Difference Between Freezing Point Depression and Boiling Point Elevation

Key Difference – Freezing Point Depression vs Boiling Point Elevation
 

Freezing point depression causes a solution to freeze at a lower temperature than the freezing point of the pure solvent due to the addition of solutes. Boiling point elevation causes a solution to boil at a higher temperature than the boiling point of the pure solvent due to the addition of solutes. Therefore, the key difference between freezing point depression and boiling point elevation is that the freezing point depression decreases the freezing point of a solution whereas boiling point elevation increases the boiling point of a solution.

Freezing point depression and boiling point elevation are colligative properties of matter. This means that they depend only on the amount of solutes, not on the nature of solute.

CONTENTS

1. Overview and Key Difference
2. What is Freezing Point Depression
3. What is Boiling Point Elevation
4. Side by Side Comparison – Freezing Point Depression vs Boiling Point Elevation in Tabular Form
5. Summary

What is Freezing Point Depression?

Freezing point depression is the decrease of the freezing point of a solvent due to the addition of a solute into the solvent. It is a colligative property. This means freezing point depression depends only on the amount of solutes, not on the nature of solute. When freezing point depression has occurred, the freezing point of the solvent decreases to a lower value than that of the pure solvent. The freezing point depression is the reason why sea water remains in the liquid state even at 0°C (the freezing point of pure water). The freezing point depression can be given as below.

ΔT_f = T_f(solvent) – T_f(solution)

Or

ΔT_f = K_fm

In this,

•    ΔT_f is the freezing point depression,
•    T_f(solvent) is the freezing point of pure solvent
•     T_f(solution)  is the freezing point of the solution (solvent + solutes)
•     K_f is the freezing point depression constant
•     m is the molality of the solution.

However, the added solute should be a non-volatile solute, if not the solute does not affect the freezing point of the solvent because it gets volatilized easily. Not only for solutions, but this concept can also be used to explain the changes in freezing point of solid mixtures. The finely powdered solid compound has a lower freezing point than the pure solid compound when impurities are present (solid-solid mixture).

The freezing point is the temperature at which the vapor pressure of a solvent and the vapor pressure the solid form of that solvent is equal. If a non-volatile solute is added to this solvent, the vapor pressure of the pure solvent decreases. Then the solid form of the solvent can remain in equilibrium with the solvent even at lower temperatures than the normal freezing point.

What is Boiling Point Elevation?

Boiling point elevation is the increase of the boiling point of a solvent due to the addition of a solute into the solvent. Here, the boiling point of the solution (after addition of solutes) is higher than that of the pure solvent. Hence, the temperature at which the solution starts to boil is higher than that of the usual temperature.

However, the added solute should be a non-volatile solute, or else, the solute will volatilize rather than dissolving in the solvent. Boiling point elevation is also a colligative property so that it only depends on the amount of solutes (not on the nature of solute).

ΔT_b = T_b(solvent) – T_b(solution)

Or

ΔT_b = K_bm

In this,

•      ΔT_b is the boiling point elevation
•      T_b(solvent) is the boiling point of pure solvent
•      T_b(solution)  is the boiling point of the solution (solvent + solutes)
•      K_b is the boiling point elevation constant
•      m is the molality of the solution

A common example of this phenomenon is the boiling point of an aqueous salt solution. A salt solution boils at higher temperatures than 100°C (boiling point of pure water).

What is the Difference Between Freezing Point Depression and Boiling Point Elevation?

Summary – Freezing Point Depression vs Boiling Point Elevation

Freezing point depression and boiling point elevation are two major colligative properties of matter. The difference between freezing point depression and boiling point elevation is that freezing point depression decreases the freezing point of a solution whereas boiling point elevation increases the boiling point of a solution.

Reference:

1.Helmenstine, Anne Marie. “What Boiling Point Elevation Is and How It Works.” ThoughtCo. Available here    
2.“Boiling-Point Elevation.” Wikipedia, Wikimedia Foundation, 13 Mar. 2018. Available here 
3.“Freezing Point Depression.” Chemistry LibreTexts, Libretexts, 19 Feb. 2018. Available here

Image Courtesy:

1.’Freezing point depression and boiling point elevation’By Tomas erderivative work (CC BY-SA 3.0) via Commons Wikimedia