Zinc, on the left side, and Copper, on the right side both have oxidation states of 0, since they're not a part of a compound.
But when they're included in a compound, they have oxidation states!
These are the oxidation states:
Zn (0) + (Cu (+2) & O (-2)) --> (Zn (+2) & O (-2)) + Cu (0)
See how Zinc, once it's apart of a compound, has an oxidation state! And Copper, since it is no longer in a compound, loses its oxidation state.
So Zinc goes from an oxidation state of 0 on the left side to an oxidation state of +2 on the right side. This means it's lost electrons.
Copper goes from an oxidation state of +2 on the left side to an oxidation state of 0 on the right side.
And Oxygen's oxidation state does not change.
Zn 0 -> +2 (lost electrons, became positive)
Cu +2 -> -2 (gained electrons, became negative)
So since Zinc lost electrons, that means it was the one oxidized.
And Copper gained electrons, so it was the one reduced.
You can remember this by using OIL RIG.
Oxidized Is Lost
Reduced Is Gained
Good luck!
