Remember that redox reactions are
spontaneous in only one direction.
Here we have an example of one spontaneous and one non-spontaneous redox reaction
First we have the reaction of zinc metal with copper ion,
then the reaction of copper metal with zinc ion.
Let's see which one is spontaneous
As you can see, the zinc metal turned dark due to the formation of solid copper on the surface.
For the zinc and copper metal reaction there was no reaction.
