A solution is known to contain either Ni2 or Cr3 . Addition of what single (1 only) reagent would identify which ion is present?

Explanation:

From the periodic table; both elements given are from the same group 3.

To test for which ion will be present in the experiment;

We added 6M aqueous sodium hydroxide  in excess.

This process is followed by the addition of hydrogen peroxide H2O2 while stirring.

Heat is then applied to the solution.

After we've carried out those process;

We will notice the following;

If is present, then a green  precipitate will be formed

If is present,then a yellow-colored will be formed.