The
melting behaviour of a sample of stuff can tell us about the
purity of the sample (that is, if it is a mixture or a pure
sample).
If it is a pure sample, we can tell the identity of the substance
from the melting point.
The same is
true for boiling behaviour. A sample of stuff that is a mixture
of substances does not have an exact boiling point. The temperature
changes as it boils away:
