What is the elevation of boiling point?

What is the elevation of boiling point?

Boiling point elevation is often referred to the increase in the boiling point of a solvent that is calculated upon the addition of a solute. In the process when a non-volatile solute is added to a solvent, the resulting solution will always have a higher boiling point than when it is to be compared with a pure solvent the simple daily example we can take is that when we mix salt in water the boiling point of that solution is higher than of normal water.

Boiling point elevation is defined as a colligative property of matter, whichmeans it is dependent on the solute-to-solvent ratiobut it is not dependent on the solute’s identity. This will also imply that the elevation in the boiling point of a solution will always depend on the amount of solute that is added to it.  This will also mean that the greater the concentration of solute in the solution, the greater will be the boiling point elevation.

It is also important to understand here that when the solute is dissolved in the solution, it also tend to change a few characteristics of the resultant solution as well. This will not only affect the boiling point but can also lead to the changes in color and the state of the substance as well.

Why Does Boiling Point Elevation Occur?

The boiling point of a liquid is said to be the temperature at which its vapour pressure will be equal to the pressure of its surrounding environment. In case of the Non-volatile substancesthey do not readily undergo evaporation and that is also one of the reason that they have very low vapour pressures that is usually assumed to be zero. In this case when we add a non-volatile solute tothe solvent, the vapour pressure of the resulting solution will always be lower than that of the pure solvent.

This can also be the reason that a greater amount of heat must be supplied intothe solutionso as to make it boil. This leads to the increase in the boiling point of the solution is often defined as the boiling point elevation. When there is an increase in the concentration of added solutethat is accompanied by a more decrease in the vapour pressure of the solution. This can even lead to a further elevation in the boiling point of the solution.

Note: The important thing to note here is that the boiling point of a liquid will always be dependent on the pressure of its surroundings. This is also said to be the reason why water boils at temperatures lower than 100oC at high altitudes just because the surrounding pressure is low.

Boiling Point Elevation Formula

The boiling point of a solution that contains a non-volatile solute can be defined as follows:

Boiling point of solution = boiling point of pure solvent + boiling point elevation (TV)

The elevation in boiling point is known to be proportional to the concentration of the solute in the solution. The equation is usually represented as:

ΔTb = i*Kb*m


  • i is called the  Van’t Hoff factor
  • Kb isdenoting  theebullioscopic constant
  • m is said to be  the molality of the solute

The most important thingto note that this formula will become less precise when the concentration of the solute is at a very high level. At the same time the formula will not t hold true for volatile solvents.

The ebullioscopy constant (Kb) can often expressed in terms of oC/molal or oC.kg.mol-1. The table below shows few of the  values of Kb for some common solvents.

Solvent Kb value (in oC.kg.mol-1)
Water 0.512
Phenol 3.04
Acetic Acid 3.07
Chloroform 3.63
Benzene 2.53

The formula of boiling point elevation can also be used so as to calculate the degree of dissociation of the solute and the molar mass of the solute.

The Relationship between Boiling Point Elevation and Vapor Pressure

There exists a relationship between the vapour pressure and the boiling point of the solutions. Vapor pressure is oftendefined as the pressure that is exerted at a given temperature by the vapor in. this is often seen in case of a thermodynamic equilibrium with its condensed phases. It is simply defined as the measure of the ability of the solvent molecules in the normal environment. The vapour pressure plays a very important role in the boiling point elevation because if the pressure is low in that case the boiling point may also come down.

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