In step 1 of the experimental procedure, copper metal is added to concentrated nitric acid. The reaction between copper metal and concentrated nitric acid is an oxidation-reduction reaction that is somewhat complicated.
4 HNO3(aq) + Cu(s) => Cu(NO3)2(aq) + 2 H2O(l) + 2 NO2(g)
Identify the element or elements that undergo REDUCTION.