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BP and FP

Ibn Alnafis MD

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I know that adding a solute to a solution increases its boiling point and depresses its freezing point. I understand the reason behind the depression of the freezing point that the solute interferes in the formation of lattice between the solvent molecules. That makes sense.

However, I don't understand how solutes increase the boiling point of a solution. Don't solutes also interfere in the Hydrogen bond formation between individual solvent molecules? shouldn't that make it easier for the solvent molecule, which now have a harder time bonding to each other through hydrogen bonding, to escape the liquid phase into the gas phase?

Also, how do volatile solutes differ from nonvolatile ones in their effect on the BP and the FP of a solution?

Thanks
 

Meredith92

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I believe solute molecules decrease the available surface area for molecules to turn into vapor at the surface of the liquid. That's how I usually think of it. It's like less can escape since a bunch of solute molecules are blocking their escape route/ take up space on the surface.

Hope that helps! Although it's probably way over simplified
 

Ibn Alnafis MD

Full Member
10+ Year Member
Dec 26, 2008
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  1. Resident [Any Field]
I believe solute molecules decrease the available surface area for molecules to turn into vapor at the surface of the liquid. That's how I usually think of it. It's like less can escape since a bunch of solute molecules are blocking their escape route/ take up space on the surface.

Hope that helps! Although it's probably way over simplified

Makes sense. Thank you.

Would this also be the case when volatile solutes added to a solution?
 

Meredith92

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I don't think so... I think EK explains it well but I don't have my book with me sorry! I feel like they usually say that this only happens with nonvolatile solutes since they just take up space at the surface and don't evaporate. Volatile solutes would leave the surface making room for the solvent to evaporate
 
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