Consider a redox reaction between Zn(s) and copper ions shown below

Zn(s) Cu2 (aq) Cu(s) Zn2 (aq)..............................(1)

Counterions are evidently contained in the solution but are not presented because they you should not participate in the reaction. The above reaction is spontaneous. If pieces of Zn metal are added to a CuSO4 solution, Cu(s) starts depositing on the Zn pieces. To get useful electrical work from such a process, it is best to place the Cu and Zn rods(electrodes) in separate compartments as indicated below