Q.10 The osmotic potential (ψ) of pure water is _____ MPa.
(A) -1
(B) 0
(C) 0.1
(D) 10
Osmotic Potential of Pure Water: 0 MPa Explained
The osmotic potential of pure water is 0 MPa, making option (B) the correct answer in this multiple-choice question.
Correct Answer
Osmotic potential, denoted as ψ_s or ψ_π, measures the effect of solutes on water’s free energy in a solution. Pure water contains no solutes, so its osmotic potential is zero (0 MPa) by definition. This serves as the reference point; any solution with solutes has a negative osmotic potential.
Option Explanations
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(A) -1 MPa: Incorrect. A negative value indicates solutes reducing water potential below that of pure water, as in hypotonic solutions. Pure water lacks solutes, so ψ cannot be negative.
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(B) 0 MPa: Correct. With zero solute concentration, no osmotic effect exists, setting ψ at 0 MPa under standard atmospheric pressure.
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(C) 0.1 MPa: Incorrect. Positive values do not apply to osmotic potential, which is always zero or negative relative to pure water. This might confuse it with pressure potential.
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(D) 10 MPa: Incorrect. High positive values relate to pressure potential (ψ_p) in turgid cells, not osmotic potential in pure water.
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Key Concepts
Water potential (ψ_w) combines osmotic potential (ψ_s) and pressure potential (ψ_p): ψ_w = ψ_s + ψ_p. For pure water at atmospheric pressure, both ψ_w and ψ_s are 0 MPa. Solutes lower ψ_s (e.g., -0.5 MPa in dilute solutions), driving osmosis from high to low potential areas. This principle explains water movement in plants, soil, and cells.
