Boiling point elevation is the phenomenon where the boiling point of a liquid increases when a non-volatile solute is added. This occurs because the solute particles disrupt the solvent molecules, requiring more heat to reach the boiling point.
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Boiling point elevation is a colligative property, which means it depends on the number of solute particles and not their identity.
The formula for boiling point elevation is $\Delta T_b = i K_b m$, where $\Delta T_b$ is the boiling point elevation, $i$ is the van't Hoff factor, $K_b$ is the ebullioscopic constant, and $m$ is the molality of the solution.
Non-volatile solutes cause boiling point elevation because they reduce vapor pressure by occupying space at the surface of the liquid.
The van't Hoff factor ($i$) accounts for ionizable substances; for instance, NaCl has an $i$ value of approximately 2 because it dissociates into two ions.
Commonly observed in everyday life, adding salt to water raises its boiling point slightly.
Review Questions
What happens to the boiling point of a liquid when a non-volatile solute is added?
Explain how molality affects boiling point elevation.
How does the presence of a non-volatile solute change the vapor pressure of a solvent?
Properties that depend on the number of solute particles rather than their identity; includes boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.