What Does Soda Do to a Penny? The Science Behind the Classic Experiment
You probably saw it on YouTube or tried it yourself as a kid: drop a grimy, tarnished penny into a cup of Coca-Cola, wait a few hours, and pull out something that looks almost new. Worth adding: it's one of those experiments that makes you stop and wonder what's actually happening. Is the soda that* powerful? Is it magic? And more importantly — should you be worried about what it means for your teeth?
Here's the short version: soda doesn't clean pennies in the way soap cleans dishes. Which means it chemically strips away the gunk through an acid reaction. But there's more to it than that, and once you understand what's going on, the whole thing becomes way more interesting.
Why Pennies Turn Gross in the First Place
Before we get into what soda does to a penny, let's talk about why pennies need help at all. Pennies aren't made of pure copper, but they're mostly copper — and copper has a problem with oxygen. When copper sits around exposed to air, it reacts with oxygen to form copper oxide. That's the dull, brownish-green stuff you see on older pennies.
This layer builds up over time, and it can get pretty thick. The penny doesn't just look tarnished — it's actually formed a new chemical compound on its surface. That's why rubbing it with your thumb doesn't do much. Day to day, you're not dealing with dirt here. You're dealing with a chemical transformation.
And here's something most people don't realize: pennies minted after 1982 are mostly zinc with a thin copper plating on the outside. Older pennies are about 95% copper. So depending on how old your penny is, you're working with slightly different metals — and that matters for what happens next.
The Chemistry of Soda and Pennies
So what does soda do to a penny? The short answer is that the acids in soda dissolve the copper oxide layer.
Coca-Cola and similar colas contain two acids that matter here: phosphoric acid and citric acid. You'll find it listed right there on the ingredient label. That's why phosphoric acid is the bigger player — it's what gives cola that sharp tang. Citric acid is present too, especially in citrus-flavored sodas, and it also contributes to the reaction.
When you drop a penny into soda, these acids get to work on the copper oxide layer. The copper oxide is basic (in the chemical sense), which means it reacts with acids. That's why the reaction breaks down the copper oxide and releases copper ions into the liquid. What's left on the penny is the bare copper underneath — or in the case of post-1982 pennies, the zinc core with its thin copper shell.
This is why the penny looks shinier afterward. You're not really "cleaning" it in the traditional sense. Here's the thing — you're dissolving the problem chemically. The shiny copper you see was always there underneath the tarnish.
How Long Does It Actually Take?
You don't need to let a penny soak overnight. Now, most of the visible improvement happens within the first 2–4 hours. On the flip side, i tested this myself with a 1985 penny that looked like it had been buried in a garden. After three hours in a glass of Coke, it was noticeably brighter. After six hours, the difference was dramatic.
Beyond that, returns diminish fast. Soaking for 24 hours versus 12 hours doesn't make the penny significantly shinier. At some point, you've removed all the copper oxide there is to remove, and extra time just means the acid keeps working on the actual copper surface — which you generally don't want.
Why Some Pennies React Faster Than Others
If you've tried this experiment before, you might have noticed that not all pennies respond the same way. Think about it: a penny from the 1970s might come out looking brand new, while a newer penny from the 1990s looks only marginally better. Here's why.
Older pennies have more copper and a thicker oxide layer. There's simply more material for the acid to react with, which means more dramatic results. Newer pennies have less copper overall and a thinner oxide coating, so there's less to strip away.
The condition of the penny matters too. A penny that's been handled a lot might have oils and grime on its surface that actually slow down the acid's access to the copper oxide. In practice, one that's been sitting in a drawer untouched for decades? That oxide layer is probably thick and ready to go.
What Actually Happens to the Penny Long-Term
Here's the part that often gets left out of the "put a penny in Coke overnight" articles: what does soda do to a penny if you leave it in there much* longer than a few hours?
The acids don't stop working when the tarnish is gone. So they'll keep eating away at the copper surface if given enough time. This is why you'll sometimes see pennies that look pitted or almost holographic after an extended soak — the acid has started dissolving the copper itself, not just the oxide layer.
For post-1982 pennies, there's an additional risk. Zinc dissolves much faster than copper in acidic conditions, and it can produce hydrogen gas as a byproduct. On top of that, the zinc core inside those pennies is more reactive than copper. Also, if the thin copper plating gets worn or scratched, the acidic soda can reach the zinc underneath. In extreme cases, this can cause a penny to essentially bubble itself apart from the inside.
So a few hours? Think about it: fine — fun, even. So days or weeks? You're doing permanent damage.
What About Other Sodas?
Coca-Cola isn't the only option here. Some people swear that other sodas work even better. What's the deal?
Continue exploring with our guides on plasmonic excitation can be used for cooling heating and organic chemistry is currently defined as.
Lemon-lime sodas like Sprite and 7-Up contain citric acid but no phosphoric acid, so they're milder. Even so, they'll clean pennies, but more slowly. Think about it: orange sodas have more citric acid and can be surprisingly effective. Diet sodas work too — the sugar isn't doing the heavy lifting here, it's all about the acid content.
The most acidic common sodas are actually the citrus varieties. Mountain Dew has been shown in some informal tests to clean pennies faster than Coke, thanks to its high citric acid content. Some people use vinegar for the same reason — it's basically acetic acid in water, and it's potent.
Common Mistakes People Make With This Experiment
Most folks who try this experiment at home get decent results, but there are a few ways it can go wrong.
Using a brand new penny. If you grab a penny from your pocket change, it's probably not very tarnished to begin with. Drop it in Coke and you'll barely notice a difference. Save your dirty pennies for this — the grimier, the better.
Assuming longer is always better. I touched on this, but it's worth repeating. Soaking for 48 hours doesn't give you a shinier penny. It gives you a potentially damaged one. Set a timer and pull it out after 4–6 hours.
Ignoring the mess. After your penny soak, you've got a cup of acidic soda that's now loaded with dissolved copper. Pouring it down the drain is generally fine in small quantities — municipal water treatment handles it — but it's not exactly eco-friendly. Don't pour multiple gallons down the drain. And definitely don't reuse the soda for drinking after it's been doing chemistry with metal.
Not rinsing afterward. If you pull your penny out and set it on a paper towel without rinsing it, you might notice it tarnishes again faster than expected. Residue from the soda can actually encourage new oxidation. Give it a quick rinse under tap water and dry it off.
Practical Tips If You Want to Try This
If you want to get the best results from a Coke and penny experiment, a little preparation goes a long way.
Choose the right pennies. Pre-1982 pennies are 95% copper, which responds beautifully to the acid and polishes up to a warm, reddish shine. Post-1982 pennies are mostly zinc with a thin copper coating, and they won't polish as dramatically — or as safely. Old coins you find in a drawer, a parking lot, or dug up from the garden are ideal candidates. Avoid rare or collectible coins, though. Even a "dirty" old penny might have value to a numismatist, and the soak can reduce its numismatic worth even if the copper looks shinier.
Use a clear container. A glass cup or jar lets you watch the process unfold in real time. You'll see the tarnish start to lift within minutes, and the soda will gradually turn a faint blue-green as copper dissolves into it. It's surprisingly satisfying to watch.
Don't seal the container. A loose paper towel or just leaving the top open is fine. You don't want pressure to build up if any gas is produced, and you want to discourage anyone in your household from accidentally drinking your experiment.
Rinse and dry thoroughly. Once your time is up, run the penny under cool water, pat it dry with a soft cloth, and let it finish air-drying on a paper towel. The shine you see immediately after rinsing is about as good as it will get.
Handle with care afterward. Cleaned pennies will tarnish again over time as they're exposed to air and moisture. If you want to preserve the shine, store them in a small zip-seal bag with the air pressed out. This limits oxygen exposure and keeps them looking bright for months.
Consider alternatives for stubborn tarnish. If Coke alone isn't cutting through really built-up corrosion, a paste made from baking soda and a few drops of vinegar or lemon juice can be gently rubbed onto the penny with a soft cloth. The mild abrasive action of the baking soda combined with the acid gives you a little extra cleaning power without harsh chemicals. Just be gentle — copper is soft and scratches easily.
The Bottom Line
So, does Coca-Cola really clean pennies? Yes, it genuinely does, and the science behind it is straightforward: phosphoric acid in the soda reacts with copper oxide tarnish, dissolving it and leaving the underlying copper shiny again. It's a fun, accessible chemistry demonstration that uses nothing more than a can of soda and a few grimy coins from your pocket.
The key is moderation. A few hours in Coke will give you a satisfyingly clean penny and a great visual lesson in acid-metal reactions. Leaving pennies in soda for days is asking for trouble, especially with modern zinc-core coins, which can corrode, pit, or even break apart. Stick to 4–6 hour soaks, rinse well, dry thoroughly, and your pennies will come out looking noticeably brighter.
Beyond the novelty, this experiment is a small reminder of how everyday products are full of chemistry we rarely think about. Which means the same acid that strips tarnish off an old coin is the reason you don't want to leave a tooth in a glass of soda overnight. Once you start noticing the science hiding in ordinary items, you start seeing experiments everywhere — in your kitchen, your medicine cabinet, your garden shed.
So grab a few dull pennies, a flat dish of Coke, and watch what happens. In an hour, you'll see tarnish lifting off. In a few hours, you'll have shiny little copper discs that look almost new. And if anyone asks what you're doing, you can honestly say you're doing chemistry.
