Two molecules are conformers if 1) they are not superimposable 2) they can be rendered superimposable by rotation of parts of their molecules around simple C-C bonds.
Two conformer molecules
Click on the right image to make the group CH2Cl turn alone around the axis C-C
As thermal agitation causes always easy rotations around single bonds, it is not possible to separate conformers molecules at normal temperature:
Conformers molecules belong to the same chemical substance.