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Electroplating uses a form of electrolysis in which the electrodes play a bigger role than just conducting the current. Using electricity, you can coat the metal of one electrode with the metal of the other! Jewelry and silverware can be silver- or gold-plated, while zinc is often used to coat iron to protect against rust. Professional electroplating requires specialized chemicals and equipment to make a high-quality coat, but in this project you can try your hand at a simple procedure that will transfer copper to a brass key. (Adult supervision and chemical safety equipment required.)
Watch us use electricity to copper-plate a brass key in this Home Science Tools video. See this project in action!
The copper sulfate solution is an electrolyte that conducts electricity from one electrode to the other. When the current is flowing, oxidation (loss of electrons) happens at the copper anode, adding copper ions to the solution. Those ions travel on the electric current to the cathode, where reduction (gain of electrons) happens, plating the copper ions onto the key. There were already copper ions present in the copper sulfate solution before you started, but the oxidation reaction at the anode kept replacing them in the solution as they were plated onto the key, keeping the reaction going.
This project has many variables, including the cleanness and smoothness of the key, the strength of the copper sulfate solution, and the strength of the current. If a black soot-like substance starts forming on the key, your solution is not strong enough for the current. Take the electrodes out and add more copper sulfate. When you put them back in, make sure the anode and cathode are as far apart as possible.
There are lots of projects you can do with electroplating! One fun idea is to use a flat piece of brass as your cathode and draw a design on it with an oil-based marker. The copper will not bond where the marker is. After you're done plating it, you can use acetone (or nail-polish remover) to wipe off the marker, leaving a design of the brass showing through the copper. You can use a little metal polish to make the copper shiny, if you want.
You may want to try this simple copper-plating experiment that doesn't use electrolysis and requires only household materials.