Why doesn't my unknown compound's melting point match my known compounds?

Question

In lab I looked up three melting points and my unknown substance had to come to a pretty close match to the known melting ranges. This didn't happen, why?

Answer 1

Your unknown compound might be an impure substance. The impurities will skew the melting point. You should find out if the substance is pure or not. Pure substances (that is, with nothing else mixed with them) will have a narrow ranging in which they melt, for example, pure Naphthalene melts between 83-85 Celsius.

Answer 2

it's a different substance??