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

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

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Impurities present in a solid organic compound tend to have 2 effects on the melting point.  First, they tend to lower the overall melting point of the compound versus the value for pure material.  Second, they tend to increase the range of the melting point values.  Pure compounds tend to melt very quickly once they reach the correct temperature.  Impure compounds tend to melt more slowly over a larger range of temperature values.  Both of these effects are due to a weaking of the molecular lattice structure of the solid.  Pure solids tend to form stronger lattice bonds that require more energy (heat) to break up.  Impurities present tend to disrupt these lattice networks, thus requiring less energy to disrupt them.

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