Isotonic solutions and colligative properties:\nTwo solutions are isotonic when they exhibit the same value of which property at the same temperature?

Introduction / Context:
“Isotonic” solutions are fundamental in bioprocessing, medicine, and membrane science. The term refers to two solutions that exert the same osmotic effect across a semipermeable membrane at a given temperature, preventing net solvent flow. This concept is distinct from equality of concentration units like molarity or normality because solute dissociation and activity play major roles.

Given Data / Assumptions:

• Comparison at the same temperature.
• Semipermeable membrane permeable to solvent but not solute.
• Possible electrolytes where van’t Hoff factor i > 1 due to dissociation.

Concept / Approach:
For dilute solutions, osmotic pressure follows the van’t Hoff relation: π = i C R T, where i is the van’t Hoff factor, C is molar concentration of solute particles, R is the gas constant, and T is absolute temperature. Two solutions are isotonic if their osmotic pressures are equal (π_1 = π_2). Equal molarity alone does not guarantee equal π when solutes differ in dissociation (different i) or when activity coefficients deviate from unity.

Step-by-Step Solution:
Define isotonic: equal osmotic pressure at the same temperature and across a given membrane.Relate to measurable quantities: π ∝ i C T; therefore equal π may arise from different C values depending on i.Conclude: the defining property is osmotic pressure, not viscosity, molarity, or critical temperature.

Verification / Alternative check:
Physiological saline (~0.9% NaCl) is isotonic to human blood plasma because its effective particle concentration (accounting for NaCl dissociation) yields an osmotic pressure similar to plasma—even though molar concentrations of individual species differ.

Why Other Options Are Wrong:
Viscosity: depends on interactions and size; not a colligative criterion.Molar concentration: ignores dissociation and activity effects.Critical temperature: a pure-component property, irrelevant to isotonicity.

Common Pitfalls:
Assuming equal normality equals isotonicity; forgetting electrolytes increase particle count via dissociation.

Final Answer:
osmotic pressure