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16.14: Boiling Point Elevation

Last updated

Mar 21, 2025
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- 16.13: Freezing Point Depression
- 16.15: Electrolytes and Colligative Properties

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Salt is often added to boiling water when preparing spaghetti or other pasta. One reason is to add flavor to the food. Some people believe that the addition of salt increases the boiling point of the water. Technically, they are correct, but the increase is rather small. You would need to add over 100 grams of $\ce{NaCl}$ to a liter of water to increase the boiling point by a couple of degrees, which is likely unhealthy.

Boiling Point Elevation

The figure below shows the phase diagram of a solution and the effect that the lowered vapor pressure has on the boiling point of the solution compared to the solvent. In this case, the solution has a higher boiling point than the pure solvent. Since the vapor pressure of the solution is lower, more heat must be supplied to the solution to bring its vapor pressure up to the pressure of the external atmosphere. The boiling point elevation is the difference in temperature between the boiling point of the pure solvent and that of the solution. On the graph, the boiling point elevation is represented by $\Delta T_b$ .

Graph showing vapor pressure vs. temperature. Two curves: vapor pressure of the pure solvent and vapor pressure of the solution. Arrows indicate solvent and solution boiling points. — Figure $\PageIndex{1}$ : The lowering of the vapor pressure in a solution causes the boiling point of the solution to be higher than that of the pure solvent.

The magnitude of the boiling point elevation is also directly proportional to the molality of the solution. The equation is:

$\Delta T_b = K_b \times \textit{m}\nonumber$

The proportionality constant, $K_b$ , is called the molal boiling-point elevation constant. It is a constant that is equal to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute. For water, the value of $K_b$ is $0.512^\text{o} \text{C}/\textit{m}$ . So, the boiling temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is $100.512^\text{o} \text{C}$ .

Summary

Boiling point elevation is the difference in temperature between the boiling point of the pure solvent and that of the solution.
The molal boiling-point elevation constant is equal to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute.
Calculations involving the molal boiling point elevation constant are outlined.

Boiling Point Elevation

Summary

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