Which statement best describes colligative properties and two examples?

• A. Colligative properties depend on the number of solute particles; examples: boiling-point elevation and freezing-point depression.
• B. Colligative properties depend on the chemical identity of solute; examples: boiling point and freezing point depression.
• C. Colligative properties are properties of the solvent only; examples: density and refractive index.
• D. Colligative properties depend on temperature alone; examples: boiling point and osmotic pressure.

Answer: (A)

Colligative properties depend on the number of solute particles present in a solution, not on what those particles are. That’s why boiling-point elevation and freezing-point depression are classic examples: the more solute particles you dissolve, the greater the impact on the solvent’s phase behavior.

When a nonvolatile solute is added, it lowers the solvent’s vapor pressure. To reach the external pressure and boil, the solution must be heated to a higher temperature, so the boiling point rises. The extent of that rise grows with the number of dissolved particles (and, for electrolytes, with the van’t Hoff factor that accounts for how many particles a dissolved formula unit yields). Similarly, solute particles interfere with the formation of the solid solvent lattice, so the solution must be cooled to a lower temperature to freeze; this freezing-point depression also scales with the particle count.

Other properties like density or refractive index are influenced by solute identity and interactions, not solely by how many particles are present, so they aren’t the defining examples of colligative behavior.