MEL Science recently posted a video of an experiment that shows how some aluminum soda cans actually contain a strange clear film. They even have the courtesy of showing you how to carry out the experiment yourself and explain a little bit about the chemistry behind the reaction.
Here’s how you can do try it at home. First up, buff off the external paint with some sandpaper. When you’re left with just a raw shiny can, lift up the can rink and stick a wooden rod through the hole. Then, place the can in a suitable beaker or jar, using the wooden rod to suspend it above the bottom. Next, fill with it drain cleaner, preferably one that contains sodium hydroxide. Finally, just leave it there for at least two hours.
Here’s the boring, but important, stuff: make sure you wear protective gloves, glasses, and a mask while doing the experiment. Also, make sure it’s carried out in a well-ventilated area, as the reaction will release hydrogen gas, and only do it under adult supervision. Last but not least, make sure you don’t drink the soda after the experiment.
After the two hours is up, remove the can and you’ll see the aluminum has completely dissolved, leaving behind a thin clear film with the drink still in it.
According to MEL Science, the chemical reaction at hand here is: 2Al + 2NaOH + 6H₂O = 2Na[Al(OH)₄] + 3H₂. The can contain aluminum, an amphoteric compound, which reacts very easily to the alkaline component in the drain cleaner.
What's the point of this film, you must be wondering? The clear film serves as a protective liner to keep the contents of the can from interacting with the aluminum. After all, some sodas are very acidic and could eat away at the metal. It also helps to act as a barrier against food-borne illnesses that could potentially make you sick.
Fortunately, this liner doesn’t seem to affect how easy it is to recycle the can. Aluminum cans used for food and drinks are widely recyclable in most countries.