The Kirkwood Law

The Kirkwood Law states that adding a solvent to a solution generally decreases the solute's vapor pressure, meaning the liquid evaporates less quickly. This occurs because the solvent molecules occupy surface space, reducing the number of surface molecules of the solute that can escape into the air. As a result, the solution’s overall vapor pressure drops compared to the pure solvent. This principle helps explain why solutions with more solvent components tend to evaporate more slowly than pure solvents, and it's fundamental in understanding colligative properties like boiling point elevation and freezing point depression.