Cleaning Coins Experiment [Procedure And Explanation]

Reviewed & Fact Checked by Rebecca Carlock

Kids are naturally curious, and as their parent or guardian, you will want to encourage that as they grow older.

A great way to do this is by doing fun science experiments with your kid at home. A lot of parents or guardians may initially be switched off to this because they assume you will need fancy kits and equipment but that’s not always true.

Cleaning Coins Experiment [Procedure and Explanation]

Sometimes, all you need is some glue, some baking soda – or a few old pennies.

If you are looking for an easy way to engage your kids with science, then why don’t you try out the classic cleaning coins experiment? It’s a great way to get their young minds’ critically thinking, and also challenges them to solve problems using logic. Plus, they’ll learn some cool facts about metals, the pH scale, and the Statue of Liberty.

So if you want to try this simple science experiment out at home, here is what you need to do:

Step One: Make Your Hypothesis

The first step you will have to do is to make your hypothesis.

Making an hypothesis is easy – it’s basically asking a question! To get your kids involved, ask them a few questions about pennies and cleaning them. For example, ask them if they know you can clean pennies, why you should clean pennies, and what would be the best thing to use to clean pennies?

This will help them to get thinking about the subject and soon, they will start to come up with their own questions. Encourage this and get them to write down their supposed answers. Later when you have your results, you will be able to look back on what they have written down and compare their results to their initial answers.

Step Two: Gather Your Materials

One of the best things about this experiment is that it does not cost a lot to set up in terms of materials. All you need is a couple of old copper coins, some different liquids and a few other tools. Here is the full list so you can tick them off with your kids as you collect them:

  • 5 old copper pennies (these coins need to be copper for this experiment to work)
  • 5 paper cups
  • Tweezers
  • 5 strips of paper (for labelling each penny)
  • Some kitchen paper or paper towels
  • A pen and paper for jotting down the results
  • 5 different liquids to use to clean your pennies

When it comes to what liquids you and your kids can use in this experiment, part of the fun is asking them what they think would be good to use and trying it out. There is a second stage to this experiment that you can also try out, and the only extra materials you will need is two more old pennies and some more paper towels.

Remember to be sensible and don’t let them near any dangerous liquids like petroleum or bleach – kids may come up with a few silly answers for fun, but as the adult, you need to draw the line at what is fun and what could be potentially dangers.

We would recommend you use any liquids from the following list for the best results:

  • Water
  • Soda/Fruit Juice
  • Ketchup
  • Vinegar
  • Vinegar and Salt Mixture

Step Three: Carry Out Your Experiment

Step Three: Carry Out Your Experiment

Now, you can get to the fun part – carrying out the experiment!

Make sure you stay with your kids during this part and engage with them. Ask them questions, have them tell you what they see happening. This will help them understand what’s going on.

Here is a step by step walkthrough of the experiment so you don’t get forget what to do or lose track of where you are:

  1. First, prepare all your cups and line them up on a counter. You should have at least five different liquids ready to pour into each individual cup, but you can always add some more for fun. Label each cup with a pen so you and your kids know what liquid is in each up.
  2. Once all your liquids have been poured into their own cup, you can have your kids drop one old coin in each liquid and leave them submerged for 10 mins.
  3. While the coins are submerged in the cups, lay out some paper towels or kitchen paper and label each towel so you know which coin is going where. This way, you won’t mix up the coins when it comes to comparing them.
  4. After the 10 minutes have passed, use the tweezers to take the coins out of each cup. If you want things to be fun and messy, you can always have your kids use their hands – as long as you are happy to deal with the consequences of ketchup-coated fingers!
  5. Wipe the coins dry with their paper towels and compare them. Have your kids write down their findings on their piece of paper ready to summarize their findings and talk through the explanation.

Once this is all done, you may be tempted to clean everything – but not yet! There is a second stage to this experiment that you can also try out while you are talking to your kids about their results from the first stage.

What You Will Need To Do Is

  1. Pop another coin in both the ketchup and vinegar and salt mixture. Leave them there for 10 minutes while you and your kids talk about the results of the first experiment.
  2. After 10 minutes, take the coins out of the cups and lay them out on some kitchen paper – but do not dry them!
  3. Leave the coins to air dry for at least 30 minutes. Check the coins, then come back again in another 30 minutes and see what has happened to the coins.

While the coins are air drying in the second stage, you can talk your kids through their findings of the first stage together!

Part Four: The Science-y Stuff

So, the results of your kid’s coin cleaning experiment should be that the ketchup and the vinegar and salt mixture were the best coin cleaners. The coins that were submerged in those cups should come out nice and shiny, while the rest are still a little dull and dirty.

So – what’s the science behind it all?

Pennies are obviously made of copper, and they start out bright and shiny when they are new. However, over time, they turn dull and dark. This is because copper reacts to oxygen in the air and it turns into a compound called copper oxide.

Copper oxide is the dull dark color that covers old coins – the older a copper coin is, the more copper oxide it has coating its surface. That is why old coins look, well, old and new coins are bright and shiny.

So when it comes to cleaning old copper coins, then you will want a solution that gets rid of copper oxide – and copper oxide dissolves easily in acids.

Therefore, acids are the best kind of liquid to use to clean coins as it easily dissolves the copper oxide that coats old coins and makes them look new and shiny again.

So, the liquids that have worked best in your experiment are the ones that are more acidic. If you line up your pennies in order from the dirtiest to the cleanest, the order should go something like this:

Water, Soda/Fruit Juice, Vinegar, Ketchup, and Vinegar and salt mixture.

Water is the worst because it is a neutral liquid, meaning that it sits smack in the middle of the pH scale. Next should either be the soda/fruit juice or vinegar (sometimes these switch depending on what type of each you have used) because their pH is usually around 3, so they are mildly acidic.

Then, you should have more acidic solutions like ketchup and the salt and vinegar mixture.

The ketchup is most likely what will surprise your kids the most (and let’s be honest – perhaps you too!). The reason why ketchup is such a good coin cleaner is because it is made from tomatoes, and tomatoes are an acidic vegetable (yes, tomatoes are a vegetable). Ketchup also contains a lot of salt, which helps add to its acidity.

This is also why the salt and vinegar mixture was a better coin cleaner than just vinegar alone. Salt and vinegar mixed together creates a chemical reaction to form hydrochloric acid, which is far more acidic than vinegar alone.

And so, these two liquids are the most acidic and therefore, the best coin cleaners!

Step Five: The Bonus Round

If you have chosen to do the extra second step with your kids, then you will have noticed upon your frequent checks that the coins will have turned a blue-green color after being left to air dry.

This is because when the hydrochloric solution cleaned the coin and air dried it, it also took with it some of the salt. Salt is another compound called sodium chloride, so it contains a chemical called chlorine (like what we use in swimming pools to keep them clean).

Now, when copper reacts to oxygen, it turns dark brown to make copper oxide. When copper reacts with oxygen and chlorine, it turns blue-green instead! This blue-green is called malachite and copper can turn into malachite over many years of exposure to chlorine and oxygen.

And the best example of this reaction is the Statue of Liberty.

When the Statue of Liberty was originally built in 1875, it was not the same blue-green color it is today. It was originally a dark brown color because it was made out of copper – and over many years, it has slowly turned blue-green because the copper has transformed into malachite!


Now, wasn’t that a lot of fun?

The classic coin cleaning experiment is an easy and affordable way to get your kids interested in science. It utilizes a lot of stuff you already have lying around your house, and does not require you to go out and buy a lot of fancy science gadgets.

It also benefits your kid in a lot of ways too. Doing a simple science experiment with them now and again helps encourage them to ask questions and learn, as well as improve important life skills like communication, critical thinking, and problem solving. This will help them develop into highly skilled individuals and prepare them for more difficult situations they are sure to encounter later on in life.

Plus, there is another great reason to try this science experiment out – you’ll be spending some quality time with your kids. Not only will you be improving them as an individual, you will also be strengthening the bond you share together.

Elena Jones