The melting point range of a pure compound is about 1-2ºC of the expected melting point. An impure solid melts within a range that is both larger than that of the pure substance (>1ºC) and begin at a lower temperature because impurities decrease the meting point. A melting range of 5º or more indicates that a compound is impure.
Since the melting points of A and B are estimated at 133 o C, the melting point range for each pure substance must be slightly different from each other. The melting point of the unknown is measured and its range is compared with the melting point ranges of pure A and B then decision can now be made about the identity of the unknown solid.
