Boiling-point elevation

Boiling-point elevation refers to the phenomenon where the boiling point of a liquid (a solvent) increases when a solute is added, forming a solution. This effect is a colligative property, meaning it depends on the number of solute particles in the solvent, not the identity of the solute itself. Boiling-point elevation is an important concept in both chemistry and physics, with practical applications ranging from cooking to industrial processes.

Overview

When a non-volatile solute is dissolved in a solvent, it causes the vapor pressure of the solvent to decrease. According to Raoult's law, the vapor pressure of a solvent above a solution is directly proportional to the mole fraction of the solvent in the solution. As the vapor pressure of the solvent decreases, more energy (heat) is required to make the solvent molecules escape into the gas phase, which results in an increase in the boiling point of the solution compared to the pure solvent.

The boiling-point elevation (\(\Delta T_b\)) can be quantitatively described by the equation:

\[\Delta T_b = i \cdot K_b \cdot m\]

where:

• \(i\) is the van 't Hoff factor, which represents the number of particles the solute splits into or forms in the solution.
• \(K_b\) is the ebullioscopic constant of the solvent, a property that depends on the nature of the solvent.
• \(m\) is the molality of the solution, defined as the number of moles of solute per kilogram of solvent.

Applications

Boiling-point elevation has several practical applications. For example, it is the principle behind adding salt to water when boiling pasta. The salt increases the boiling point of the water, allowing the pasta to cook at a higher temperature. In colder climates, salts are used to lower the freezing point of water (freezing-point depression) on roads, which is a related colligative property. In industrial settings, boiling-point elevation is used in processes such as distillation and in the design of antifreeze solutions for engines.

Factors Affecting Boiling-Point Elevation

Several factors can affect the boiling-point elevation of a solution:

• The nature of the solvent: Different solvents have different ebullioscopic constants (\(K_b\)).
• The concentration of the solute: Higher concentrations of solute particles increase the boiling point.
• The identity of the solute: While the type of solute does not directly affect the boiling point elevation, it influences the van 't Hoff factor (\(i\)), especially if the solute dissociates or associates in the solution.

Examples

An example of boiling-point elevation can be observed by adding table salt (sodium chloride, NaCl) to water. Sodium chloride dissociates into sodium (Na\(^+\)) and chloride (Cl\(^-\)) ions, effectively doubling the number of solute particles in the solution and causing a noticeable increase in the boiling point of the water.

See Also

• Colligative properties
• Freezing-point depression
• Raoult's law
• Vapor pressure

References

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  Diagram showing freezing point depression and boiling point elevation

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  Room of freedom in pure compounds

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  Room of freedom in impure compounds