ACS General Chemistry 2 Practice Exam

If a nonvolatile solute is added to a solvent, what happens to the freezing point?

Freezing point increases Freezing point decreases Freezing point stays the same Freezing point becomes undefined

Correct Answer:

Freezing point decreases

Explanation:

This question tests how adding a nonvolatile solute affects the freezing point. Dissolving particles in a solvent lowers the solvent’s mole fraction and lowers its vapor pressure, which shifts the equilibrium between the solid and liquid to require a lower temperature for freezing. In other words, the solution’s liquid phase remains stable down to a lower temperature because the dissolved particles disrupt the orderly formation of the solid. This freezing-point depression is a classic colligative property and scales with the number of dissolved particles (ΔTf = iKf m for ideal solutions). So, adding a nonvolatile solute causes the freezing point to decrease.

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