Diving deeper: secrets hiding in aluminum cans
What’s hiding inside aluminum cans? Just the drink, or…?
- Wear protective gloves, glasses, and a mask.
- Work in a well-ventilated area.
- Perform this experiment under adult supervision only!
Reagents and equipment
- various soft drinks in aluminum cans;
- drain cleaner;
- wooden rods;
- glass container.
Buff the top layer of paint off of the aluminum cans. Open the cans and use the wooden rods to suspend them in suitably-sized beakers. Add some drain cleaner (usually a 10% sodium or potassium hydroxide solution). Observe a tumultuous reaction and release of gas. Wait two hours. Take the cans out of the liquid. The aluminum may have dissolved, but the drinks haven't leaked out!
These ordinary aluminum cans are concealing a devious secret – they are covered with a protective layer not only on the outside, but on the inside as well! When we remove their paint coatings, we expose the aluminum, which easily reacts with a drain cleaner that contains an alkaline component. But even when the aluminum has dissolved completely, the drinks will not leak. The cans contain a secret second material – a layer of plastic that keeps the drinks themselves from interacting with the aluminum.
Carbonated drinks contain what are known as “acidity regulators,” the most popular of which are orthophosphoric and citric acids. They are used to acidify foods, giving them a sour and slightly bitter taste. These acids can react with various metals, including the aluminum comprising the soda cans. A special plastic coating is therefore applied to the inside of the cans to prevent the two from interacting. This plastic is most often an epoxy resin (polyethylene, polypropylene, polyester, or other polymers may be used in some cases). These extremely-prevalent epoxy resins contain bisphenol A (BPA), a substance that usually evokes a mixed reaction from consumers. Bisphenol A has the ability to mimic natural human hormones; namely, it is structurally similar to estrogen. Over the past 20 years, it has undergone thorough investigation to determine any effects it may have on the human body. According to extensive research and risk assessments conducted by both governmental and independent organizations worldwide, bisphenol A is safe at normal everyday levels of exposure from food and beverage containers. A Canadian Department of Health report concluded that an average adult would need to consume 940 canned drinks in one day to even get close to the “tolerable daily intake” limit for BPA exposure.
When the aluminum cans are processed for recycling, the films burn up at the high temperatures in the convection furnaces.
2Al + 2NaOH + 6H₂O = 2Na[Al(OH)₄] + 3H₂