osmosis and is of great practical utility. Reverse osmosis is used in desalination of sea water.”