Materials
- Goggles, gloves
- 1.0 mol/L copper sulfate solution
- Sodium chloride
- Aluminum can
- Drafting compass or another tool with a sharp point
- Stencil
- Masking tape
- Petri dish — larger diameter than the can — or any shallow dish of suitable size
- Filing hand-tool
Let’s do it
Aluminum cans are used for juices, soda and beer. The cans receive, on the inside, a plastic film that prevents the contact of the liquid with the metal. On the outside, the cans are painted with the brand's logo and other information. This external paint protects the aluminum surface from corrosion.
If we scratch the surface of the can with a pointy tool (we used the spike of a drafting compass), the aluminum surface will be exposed. If we find a suitable oxidizing agent, we can corrode the aluminum only in the parts of the can that were scratched.
Safety
We selected copper sulfate as the oxidizer. It is less toxic and dangerous than other alternatives, such as sodium hydroxide, to react with the aluminum. Still, gloves and goggles should be worn during the experiment. Students should be instructed to be careful with the pointy tool used to scratch the cans.
Select a stencil to help you scratch the can. You can pick a drawing, print it on an overhead projector transparency and cut it out. Many free drawings are available on the internet. You can modify the activity for Halloween, Christmas, or any other theme. I usually ask the students in advance to bring their own stencils, ready for use. I also keep a small number of options ready for those who forget to bring their own.
After the contour of the drawing is scratched on the can, cover its top and bottom parts with masking tape. This will protect these parts, since they are not covered by paint. Now the can is ready for "carving". Put the copper sulfate solution in a shallow dish. Place the can sideways, with the drawing facing down in the solution for a few seconds. You will notice that nothing happened at all! Put a teaspoon of sodium chloride in the copper sulfate solution and stir until it is dissolved. Place the can in the solution again for a few seconds. Now you will see a reddish-brown solid deposited on top of the drawing. Keep the part of the can with the scratched drawing in the solution for a few minutes. You can make the reaction go faster by removing the copper deposit with paper towels. After some time, all the aluminum along the edges of the drawing will have reacted and dissolved. The internal plastic covering will be still there, and you can remove it with the help of the pointy end of the compass.
Wash the can and let it dry. You can paint the can using spray paint. Use a file to remove the top part of the can. Place a small candle inside. The top of the can was originally inserted by folding the edges so that the top does not become sharp when it is removed. If there are sharp edges, you can simply file them down. Your lantern is ready!
What's going on
When we place the can in the copper sulfate for the first time, we could expect it to react right away. After all, looking at the reaction:
2Al(s) + 3Cu2+(aq) → 2Al3+(aq) + 3Cu(s)
and at a table of reduction potentials, everything seems right. Aluminum is indeed easier to oxidize than copper. Then why does nothing happen? Even though we removed the paint covering the aluminum in the can, there is still a layer of aluminum oxide protecting the aluminum surface. We have to remove this layer for the copper ions to reach the aluminum metal. That is where the chloride ions enter. They help remove the oxide layer and let the copper ions react with the aluminum. You could use copper chloride directly, but I like to show the students that the potentials are only one part of predicting if a reaction will happen. You may also notice that during the reaction some bubbles can be seen. The aluminum also reacts with water under the reaction conditions to release hydrogen gas.
The students can take their lanterns home and use them for decoration.
[Editor’s note: The reactions in this activity are further explored in the article “Chemistry using minimum-cost resources”, of this issue.
As well, we invite readers to send in photos of their students’ lanterns. We look forward to seeing your creative glowing photos.]
