[jhanvi_y]

Hi, I was reading one of my books about testing for purity. Under "boiling point," two schemes were given:
(1) Distillation Method
and
(2) Capillary-Tube Method.

I had a question on the distillation method.

1-What exactly does it mean here by "distillation?"

This is what the book said word-by-words:

"If enough liquid is available, its boiling point is determined in an ordinary distillation apparatus. A pure liquid will distil at a constant tempreature which is its boiling point. In case the liquid is impure, the boiling point will rise during distillation."

2-How can the boiling point "rise" while distillation?

Could someone please clarify these points for me?

Thanks!!
~Jhanvi

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[Bakegaku]

2:

if a liquid is impure, then the concentration of the solute will increase as the liquid is boiled off.  If the solute affects the liquid so that it increases its boiling point with concentration, then the boiling point will rise with concentration as the liquid is boiled.

[Donaldson Tan]

When you distill a mixture, you are seperating components based on their different boiling point. Consider a mixture of 2 miscible liquids A (bp 70C) and B (bp 120C). When you distill the mixture, you will discover that your mixture first boils at 70C, then it boils at 120C. The fact that you observe more than 1 boiling point indicates there is more than 1 type of substance present in the mixture.

However, in reality, the actual recorded boiling point will not be 70C or 120C because more than 1 component is present in the liquid phase. If liquids A and B forms intermolecular bond stronger than the intermolecular bonding among A molecules and among B molecules, then there will be an elevation in the boiling point of the mixture. If the intermolecular bond is weaker, then there will be a drop in the boiling point of the mixture. It all depends on the mole fraction and the strength of intermolecular bonding between components A and B.