When you add a solute to a solvent, it raises its boiling point. The boiling point of a mixture containing a non-volatile solute is greater than that of the pure solvent. The solute-to-solvent ratio determines the elevation of a boiling point but not the identity of the solute. Thus, the amount of solute added to a solution determines how high the boiling point of the solution rises. In other words, the boiling point increases in proportion to the solute concentration in the solution. 

Types of Colligative Properties

• Osmotic pressure
• Relative lowering of vapour pressure
• Elevation in boiling point
• Depression in freezing point.

Elevation in Boiling Point

The vapour pressure rises when a liquid is heated, and the liquid boils when it equals the atmospheric pressure. 

Thus, we need to increase the solution’s temperature to make the atmospheric pressure proportional to the vapour pressure. The meaning of an elevation in boiling point is understood as the disparity between the boiling point of the pure solvent and the boiling point of the solution. 

So now, suppose Tb is the solvent’s boiling point, and T is the solution’s boiling point. Thus, the disparity in the boiling points (ΔT) is known as the elevation in boiling point.

What Causes the Elevation of the Boiling Point?

• When one introduces a solute to a solvent, the vapour pressure drops. This phenomenon occurs as a result of the solute dissolving solvent molecules. 
• It occurs in both electrolytic and non-electrolytic solutions when the solute replaces part of the solvent molecules at the liquid’s surface. 
• Because there are fewer solvent molecules at the surface, less will be evaporation, lowering the vapour pressure. 
• A greater temperature is necessary to equalise the vapour pressure with atmospheric pressure; thus, it is essential to consider a higher boiling point.

Formula for Elevation in Boiling Point 

The elevation in the boiling point of a solution is represented by (ΔTb). It is calculated with the following formula:

ΔTb = i*Kb*m

Where,

• Kb is the ebullioscopic constant
• m is the molality of the solute
• i is the Van’t Hoff factor

Kb is frequently expressed in oC/molal or oC.kg.mol-1.

This formula does not apply to volatile solvents and becomes less precise when the solute’s concentration is very high. 

The formula for elevation in boiling point can be used to calculate the degree of dissociation and molar mass of the solute.

The Boiling Point Elevation and Vapour Pressure Relationship

The rising of the boiling point can be described in terms of vapour pressure. 

Vapour pressure is a quantity of the solution molecules’ proclivity to escape by entering the gas phase. 

When the vapour pressure of a liquid equals the air pressure, it boils.

Kb = RTb2M/ΔHv, 

Where,

• R is the gas constant
• Tb is the boiling temperature of the pure solvent [in K], 
• M is the molar mass of the solvent
• ΔHv is the heat of evaporation per mole of the solvent.

Conclusion

When a certain solute is introduced to a solvent, the solvent’s boiling point is raised. It must be a non-volatile solute. The molal concentration of the solution’s solute and the increase in boiling point are directly proportional.

Tb = Kbm = (1000 w2) (w1 M2) 

As a result, an increase in boiling point is indicated by:

ΔTb = (Kb × 1000 × w2) ÷ (w1 × M2)

As a result, the solute’s molecular weight becomes:

M2 = (Kb × 1000 × w2) ÷ (w1 × △Tb)