Why can the boiling point and the melting point be used to help identify a substance?

The boiling and melting points are almost unique to individual substances. If it should happen that two possible substances have the same melting point, they can still be identified by the method of mixed melting points. If substances A and B have the same melting point and you mix them, the mixture will melt belowthe tabulated temperature. Thus if you mix your unknown with a sample of what you think it is, if you are right it will still melt sharply at the expected temperature, but if you are wrong it will melt gradually and at a lower temperature.