Question

Given below are two statements:
Statement I: Osmotic pressure is always negative.
Statement II: Osmotic potential is also always negative.
In light of the above statements, choose the correct answer from the options given below

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Remember this simple relationship:
- Osmotic Pressure (\(\pi\)): A positive force (\(+\)).
- Osmotic Potential (\(\psi_s\)): A negative potential (\(-\)).
The two values are equal in magnitude but opposite in sign: \(\psi_s = -\pi\).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Osmotic pressure and osmotic potential (solute potential) are thermodynamic parameters used to describe the water relations of solutions and living plant cells.
While they describe the same physical phenomenon (the effect of dissolved solutes on water movement), they are defined differently and use opposite signs.

Step 2: Detailed Explanation:
Let us analyze both statements in detail:
- Statement I: "Osmotic pressure is always negative."
This statement is incorrect.
Osmotic pressure (\(\pi\)) is defined as the external physical pressure that must be applied to a solution to prevent the inward flow of water across a semipermeable membrane.
Because it represents a physical, positive pushing force, osmotic pressure is always expressed as a positive value:
\[ \pi > 0 \quad (\text{for any solution containing solutes}) \] Pure water has an osmotic pressure of zero.
- Statement II: "Osmotic potential is also always negative."
This statement is correct.
Osmotic potential (\(\psi_s\)), also known as solute potential, is a measure of the reduction in water potential caused by the addition of solute molecules.
Pure water has the maximum possible water potential, which is defined as zero (\(\psi = 0\)).
When solutes are dissolved in water, they bind with water molecules, reducing their free energy and ability to move. This lowers the water potential below zero.
Therefore, the osmotic potential of any solution is always negative:
\[ \psi_s < 0 \] The relationship between the two is given by:
\[ \psi_s = -\pi \] Thus, Statement I is false, but Statement II is true.

Step 3: Final Answer:
The correct option is (D).