One answer is the simple electrochemical
cell. On the left side, a piece of metallic zinc is
immersed in zinc sulfate solution, and on the right, copper
is immersed in copper sulfate. The two pieces of metal are
connected by a wire, and the two solutions are connected by
a porous barrier that allows
the migration of ions but prevents
the bulk mixing of the two solutions.
At the left metal rod, called an electrode, zinc atoms give
up electrons and enter the solution as zinc ions. This electrode
is slowly eaten away. At the copper electrode on the right,
copper ions from solution combine with electrons and plate
out on the electrode as metallic copper.
This electrode slowly increases in bulk as the reaction progresses.
The electrons needed to reduce the
copper ions at the right come from
of zinc atoms at the left, but to do so they must travel
through the external wire circuit.
In the two solutions, as zinc ions enter the left compartment
and copper ions are removed from the right, negative sulfate
ions must migrate slowly through the porous barrier from right
to left, and positive zinc ions from left to right, to preserve
electrical neutrality in the two solutions.
The two electrodes, connecting wire, and porous barrier form
a closed circuit, with negative
electrons moving from left to right through the wire, and
positive and negative ions moving through the porous barrier.