Elevation of boiling point

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Boiling-point elevation describes the phenomenon that the boiling point of a liquid (a solvent) will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent.

The extent of boiling-point elevation can be calculated by applying Clausius–Clapeyron relation and Raoult's law together with the assumption of the non-volatility of the solute. The result is that in dilute ideal solutions, the extent of boiling-point elevation is directly proportional to the molal Concentration of the solution according to the equation

ΔT_b = K_b • b_B

where

ΔT_b, the boiling point elevation, is defined as T_{b (solution)} - T_{b (pure solvent)}.

K_b, the ebullioscopic constant, which is dependent on the properties of the solvent. It can be calculated as:

K_b = RT_b²M/ΔHv, where

R is the gas constant,

T_b is the boiling temperature of the pure solvent [in K],

M is the molar mass of the solvent,

ΔH_v is the heat of vaporization per mole of the solvent.

b_B is the molality of the solution, calculated by taking dissociation into account since the boiling point elevation is a colligative property, dependent on the number of particles in solution.

Example:

The boiling point of salted water is higher than the boiling point of pure water.

Factors affect boiling point

The boiling point of a liquid depends on following factors:

temperature,

atmospheric pressure and

the vapor pressure of the liquid.

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