boiling point & melting point with impurities

[lmcdavidkim]

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melting point of an organic compound decreases with impurity.
boiling point of an organic compound increases with impurity.

is that how it goes? ignoring special cases like alcohol. thanks

 

[Mornhavon]

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[sciencebooks]

melting point of an organic compound decreases with impurity.
boiling point of an organic compound increases with impurity.

is that how it goes? ignoring special cases like alcohol. thanks

Yep, it comes down to colligative properties. (So there's also that vapor pressure decreases and osmotic pressure increase.)

 

[drechie]

now, what exactly are "impurities?" i found a similair question on mcat, when it asked use to deduce boiling points for several acids with given pka's. the answer solution it basically said that the acid with the lowest pka (thus the STRONGEST acid) had the higher boiling point. What are impurities?

 

[DrknoSDN]

Impurities are anything that prevent normal molecular bonding that would occur if only a single type of molecule was present.

With melting point impurities are something that prevents perfect crystal formation. A completely pure liquid would form a low-energy crystal structure based on the molecular shape. Adding impurities blocks some of the normal bonding so it melts easier (lower melting point).
With boiling point impurities like dissolved salts prevent interaction between the vapor and the surface liquid molecules.

Relating pKa seems strange unless all the acids had the ability to hydrogen bond or were vary similar in the rest of the structure.
For example Acetic acid can hydrogen bond (pKa = 4.75) and boils at 118C
Hydroiodic acid by contrast cannot hydrogen bond (pKa of -9.3) is one of the strongest acids and boils at a very low -35C

No relation between pKa and boiling point between acids of different structures.