A plant for the desalination of sea water stops working due to which of the following reasons?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The pressure applied on the saline water was less than osmotic pressure.
Osmosis is the flow of solvent through a semipermeable membrane from pure solvent into a solution. It stops when a pressure equal to the osmotic pressure is applied on the solution side. If a pressure larger than the osmotic pressure is applied, the flow is reversed and pure water is pushed out of the solution: this is reverse osmosis, used to desalinate sea water.
If the applied pressure falls below the osmotic pressure of the sea water, the flow is no longer reversed. Water would instead pass into the sea water by ordinary osmosis, and the plant stops giving fresh water.
Option (2) The pressure applied on the saline water was greater than osmotic pressure. — Incorrect. A pressure greater than the osmotic pressure is the normal working condition of the plant, so it would keep running.
Option (3) The osmotic pressure application has no significance. — Incorrect. The osmotic pressure sets the minimum pressure needed for reverse osmosis; it is the deciding factor.
Option (4) Salinity of water decreased and osmotic pressure was more than pressure applied. — Incorrect. Osmotic pressure is proportional to the concentration of solute, so a decrease in salinity lowers the osmotic pressure. That makes reverse osmosis easier, not harder.
NCERT (2026-27 edition):
Reverse Osmosis and Water Purification
