If you observed the sister chromatids separating during cell division, what phase of mitosis would you be observing?

This is the anaphase of mitosis. Anaphase is characterized by the separation of sister chromatids or "pulling away" of the sister chromatids from the metaphase plate to the opposite spindle poles. This is also the phase where chromosomes reach its maximum condensation meaning that the process of the reformation of the nucleus will be shorter and easier.