In the following diagram, to prevent osmosis, if excess pressure is applied on one side of the semipermeable membrane (SPM), then I, II, III and IV are, respectively,
(A) Solution
(B) \( P_{atm} \)
(C) \( P_{atm}+\Pi \)
(D) Solvent
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (B), (A), (D)
In osmosis the solvent flows through the semipermeable membrane from the pure solvent into the solution. The flow can be stopped by pressing on the solution side with an extra pressure equal to the osmotic pressure, \( \Pi \). So the solution side carries \( P_{atm} + \Pi \) and the solvent side only the atmospheric pressure \( P_{atm} \).
In the diagram, piston I presses on the liquid in vessel III and piston II on the liquid in vessel IV:
I is the piston with the extra pressure: \( P_{atm} + \Pi \), (C)
II is open to the atmosphere only: \( P_{atm} \), (B)
III, under the extra pressure, holds the solution: (A)
IV holds the solvent: (D)
Applying more than \( \Pi \) on the solution side would push the solvent the other way, which is reverse osmosis, used to desalinate sea water.
NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – “… The osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis, i.e., to stop the passage of solvent molecules through a semipermeable membrane into the solution. This is illustrated in Fig. 1.10.”
