Boiling Point Elevation-Definition, Formula with Examples

In actual practice, the boiling point of a solution is affected by a boiling point elevation and a liquid head. As the vapor pressure of most aqueous solutions is less than that of water at any given temperature, the boiling point of the solution is higher than that of pure water at a given pressure.  

The difference between the boiling point of a solution and that of pure water at any given pressure is known as the boiling point rise/elevation of the solution. The boiling point elevation is a smell for dilute solutions and large for concentrated solutions of inorganic salts.

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Rule of Boiling Point Elevation

The boiling point elevation of strong solutions can be obtained from an empirical rule known as Duhrings' rule. It states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure. So when the boiling point of a solution is plotted against the boiling point of water, we get a straight line.

We get a straight line. Fig shows such a plot for an aqueous solution of caustic soda of various concentrations.

For strong solutions, the boiling point elevation is best found from an empirical rule known as the Duhrings rule which states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure. Thus if the boiling point of the solution is plotted against that of water at the same pressure a straight line results. Different lines are obtained for different concentrations. Over wide ranges of pressure, the rule is not exact, but over a moderate range, the lines are very nearly straight, though not necessarily parallel.

In fig. is a set of Duhrings rules for solutions of sodium hydroxide in water. The use of this figure may be illustrated by an example.

If the pressure over a 25 percent solution of sodium hydroxide is such that water boils at 180°F by reading up from the x-axis at 180°F to the line for 25 percent solution and then horizontally to the y axis it is found that the boiling point of the solution at this pressure is 200°F. The boiling point elevation for this solution at this pressure is therefore 20°F.

Boiling Point Elevation Formula

The boiling point of a solution carries a non-volatile solute that can not be extracted as 

BPE of solutions = BP of pure solvent + BPE 

The elevation in BP (∆Tь) is directly proportional to the concentration of the solute in the solutions.

 ∆Tь = v • Kь • m 

Where ∆Tь = is the boiling point elevation

v = is the vants Hoff factors

Kь = is the ebullioscopic constant

m = is the molality of the solute. 

Example of boiling point elevation

Ex. What is the boiling point elevation of the solution and driving force for heat transfer?

Data: The solution boils at a temperature of 380K and the boiling point of water at a pressure in the vapor space is 373K. The temperature of condensing is 399K.

Solution (Ans.) The boiling point of solution T = 380K

The boiling point of water T1 = 373K

Boiling point elevation = T - T1 = 380 - 373K

= 7 K ans.

Saturation temperature of condensing steam = Ts 

= 399 K 

= Ts - T = 399 - 380

= 19 K     Ans. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane