Mixture is "a material of variable composition that contains two or more (pure) substances" (S.S.Zumdahl).
Mixtures (like petroleum) do   not  have fixed boiling points (their temperatures increase during boiling).

All substances of your daily life are mixtures: water, air, soil, petrol, milk.
In homogen (the same throughout) mixtures it cannot be distinguished between its components eaven by the strongest micoscopes.
In hetrogen (containing regions of with differing properties) mixturesit can be distinguished between its components (i.e. with a microscope).