First 10 drops of copper (II) sulfate was placed into two wells, and then a piece of string that was completely saturated with the copper (II)...

First 10 drops of copper (II) sulfate was placed into two wells, and then a piece of string that was completely saturated with the copper (II) sulfate, acted as a salt bridge. A piece of pencil lead was placed into each well and clip from a 9 volt battery was hooked to each of the pieces of lead contained within the two wells. This was done at 7:21pm. It was first observed that the black clip sparked upon immediate contact with the pencil lead. Then after ten to fifteen minutes it was noted that the pencil lead that was held by the red clip turned a black color and the pencil lead held by the black clip turned a red color. There was no changes in color of the copper (II) sulfate, as well as no changes in odor. There was also no precipitation or gas noted either. Why was this chemical reaction observed?