What is the effect of impurities on the boiling point of a compound?

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ncchemist | eNotes Employee

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In general, adding impurities to compounds elevates their boiling points and lowers their freezing points.  You can think of this as extending the liquid range in both directions.  There are some examples where this does not hold true.  For example, if you add a small amount of water as an impurity to ethanol and heat it, you will get a mixture that is 95% ethanol and 5% water that boils over.  This is called an azeotrope and actually has a slightly lower boiling point than pure ethanol.  But azeotropic mixtures are usually limited to talking about mixtures of volatile organic solvents, not impurities in small molecules.