Why is MP of impure substance always lower than pure?

[csx]

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couldn't it be HIGHER if the impurity is of something with a higher MP??

 

[3655]

For example if a liquid has a lot of impurities, when freezing the parts that are pure freeze first trapping the impurities inside, making it difficult to completely freeze so it freezes at a lower temperature. I don't know if that makes sense. You have to think of it from the substance point of view not the impurity. That's how i think of it. If someone else can better explain it please do.

 

[Czarcasm]

On a molecular level, I believe it disrupts the formation of a crystal lattice structure, so more heat needs to be removed to attain that structure (why freezing point decreases). Some people say boiling point increases because of increased molecular interactions, but I don't think that's necessarily always true, especially if the solute's polarity is very similar to the solvent it's dissolved in. I was told the reason for increased boiling point had to do with increased entropy of the solvent, so it's more thermodynamically stable and therefore, for the liquid to evaporate into gas molecules, more energy must be put in to disrupt that stability.