Two molecules are superimposable ("identical") if they can be brought into coincidence by rotation or (and) translations of entire molecules without changing the relative arrangement of the atoms.
Two superimposable molecules
Click on the right image to make the molecule turn about the C-C axis !
It is obviously not possible to separate superimposable molecules by conventional separation methods (distillation, chromatography ..)
Superimposable molecules belong to the same chemical substance .