Question

What is the effect of a small amount of impurity on the melting point of an organic compound?

Answer 1

Impurities either increases or decrease the melting point of a substance.

The impurities tend to lower the overall melting point of the compound versus the value for pure material. Because there are unwanted particles forcing their way into the solid crystal structure and making it weaker.

The impurities may also tend to increase the range of the melting point values. Pure compounds tend to melt very quickly at the melting point temperature. Impure compounds tend to melt more slowly over a larger range of temperature values.

Both of these effects are due to a weakening of the molecular lattice structure of the solid. Pure solids may form stronger lattice bonds that require more energy to break up. Impurities present tend to disrupt these lattice networks, thus requiring less energy to disrupt them.