A student received an impure sample of benzoic acid and reported the melting point as "mp = 121.5C." The student reported the sample as pure since the melting point wasn't too far off. What mistake was made that could have proven the sample's impurity?

A student received an impure sample of benzoic acid and reported the melting point as " mp=121.5 'C." The student reported the sample as pure since the melting point wasn't too far off. What mistake was made that could have proven the sample's impurity? The student did not note the full range, which may have been wide. The student should have noticed that the mp was low, since benzoic acid should melt at 122C. The student has the sample wrong. It was actually Fluorene. The student should have done a mixed melting point with something like Benzamide to prove they were not the same