Exploring the Classic Penny Experiment for Kids at Home

Table of Contents

1. Introduction
2. The Science of the "Dirty" Penny
3. Experiment 1: The Ultimate Penny Cleaning Challenge
4. Experiment 2: The Mystery of the Green Penny
5. Experiment 3: Surface Tension and Water Drops
6. Experiment 4: The Dancing Penny (Air Expansion)
7. Integrating "Edutainment" into Daily Learning
8. Tips for Parents and Educators
9. Extension Activity: Copper Plating a Nail
10. Historical Connections: The Story of the Penny
11. Conclusion
12. FAQ

Introduction

Finding a handful of dull, brown pennies at the bottom of a purse or tucked into the crevices of the sofa is a familiar experience for most of us. To a child, these tarnished coins might look like old junk, but to an educator or a curious parent, they represent a perfect, low-cost laboratory. We love finding ways to turn everyday objects into "aha!" moments that bridge the gap between playing and learning.

At I'm the Chef Too!, we believe that the best way to teach STEM is through hands-on experiences that children can actually touch, see, and sometimes even taste. While you won't be eating these pennies, the chemistry involved is very similar to the reactions we see in the kitchen every day. This article explores several variations of the penny experiment for kids, providing step-by-step instructions and the clear science behind the magic. For more hands-on chemistry ideas, explore these easy kitchen chemistry experiments for kids.

Our goal is to help you transform a rainy afternoon or a classroom science block into a vibrant exploration of chemistry and physics. By the end of these activities, your children will understand why metal changes color, how water molecules stick together, and how simple household acids can act like powerful cleaners. These experiments are designed to spark curiosity and build the confidence kids need to view themselves as young scientists.

The Science of the "Dirty" Penny

Before we start scrubbing, it is important to understand why pennies look the way they do. When a penny is first minted, it is made primarily of copper (with a zinc core in modern versions), which has a bright, reddish-gold shine. Over time, that shine fades into a dull, dark brown. This is not actually dirt or grime in the traditional sense; it is the result of a chemical reaction.

What is Copper Oxide? When copper is exposed to the oxygen in the air, the two elements bond together to form a new substance called copper oxide. This process is called oxidation. Think of it like rust on a car or a brown spot on a sliced apple. The copper oxide creates a thin, dark layer over the coin, protecting the metal underneath from further damage but hiding its natural luster.

Why Use Pennies for STEM? Pennies are an ideal teaching tool because they are accessible and provide immediate visual feedback. When we introduce a cleaning agent, the change happens almost instantly, which is vital for keeping younger children engaged. It allows us to discuss concepts like atoms, molecules, and chemical bonds without needing a professional laboratory.

Key Takeaway: The "dirt" on a penny is actually copper oxide, a chemical compound formed when copper reacts with oxygen in the air.

Experiment 1: The Ultimate Penny Cleaning Challenge

This is the most popular penny experiment for kids because it feels like a magic trick. By using simple kitchen pantry staples, you can strip away years of oxidation in seconds. This activity is perfect for teaching the scientific method: asking a question, making a hypothesis, and observing the results.

Materials Needed

• 10–15 dull, tarnished pennies
• 1/4 cup white vinegar (acetic acid)
• 1 teaspoon table salt (sodium chloride)
• A shallow glass or plastic bowl
• Paper towels
• A spoon for stirring
• Optional: Other liquids for comparison (water, dish soap, lemon juice, ketchup)

Step-by-Step Instructions

Step 1: Prepare the cleaning solution. Pour the vinegar into the bowl and add the salt. Stir the mixture until the salt has completely dissolved. You are creating a mild acid solution that is specifically designed to break down copper oxide.

Step 2: Make a hypothesis. Ask your child what they think will happen when the penny touches the liquid. Will it change color? How long will it take? If you are using comparison liquids like plain water or soap, ask them to guess which one will be the most effective "shiner."

Step 3: The "Half-and-Half" test. Hold one dirty penny by the edges and dip it halfway into the solution. Count to ten slowly. When you pull the penny out, you will see a sharp, distinct line where the bottom half is bright and shiny while the top half remains dull.

Step 4: Submerge the rest. Drop the remaining pennies into the bowl. You can watch them start to brighten almost immediately. Let them soak for about five minutes to ensure the reaction is complete.

Step 5: Rinse and dry. Remove the pennies and rinse them under cool, running water. This stops the chemical reaction. Pat them dry with a paper towel and admire the "new" coins.

Why Does It Work?

The combination of vinegar (an acid) and salt (which contains chloride) creates a powerful environment for dissolving copper oxide. The acid attacks the oxygen bonds in the oxide layer, while the salt helps speed up the process. Once the copper oxide is dissolved into the liquid, the pure, shiny copper underneath is revealed.

Quick Answer: The vinegar and salt solution dissolves the copper oxide layer on the penny, revealing the shiny copper underneath through a chemical reaction.

What to do next:

• Try using ketchup instead of vinegar and salt. Since ketchup contains both vinegar and salt, it works surprisingly well!
• Test different types of coins, like nickels or quarters, to see if the reaction is the same (spoiler: it isn't, because they aren't made of copper).
• Use a toothbrush to see if scrubbing makes the pennies even shinier.

For another collection of household science activities, try these fun and easy experiments for kids at home.

Experiment 2: The Mystery of the Green Penny

If you have ever wondered why the Statue of Liberty is green, this experiment will give you the answer. This is a follow-up to the cleaning experiment and teaches children about long-term chemical reactions and the formation of a "patina."

Materials Needed

• The "cleaned" pennies from the previous experiment
• Paper towels
• A shallow tray or plate
• Extra vinegar

Step-by-Step Instructions

Step 1: Set the stage. Place a paper towel on a plate and soak it with white vinegar until it is quite wet but not swimming in liquid.

Step 2: Position the pennies. Take the pennies that you just cleaned in the previous experiment. This time, do not rinse them with water. Place them directly onto the vinegar-soaked paper towel.

Step 3: Wait and observe. This experiment requires patience. Leave the pennies exposed to the air for at least 24 hours. Check on them every few hours to see if any colors are starting to appear.

Step 4: Analyze the results. After a day, the pennies will no longer be shiny. Instead, they will be covered in a crusty, bluish-green substance.

The Science of Verdigris

When you don't rinse the salt and vinegar off the penny, the copper remains "active." The vinegar helps the copper react even faster with the oxygen in the air and the chlorine from the salt. This creates a new compound called malachite or verdigris. This is exactly what happened to the Statue of Liberty, which is covered in a thin layer of copper. Over decades of exposure to air and salty sea spray, it turned from a "penny-brown" statue to the iconic green landmark we see today.

Bottom line:

Rinsing a penny stops the reaction, keeping it shiny. Leaving it unrinsed allows the copper to react with the air, turning it green—just like the Statue of Liberty.

Experiment 3: Surface Tension and Water Drops

Moving away from chemistry and into the world of physics, the water drop experiment is a fantastic way to teach kids about the hidden "skin" of water. This is a high-engagement activity because it often turns into a friendly competition to see who can get the most drops on their penny. You can extend the lesson with these surface tension experiments for kids.

Materials Needed

• One clean penny
• An eye dropper or pipette
• A small cup of plain water
• A small cup of soapy water
• Paper towels (for the inevitable spills)

Step-by-Step Instructions

Step 1: Level the penny. Place your penny flat on a table. Make sure the surface is as level as possible so the water doesn't slide off prematurely.

Step 2: Make a guess. Ask your child to guess how many drops of water will fit on the head of the penny. Most kids guess 5 or 10. They are usually shocked by the real answer.

Step 3: Add water slowly. Using the dropper, add water to the center of the penny one drop at a time. Count them out loud. Watch the water as it begins to rise above the edges of the penny, forming a shimmering dome.

Step 4: The breaking point. Continue adding drops until the "bubble" finally bursts and the water spills over the side. Record the total number of drops.

Step 5: The soap variable. Repeat the experiment using the soapy water. Ask if they think the soap will allow for more or fewer drops.

What is Surface Tension?

Water molecules are very "sticky." They like to cling to each other through a force called cohesion. On the surface of the water, the molecules have nothing above them to grab onto, so they cling even more tightly to the molecules beside and below them. This creates a strong "film" or "skin" called surface tension. This tension allows the water to pile up into a dome shape on the penny without falling off.

Why did the soap change things? Soap is a "surfactant," which means it breaks the surface tension. It gets in between the water molecules and stops them from sticking together as tightly. Because of this, a penny will usually hold significantly fewer drops of soapy water than plain water.

Key Takeaway: Surface tension acts like an invisible skin that allows water to form a dome on a penny, but soap weakens that skin and causes the water to spill sooner.

Experiment 4: The Dancing Penny (Air Expansion)

This experiment introduces children to the concepts of thermodynamics and air pressure. It is a quick, visual demonstration of how heat can move objects.

Materials Needed

• An empty glass bottle (like a soda or juice bottle)
• A penny
• A small cup of water
• Your own two hands

Step-by-Step Instructions

Step 1: Prep the bottle. Place the empty glass bottle in the freezer for about 10–15 minutes. You want the air inside the bottle to be very cold.

Step 2: Seal the top. Take the bottle out of the freezer. Dip the penny in water so it is wet, then place it flat over the mouth of the bottle. The water creates an airtight seal.

Step 3: Apply heat. Wrap your warm hands tightly around the body of the bottle. Hold it there and watch the penny closely.

Step 4: Watch the "dance." After a minute or two, the penny will start to jitter, hop, or "dance" on top of the bottle.

The Science of Expanding Air

When air gets cold, its molecules slow down and move closer together, taking up less space. When you warm the bottle with your hands, you are warming the air inside. Warm air molecules move faster and push outward, taking up more space. This is called thermal expansion. As the air expands, the pressure inside the bottle builds up. Eventually, the pressure becomes strong enough to lift the penny for a split second to let some air escape, creating the "hopping" or "dancing" effect.

Integrating "Edutainment" into Daily Learning

At I'm the Chef Too!, we see every kitchen activity as a chance to weave together STEM, art, and the joy of discovery. These penny experiments are the perfect introduction to our philosophy of "edutainment." While we focus on the science of the coins, the underlying skills—observation, measurement, and patience—are the same ones kids use when they are baking a cake or mixing colors for a painting.

When children see that science isn't just something in a textbook but something happening in their pockets or on their kitchen counters, their perspective changes. They stop asking "Why do I have to learn this?" and start asking "How does this work?" This shift in mindset is the foundation of lifelong learning.

For families who want to take this hands-on approach even further, our monthly STEM cooking adventures provide everything needed to explore these concepts through food. For example, understanding chemical reactions by cleaning a penny is the perfect precursor to an activity like our Erupting Volcano Cakes kit, where kids see a much more "delicious" version of a chemical reaction in action.

Myth vs. Fact

• Myth: Pennies are dirty because people have touched them.
• Fact: While they do carry germs, the dark color is actually a chemical compound (copper oxide) caused by the air.
• Myth: Only strong chemicals can clean metal.
• Fact: Mild food-grade acids like vinegar, lemon juice, and even the acid in ketchup are strong enough to break down metal oxides.

Tips for Parents and Educators

Running a penny experiment for kids is generally simple, but a few small adjustments can make the experience much smoother and more educational.

1. Manage the Mess

While these experiments aren't as messy as baking, vinegar and salt can be sticky and have a strong smell.

• Work on a tray or a plastic tablecloth.
• Keep plenty of paper towels nearby.
• Ensure the children wash their hands after handling the coins and the vinegar solution to avoid eye irritation.

2. Encourage Documentation

Even for preschoolers, keeping a "lab notebook" is a great habit.

• Have them draw a picture of the "before" and "after" penny.
• Ask them to write down their guess (hypothesis) before each step.
• If you have a magnifying glass, encourage them to look at the surface of the penny up close to see the texture of the copper oxide or the malachite.

3. Ask Open-Ended Questions

Instead of just explaining the science, try to lead them to the answer.

• "What do you think made the water fall off the penny?"
• "If the vinegar cleaned the penny, do you think lemon juice would work too? Why?"
• "Why do you think the penny didn't turn green as soon as we put it on the paper towel?"

4. Adjust for Different Ages

• Toddlers and Preschoolers: Focus on the "magic" of the color change and the sensory experience of the water drops. Use simple words like "acid" and "sticky water."
• Elementary Kids: Introduce the scientific method. Have them measure the ingredients exactly and record their data in a chart.
• Middle Schoolers: Dive deeper into the chemistry. Discuss the periodic table, the specific chemical formula for copper oxide, and the concept of ions in the vinegar and salt solution.

Extension Activity: Copper Plating a Nail

If your students are fascinated by the cleaning experiment, you can take it a step further by showing them how metal can move from one object to another. This is a brilliant way to demonstrate that the copper oxide didn't just disappear—it's still in the liquid!

What to do:

1. After you have cleaned about 20–30 pennies in your vinegar and salt solution, remove the pennies.
2. Take a clean, steel nail or a galvanized screw and place it into the "used" blue-tinged liquid.
3. Let it sit for 15–30 minutes.
4. When you pull the nail out, it will be coated in a thin layer of copper!

The Science: When you cleaned the pennies, the copper atoms were pulled into the liquid. These atoms are looking for a new home. Because the iron in the nail is more "active" than copper, the atoms swap places. The iron dissolves slightly into the vinegar, and the copper atoms settle onto the surface of the nail. This is a basic form of electroplating without the need for a battery!

Historical Connections: The Story of the Penny

Integrating history into your STEM lesson adds a layer of "art and humanities" that makes the learning more well-rounded. Understanding the object you are experimenting with makes the experiment feel more significant.

• 1793: The first official US pennies were minted. They were much larger than today's pennies and were made of 100% pure copper.
• 1857: The penny was made smaller to the size we recognize today.
• 1909: Abraham Lincoln was placed on the penny to celebrate the 100th anniversary of his birth. He was the first president to ever appear on a US coin.
• 1943: Because copper was needed for equipment during World War II, the US Mint made "steel pennies" for one year. They were silver in color and are highly collectible today.
• Today: Modern pennies are actually 97.5% zinc with a very thin copper coating. This is why they feel lighter than older pennies!

By sharing these facts, you transform a simple science lesson into a story about our country and how we use resources. It also explains why older pennies might react differently or more intensely in your experiments than brand-new ones.

Conclusion

The humble penny experiment for kids is a testament to the fact that you don't need expensive equipment to teach profound scientific concepts. Whether you are revealing the hidden shine of an old coin, watching a green patina form over time, or marveling at the strength of surface tension, you are building the critical thinking skills that define a scientific mind.

We believe that learning should be an adventure that happens right at the kitchen table. By combining the arts, STEM, and a bit of "edutainment," we can turn any mundane object into a source of wonder. These moments of screen-free, hands-on discovery are where genuine family bonds and confidence are built.

If you enjoyed these activities, you can explore our full kit collection or continue with more kid-friendly chemical reaction experiments. Our mission is to make learning as exciting as it is educational, providing parents and educators with the tools to inspire the next generation of creators and thinkers.

"The most important thing a child can learn is how to ask 'why'—and the most important thing a parent can do is help them find the answer together."

Next time you find a dull penny, don't just spend it. Use it as an invitation to explore the invisible world of atoms and forces that shape our everyday lives.

FAQ

Why does ketchup clean pennies?

Ketchup contains both vinegar and salt, which are the two key ingredients needed to dissolve copper oxide. The acetic acid in the vinegar and the sodium chloride in the salt work together to break the chemical bonds of the tarnish, making ketchup a surprisingly effective (though slightly messier) penny cleaner.

Is the penny experiment safe for preschoolers?

Yes, the penny experiment is very safe, provided there is adult supervision. White vinegar and table salt are non-toxic kitchen staples, though they can sting if they get into a small cut or the eyes. Ensure children wash their hands after the activity and remind them not to put the coins in their mouths, as they can be a choking hazard.

How long does it take for a penny to turn green?

While you might see slight changes within a few hours, it typically takes 24 to 48 hours for a distinct bluish-green patina (malachite) to form. The speed of the reaction depends on how much oxygen is reaching the penny and how much vinegar and salt solution remains on the surface.

Does this experiment work on all coins?

The cleaning and greening experiments work best on pennies because they have a copper exterior that reacts specifically with vinegar and salt. Other US coins, like nickels, dimes, and quarters, are made of a nickel-copper alloy that does not oxidize in the same way, so you won't see the same dramatic results. Teachers planning a larger hands-on activity can also explore school and group programmes for classroom and group learning.