How does the melting point indicate the purity of a compound?

A sharp melting point is an indicator of higher purity because pure substances typically have a well-defined melting point range. When a compound is pure, it will melt at a specific temperature under standard conditions. Impurities disrupt the ordered structure of the solid crystal lattice, which can cause the melting point to decrease as well as broaden the range over which melting occurs. This phenomenon occurs because impurities introduce defects in the crystal structure and may alter the energy required to break the bonds holding the solid together, leading to a mixture of solid and liquid phases over a wider range of temperatures. Therefore, observing a sharp melting point suggests that the sample is predominantly composed of a single, pure substance rather than a mixture, confirming its purity.