Which of the following best explains why colligative properties are affected differently when a volatile solute is used compared to a nonvolatile solute?

• A. A volatile solute decreases the solubility of the solvent, reducing colligative properties.
• B. A volatile solute contributes to the vapor pressure of the solution, altering properties such as boiling point elevation and freezing point depression.
• C. A volatile solute increases the number of ions in solution, which enhances colligative effects.
• D. A volatile solute forms a precipitate, which prevents colligative properties from being observed.

Answer: (B)

Verified step by step guidance

1. Understand that colligative properties depend on the number of solute particles in a solution, not their identity, and typically assume the solute is nonvolatile (does not evaporate).
2. Recognize that a nonvolatile solute lowers the vapor pressure of the solvent by simply adding particles that do not escape into the gas phase, leading to predictable changes in boiling point and freezing point.
3. Consider that a volatile solute can evaporate and thus contributes to the total vapor pressure of the solution, unlike a nonvolatile solute which does not.
4. Analyze how the presence of a volatile solute changes the vapor pressure in a way that affects colligative properties differently, because the solute's own vapor pressure adds to the solvent's vapor pressure.
5. Conclude that the key difference is that a volatile solute alters the vapor pressure directly, which modifies boiling point elevation and freezing point depression differently than a nonvolatile solute would.