How do you differentiate between pure substances and mixtures?
Well, pure substances have constant physical properties..............
And physical properties include melting point, and boiling point.
Melting point is an extremely useful physical property. And usually knowledge of melting point of an unknown organic compound, and a few of its crystalline derivatives allows conclusive identification of the unknown compound.
On the other hand, mixtures have non-definite melting points, and occur over a range..........