Colligative effects of a non-volatile solute: Adding a non-volatile solute to a solvent causes which primary change in the solvent at a given temperature?

Introduction / Context:Colligative properties arise from solute particle count rather than identity. Recognizing the first-principles effect—vapor pressure lowering—helps explain boiling point elevation and freezing point depression that follow from phase equilibria shifts.

Given Data / Assumptions:

• Solute is non-volatile and does not associate/dissociate strongly.
• Temperature initially fixed while assessing immediate effect.

Concept / Approach:Raoult’s law for the solvent: P_solvent = x_solvent · P°_solvent. Adding non-volatile solute reduces x_solvent, thereby lowering the solvent’s equilibrium vapor pressure. The reduced vapor pressure leads to boiling point elevation (higher T needed for P_vap = P_ext) and freezing point depression (liquid phase stabilized), but those require temperature change to observe.

Step-by-Step Solution:Add solute → x_solvent decreases.By Raoult’s law, P_solvent decreases at the same T.Therefore, the immediate, primary change is vapor pressure lowering.

Verification / Alternative check:Boiling point elevation and freezing point depression equations (ΔT_b = K_b · m, ΔT_f = K_f · m) are consequences of the vapor pressure change and phase equilibrium, supporting this cause–effect chain.

Why Other Options Are Wrong:Freezing point elevation and boiling point depression are reversed in sign; claiming all as listed is incorrect; surface tension behavior is system-specific and not a defining colligative property.

Common Pitfalls:Confusing the primary at-constant-T effect (vapor pressure) with the induced temperature shifts.

Final Answer:vapor pressure lowering