The law of the conservation of mass must be obeyed in a chemical reaction. what do the atoms do during a reaction that shows they obey the conservation of mass?

A balanced equation is a prime example of the law of the conservation of mass as the number of atoms in the reactants is consistent with the number of atoms in the reactants meaning the amount of matter has not changed and no mass has been created or destroyed hence obeying the law.

The teacher is mostly talking about the distillation of a mixture