Isotonic solutions must have the same
(A) Solute
(B) Density
(C) Elevation in boiling point
(D) Depression in freezing point
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C) and (D) only
Two solutions with the same osmotic pressure at a given temperature are called isotonic. Osmotic pressure is a colligative property: \( \pi = iCRT \), so at the same temperature isotonic solutions have the same effective concentration of solute particles, \( iC \). All colligative properties depend only on the number of solute particles in a given amount of solvent, so in the same solvent the other colligative properties are also the same.
(A) Solute — Incorrect. The solutes can differ; only the particle concentration must match. For example, 0.1 M glucose and 0.1 M urea are isotonic, and so are 0.05 M NaCl (which gives 2 ions, i = 2) and 0.1 M glucose.
(B) Density — Incorrect. Density depends on the mass and nature of the solute, not on the number of particles, so isotonic solutions can have different densities.
(C) Elevation in boiling point — Correct. \( \Delta T_b = iK_bm \); with the same particle concentration in the same solvent, the elevation is the same.
(D) Depression in freezing point — Correct. \( \Delta T_f = iK_fm \) is the same for the same reason.
NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – “… Two solutions having same osmotic pressure at a given temperature are called isotonic solutions. When such solutions are separated by semipermeable membrane no osmosis occurs between them.”