what is meant by the statement that a pure crystalline organic compound substance has a sharp melting point?

Answer 1

If you heat up a sample of a pure crystalline organic compound at a steady, slow rate and watch it melt, the temperature should not rise by more than 2C between it being totally solid to it being totally liquid. The same is true if you them let it cool down and crystallise. An impure solid will do the same over a much wider temperature range - say 10C or more.