1. What is the difference between the melting point of pure and impure naphthalene?

What is the difference between the melting and freezing point of a substance? Also, what do freezing point and melting point of a substance have in common?

A mixture is prepared that is 5.0% (w/w) of an unknown substance mixed with naphthalene (molar mass = 128.2 g mol-1). The freezing point of this mixture is found to be 77.3 °C. What is the molar mass of the unknown substance? (The melting point of naphthalene is 80.2 °C and Kf = 6.9 °C m-1.)