Boiling point elevation

Definition

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.

5 Must Know Facts For Your Next Test

1. Boiling point elevation is a colligative property, which means it depends on the number of solute particles and not their identity.
2. 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.
3. Non-volatile solutes cause boiling point elevation because they reduce vapor pressure by occupying space at the surface of the liquid.
4. 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.
5. 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?

Related terms

Freezing Point Depression: A colligative property where adding a solute to a solvent decreases its freezing point.

Vapor Pressure Lowering: A decrease in vapor pressure of a solvent due to the addition of a non-volatile solute.

Colligative Properties: 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.