The classic demonstration of cleaning pennies with Coca-Cola has fascinated kids and adults alike for generations. It seems almost magical: dull, tarnished pennies submerged in a fizzy brown liquid emerge gleaming and bright. But what’s the science behind this seemingly simple experiment? It’s more than just a party trick; it’s an engaging way to introduce basic chemistry concepts to children. Let’s delve into the fascinating chemical reactions that make this possible.
The Chemistry of Tarnished Pennies
To understand why Coke cleans pennies, we first need to understand what makes them look dirty and dull in the first place. The culprit is a chemical process called oxidation.
Understanding Oxidation and Copper Oxide
Pennies, primarily made of copper (although more recent pennies are mostly zinc with a copper coating), react with the oxygen in the air. This reaction, sped up by moisture and other environmental factors, leads to the formation of copper oxide on the penny’s surface. Copper oxide is a blackish-brown compound that dulls the shiny copper beneath. Think of it like rust on iron, but instead of iron oxide (rust), we have copper oxide. This process happens gradually over time, explaining why older pennies tend to be much darker and more tarnished than newer ones.
The chemical equation for this process is simplified as:
2Cu (Copper) + O2 (Oxygen) → 2CuO (Copper Oxide)
It’s important to realize that this is a natural process. Copper, left exposed to the elements, will always undergo oxidation. It’s not necessarily harmful, but it does affect the appearance of the metal.
Other Culprits: Sulfides
In addition to oxidation, pennies can also react with sulfur compounds in the air, leading to the formation of copper sulfide. This compound is also dark and contributes to the tarnished look. Sulfur compounds are often present in polluted air, further accelerating the tarnishing process in urban environments. The presence of sulfides explains why some pennies might appear almost black.
The Role of Coke: Acids and Cleaning Action
Now that we understand what makes pennies dirty, we can explore how Coke, a seemingly innocuous soft drink, can actually clean them. The secret lies in the acids present in Coke.
Phosphoric Acid: The Key Ingredient
The primary acid responsible for cleaning pennies in Coke is phosphoric acid (H3PO4). This acid is added to Coke for flavor and as a preservative. Phosphoric acid reacts with the copper oxide, breaking it down and dissolving it. In essence, the acid attacks the tarnish, converting the copper oxide back into copper ions which dissolve into the Coke solution.
The simplified chemical equation is:
CuO (Copper Oxide) + 2H3PO4 (Phosphoric Acid) → Cu3(PO4)2 (Copper Phosphate) + H2O (Water)
The copper phosphate formed is soluble in the Coke solution, effectively removing the tarnish from the penny’s surface. The concentration of phosphoric acid in Coke is carefully controlled to ensure it is safe for consumption, but it’s still strong enough to react with and dissolve the copper oxide.
Carbonic Acid: A Supporting Player
While phosphoric acid is the primary cleaning agent, carbonic acid (H2CO3) also plays a role. Carbonic acid is formed when carbon dioxide (CO2) dissolves in the water within Coke. This is what gives Coke its fizz. Although carbonic acid is a much weaker acid than phosphoric acid, it still contributes to the overall acidity of the solution and helps in the cleaning process.
Citric Acid: Possible Additional Contributor
Some formulations of Coke may contain small amounts of citric acid. If present, this acid can also contribute to the cleaning action by chelating (binding to) the copper ions, further assisting in the removal of the tarnish.
The Experiment: Setting it Up and Observing the Results
Conducting the penny-cleaning experiment is simple and requires minimal materials.
Materials Needed
- Dirty, tarnished pennies
- A glass or bowl
- Coca-Cola (or other similar cola drink)
- Paper towels
Step-by-Step Instructions
- Pour a small amount of Coke into the glass or bowl.
- Place the tarnished pennies into the Coke, ensuring they are fully submerged.
- Let the pennies soak in the Coke for several minutes (15-20 minutes is often sufficient, but heavily tarnished pennies may require longer).
- Remove the pennies from the Coke and rinse them with water.
- Dry the pennies with a paper towel.
- Observe the difference between the cleaned pennies and the uncleaned ones.
Observations and Explanations for Kids
Children will immediately notice the difference between the pennies before and after the experiment. The pennies submerged in Coke will appear much brighter and shinier. Explain to them that the Coke acted like a cleaner, removing the “dirt” (copper oxide) from the surface of the pennies.
You can further explain that the Coke contains special ingredients (acids) that react with the “dirt” and wash it away. Encourage them to ask questions and explore the concept of chemical reactions in a simple and understandable way. This experiment serves as a great introduction to the world of chemistry.
Safety Considerations
While the penny-cleaning experiment is generally safe, it’s important to take a few precautions.
Adult Supervision
Children should always be supervised by an adult when conducting this experiment. While Coke is safe to drink, it’s important to emphasize that it should not be ingested in large quantities, especially after being used to clean pennies.
Proper Disposal
The Coke used in the experiment should be disposed of properly. It can be poured down the drain, but it’s best to avoid prolonged contact with skin.
Hand Washing
After handling the pennies and Coke, children should wash their hands thoroughly with soap and water. This will remove any residue and ensure hygiene.
Beyond Pennies: Other Applications of Acids
The penny-cleaning experiment provides a tangible example of the power of acids. It opens the door to discussing other applications of acids in everyday life.
Acids in Cleaning Products
Many household cleaning products contain acids to help remove dirt, grime, and stains. For example, vinegar (acetic acid) is a common ingredient in all-purpose cleaners. Toilet bowl cleaners often contain stronger acids to dissolve mineral deposits and kill bacteria.
Acids in Food Processing
Acids are also used in food processing for various purposes, such as preserving food, adding flavor, and controlling acidity. For example, citric acid is used in many processed foods and beverages as a preservative and flavoring agent. Lactic acid is used in the production of yogurt and cheese.
Acids in Industrial Applications
Acids have numerous industrial applications, including the production of fertilizers, plastics, and pharmaceuticals. Sulfuric acid, for example, is one of the most widely used industrial chemicals.
Variations on the Experiment
There are several variations on the penny-cleaning experiment that can be explored to further enhance learning.
Comparing Different Cola Drinks
Try using different brands of cola drinks to see if they clean pennies equally well. This can lead to a discussion about the differences in the composition of different cola drinks and the relative concentrations of acids.
Testing Different Liquids
Experiment with other acidic liquids, such as lemon juice, vinegar, or even diluted hydrochloric acid (with extreme caution and adult supervision). Compare the results and discuss the relative strength of the different acids.
Investigating the Effect of Soaking Time
Vary the amount of time the pennies are soaked in Coke and observe how it affects the degree of cleaning. This can help illustrate the concept of reaction rates and the importance of time in chemical reactions.
The Long-Term Implications of Copper Tarnish
While a tarnished penny may seem like a minor issue, the processes involved are relevant to broader concerns about metal corrosion and preservation.
Protecting Copper Statues and Buildings
Copper is a common material used in statues and architectural elements. The Statue of Liberty, for example, is clad in copper. Over time, copper exposed to the elements develops a green patina, which is a form of copper carbonate. This patina actually protects the copper underneath from further corrosion.
Preventing Corrosion in Plumbing
Copper pipes are widely used in plumbing systems due to their durability and resistance to corrosion. However, under certain conditions, copper pipes can corrode, leading to leaks and water contamination. Understanding the factors that contribute to copper corrosion is crucial for maintaining the integrity of plumbing systems.
Conserving Historical Artifacts
Museums and historical societies invest significant resources in preserving copper artifacts. The same principles used in cleaning pennies can be applied, with careful consideration, to cleaning and preserving valuable historical objects.
Conclusion: A Simple Experiment with Powerful Lessons
The seemingly simple experiment of cleaning pennies with Coke is a fantastic way to introduce basic chemistry concepts to children in an engaging and memorable way. It demonstrates the power of acids, introduces the concept of oxidation, and encourages scientific inquiry. By understanding the science behind the sparkle, we can appreciate the fascinating world of chemistry that surrounds us. It’s a fun, educational, and safe activity that can spark a lifelong interest in science.
Why does Coke make pennies look cleaner?
The cleaning action of Coke on pennies is primarily due to its acidity. Coca-Cola contains phosphoric acid and carbonic acid, both of which contribute to its relatively low pH. This acidic environment dissolves the layer of copper oxide, also known as tarnish, that forms on the surface of pennies over time.
The copper oxide reacts with the acids in Coke, breaking down the bonds that hold the tarnish to the penny’s surface. As the copper oxide dissolves, it reveals the shiny, clean copper underneath, giving the appearance of a much brighter and cleaner penny. The other ingredients in Coke, such as sugars and carbonation, play a less significant role in the cleaning process, but the acidity is the main factor.
What type of acid in Coke is responsible for cleaning pennies?
While multiple acids contribute, phosphoric acid is a key player in Coke’s penny-cleaning ability. Its strength and ability to react with metal oxides make it particularly effective at dissolving the tarnish that accumulates on pennies. Carbonic acid, formed from dissolved carbon dioxide, also contributes to the acidic environment.
However, the phosphoric acid provides a more substantial chemical reaction with the copper oxide. While weaker acids could technically clean pennies, the phosphoric acid in Coke speeds up the process and is a significant reason why the beverage is more effective than other less acidic solutions. It breaks down the dull copper oxide layer, revealing the shiny copper beneath.
Does Coke damage the penny in the cleaning process?
While Coke effectively cleans pennies, prolonged exposure can potentially damage the metal. The acidic nature that dissolves the copper oxide can also, over time, erode the underlying copper metal itself, leading to a slight loss of mass and potentially altering the penny’s surface texture if left in the Coke for excessive periods.
The damage is typically minimal with short exposure times (a few minutes), making Coke a relatively safe method for cleaning pennies. However, soaking pennies in Coke for hours or days is not recommended. This prolonged exposure could accelerate the etching process, resulting in visible changes to the penny’s surface and a reduction in its original detail.
Are there alternative cleaning methods for pennies besides using Coke?
Yes, various other cleaning methods can effectively remove tarnish from pennies. A simple solution of vinegar and salt is a popular alternative. The acetic acid in vinegar, combined with the chloride ions from salt, forms hydrochloric acid, which dissolves copper oxide similarly to the acids in Coke.
Another alternative is a commercially available metal cleaner specifically designed for copper or brass. These cleaners often contain chelating agents that bind to the copper oxide and lift it away from the surface. Furthermore, mechanical cleaning using a soft cloth and a polishing compound can also be effective, although it may require more effort than chemical methods.
Why does Coke work better on old pennies compared to newer ones?
Coke tends to show more dramatic results on older pennies due to the greater amount of accumulated tarnish on their surface. Older pennies have had more time to react with oxygen and other elements in the atmosphere, resulting in a thicker layer of copper oxide.
Newer pennies, especially those that have been kept in relatively clean environments, may have only a very thin layer of tarnish or none at all. Therefore, the cleaning action of Coke will be less noticeable on these newer pennies. The more tarnish present, the more visible the cleaning effect.
Is the type of Coke (e.g., Diet Coke, Coke Zero) important for cleaning pennies?
While all Coke products contain acids, the sugar content might slightly influence the results. Regular Coke, with its higher sugar content, can leave a slightly sticky residue on the penny after cleaning. Diet Coke or Coke Zero, which use artificial sweeteners instead of sugar, may leave a cleaner finish.
However, the primary cleaning action comes from the acids, particularly phosphoric acid, which are present in all Coke formulations. Therefore, the type of Coke used will likely not make a significant difference in the overall cleaning effectiveness. The acidity remains the critical factor.
Does Coke clean other metal items besides pennies?
Coke can clean other metal items that are tarnished or corroded, particularly those made of copper, brass, or bronze. The acids in Coke will react with the metal oxides on the surface of these items, dissolving the tarnish and revealing the cleaner metal beneath.
However, caution should be exercised when cleaning delicate or valuable metal items with Coke. Prolonged exposure to the acidity could potentially damage or etch the surface of the metal. It’s always best to test the cleaning action on a small, inconspicuous area first before applying it to the entire item and to limit the exposure time.