Which of the following is not a colligative property of solutions?

• A. pH
• B. Freezing point
• C. Osmotic pressure
• D. Vapor pressure

Answer: (A)

Colligative properties depend on how many solute particles are present in solution, not on what those particles are. This is why freezing point lowers, vapor pressure drops, and osmotic pressure rises in proportion to the number of dissolved particles (with the appropriate i factor for electrolytes). For example, freezing point depression follows ΔTf = iKf m, vapor pressure lowering follows Raoult’s law (P_solution = X_solvent P°), and osmotic pressure is given by π = iCRT. Hydrogen ion concentration, reflected in pH, does not follow this pattern because it is governed by acid–base equilibria and activity of ions, which depend on dissociation behavior and chemical interactions rather than just particle count. So pH is not a colligative property.