How can melting point determinations be used in identifying and/or characterizing unknown compounds?
Determining the melting point of a compound is one way to test if the substance is pure, but it doesn't necessarily identify the compound.
A pure substance generally has a melting point range of less than 2 °C.
However, other compounds may also have melting points within 2 °C of your sample.
One way to become more certain is to determine the mixed melting points of mixtures of your sample with other compounds.
Impurities tend to depress and broaden the melting range, so your pure sample should have a higher and narrower melting range than the mixture.
For example, you may have an unknown compound you think is cinnamic acid. It melts at 133-134 °C.
But 2-furancarboxylic acid and decanedioic acid also melt near 134 °C.
So, you make separate mixtures of your unknown with the known samples of cinnamic acid, 2-furancarboxylic acid, and decanedioic acid.
If your unknown is cinnamic acid, mixing it with pure cinnamic acid will not change the melting point.
However, the other compounds will be "impurities" in your unknown, and the mixed melting points will be lower and have wider melting point ranges.