Tag Archives: vapour pressure

[Explanation] Boiling Point Elevation: Cause & Effect


Boiling point elevation describes the phenomenon that the boiling point of liquid (a solvent) will be higher when a non volatile solute is dissolved in it than when it is pure.

Cause and Effect :

Boiling point elevation is the reduction of chemical potential of the liquid solvent as a result of the presence of a non-volatile solute. The physical basis of lowering of chemical potential is entropic.
In the absence of solute, the pure liquid solvent has an entropy that reflects its disorder. Its vapour pressure reflects the tendency of the liquid to change towards greater entropy, which can be achieved if the liquid is vaporized to form a more chaotic gas. When a solute is present, there is an additional contributions to the entropy of the liquid. Since there is already an additional randomness in the liquid, the system reaches its maximum entropy. When less liquid evaporates then the solute, and the randomness it causes is absent. The effect of the solute is a lower vapour pressure and hence a higher boiling point.
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