Question

What do you understand by elevation of boiling point?

Answer 1

The increase in the boiling point of a solvent on addition of a solute to it is called the elevation of boiling point for that solvent.

A graphical representation of the elevation of the boiling point is provided in Fig. The vapour pressure variation with temperature for pure solvent, solution I and solution II is shown in this figure. Solution II has a higher concentration as compared to that of solution I. From the graph it is clear that the pure solvent boils at temperature T_b, corresponding to point A, when its vapour pressure equals the atmospheric pressure. Solution I reaches its boiling point T₁ at point B when its vapour pressure equals air pressure. The graph clearly indicates that T₁ is greater than T_b. Consequently, the boiling point of solution I exceeds that of the pure solvent. The difference = `Delta T_(b_1)` between the two boiling points T₁ and T_b represents the rise of the boiling point for solution I. The boiling point of solution II remains elevated due to its larger concentration. It vaporises at temperature T₂, corresponding to point C on the graph, when its vapour pressure equals atmospheric pressure. The difference = `Delta T_(b_2)` between temperatures T₂ and T_b represents the rise of the boiling point for solution II. Thus,

Elevation of boiling point for solution I = T₁ − T_b

= `Delta T_(b_1)`

Elevation of boiling point for solution II = T₂ − T_b

= `Delta T_(b_2)`