Let us suppose we have a liquid with non-volatile solute, and the solution is raised to its boiling point by applying heat.

Since the solvent is being evaporated, the concentration of solute will increase. Thus, the vapour pressure of solution will decrease and the boiling point must increase. Since we are still applying heat, the temperature will keep increasing as the boiling point increases.

But we have learnt that the temperature of a boiling liquid is constant until all liquid is evaporated. Is this only applicable for pure liquid and not for a solution with non-volatile solute?


  1. Will the temperature of the boiling solution be constant or increasing?
  2. Will the vapour pressure of the solution stay constant (equal to atmospheric pressure) even if vigorously heated so that the solution evaporates very quickly?

"The temperature of a boiling liquid is constant until all liquid is evaporated" is true only if the nature of the liquid doesn't change. When you have a changing concentration of a solute (because of the evaporation of the solvent), you don't have the "same" liquid as time goes on ... therefore there is no contradiction.

The second question is slightly harder to answer - but unless "vigorously heated" means "explosively heated" I suppose the answer is still "yes".


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