student recrystallized some impure benzoic acid and isolated it by filtration. He scraped the purified benzoic acid off the filter paper after it had dried and took the melting point as a test for...

student recrystallized some impure benzoic acid and isolated it by filtration. He scraped the purified benzoic

acid off the filter paper after it had dried and took the melting point as a test for purity. He was surprised that most of the white solid melted sharply between 121 and 122oC but that a small amount remained unmelt even at temperatures above 200oC. Explain this behaviour.