Some pennies are bright and shiny, while others look dark and dirty. Through a penny cleaning science project, you can study atoms, chemical changes and observing processes through the scientific method, all of which are science lesson benchmarks for eighth grade.

## Preparation

### Step 1

Collect pennies of varying color. Observe the pennies with parents or project partners and ask why they think some pennies look really dirty while others are a gorgeous copper color.

### Step 2

Research what materials pennies are made of. Find a source that explains everything is made of atoms. The pennies are made of copper atoms, but sometimes the copper atoms combine with other atoms to make molecules. When copper atoms combine with oxygen atoms in the air, they make a molecule called copper oxide, which makes the penny look dirty.

### Step 3

Ask yourself what might clean the copper oxide. Create a list of suggestions. Narrow the list to three to five options. You might want to add a vinegar and salt mixture or ketchup as one of the possibilities because those substances have been used in previous penny-cleaning experiments.

### Step 4

Create your research question for the experiment. For this experiment, you can ask, “What substance will make the pennies look the cleanest?”

### Step 5

Come up with a hypothesis, which is an educated guess. A possible hypothesis is “The ketchup will clean the copper oxide the best.” Next, you will do your own experiments at home and share your results at the science fair.

## The Experiment

### Step 1

Label clear plastic cups or glass jars with each substance you will test. For example, have a cup each for water, ketchup, lemon juice and vinegar and salt. Alternatively, from your research you know that ketchup will clean the penny because it has vinegar and salt in it. You can test ketchup, vinegar alone, saltwater alone, tomato paste alone and vinegar mixed with salt to determine which one works the best.

### Step 2

Fill each cup with a few tablespoons of each substance.

### Step 3

Place a dirty penny of comparable color inside each cup, making sure the substance covers the penny completely.

### Step 4

Wait 30 seconds to five minutes and pull the pennies out of the cups. Rinse each penny in water, and set them on paper towels in front of the labeled cups.

### Step 5

Compare the brightness of each penny and record the data into a notebook. Which substance worked the best? Was it the ingredient you expected? Was your hypothesis correct?

### Step 1

Explain in your science project report why certain substances cleaned the dirty pennies. Copper oxide dissolves in weak acids, and since vinegar is an acid, it works well to clean the pennies. Since ketchup contains vinegar, it also works. Acids like lemon or orange juice will also dissolve the copper oxide.

### Step 2

Explain in your report why water alone didn’t clean the pennies. Water is not an acid but rather has a fairly neutral pH, so it is not strong enough to cause the chemical reaction.

### Step 3

Explain why the combination of vinegar and salt was the winner of the experiment if it indeed did the best job. In vinegar, salt breaks down into sodium and chloride ions, which combine with the copper ions to remove the copper oxide from the penny’s surface. The tomato paste or saltwater alone probably didn’t clean the penny very well, but the combination of these ingredients, such as that found in ketchup or the vinegar and salt mixture, makes a more effective cleaner.

### Step 4

Put all your data together in a report and create a visual aid like a poster board that shows your results from the experiment and why and how you came up with those outcomes.