That last group includes what we call the "non-trivial" redox reactions. Many of these occur among ions in solution and the reactions are often complex enough that they cannot be easily balanced by "inspection" (that's what we've been doing so far).

For example, copper metal will react with nitrate ions in an acidic solution to produce nitrogen dioxide and copper(II) ions. If we write this out in symbols we are faced with a bewildering array of possibilities for beginning to balance the reaction:

Cu + NO₃^- + H⁺ → Cu²⁺ + NO₂

In fact, we have an immediate problem since there are no hydrogen atoms at all among the products! Although one can eventually learn a repertoire of such reactions and how to balance them, it is easier in the beginning to master a series of steps that will generally yield the correct result.