The penny cleaning experiment is a classic school project because it turns an old, dull coin into a bright copper-colored penny using simple kitchen materials. Pennies look dirty over time because copper on the surface reacts with oxygen and other substances in the air. Vinegar and salt work together to loosen and remove this tarnish.
The project is easy to see, safe with adult supervision, and a great way to learn that chemistry can happen all around us.
In the cup, vinegar provides acetic acid and salt provides chloride ions that help dissolve copper oxide from the penny's surface. When the dark coating is removed, the shiny copper underneath becomes visible. A good project setup compares pennies cleaned for different amounts of time, such as 1 minute, 3 minutes, and 5 minutes.
Students can record observations, make a simple data table, and explain how the solution changed the penny's surface.
Understanding The Penny Cleaning Experiment
A strong experiment separates what you change from what you measure. The changed factor might be soaking time, amount of salt, or the kind of liquid used. The measured result could be brightness, amount of dark coating removed, or a score based on photographs.
Keep the other conditions the same. Use pennies of similar dullness, equal amounts of solution, identical cups, and the same rinsing method.
Test more than one penny in each condition because one coin may have scratches or a different amount of tarnish. This makes the conclusion more reliable.
The cleaning does not create new copper. It reveals copper that was hidden beneath a thin surface layer. That layer can contain copper compounds formed as the metal meets air, moisture, skin oils, and pollutants.
Acids can react with parts of this layer, allowing particles and dissolved copper compounds to move into the liquid. Salt changes how easily copper compounds dissolve. This is why the combined mixture often works better than plain vinegar.
The exact color of a dirty penny can vary. Brown, black, green, or blue-green areas may come from different copper compounds and different conditions during storage.
Careful observation matters more than simply deciding that one penny looks cleaner. Photograph each coin before treatment, immediately after treatment, and after it dries. Put the coins on the same background under the same light.
A wet coin can look much shinier than a dry one, so compare dry coins for a fair result. Record changes in the liquid too. A blue or green tint can suggest that copper-containing material has entered the solution.
Note bubbles, loose particles, or patches that remain dark. These details help connect visible results to chemical changes rather than treating the experiment as a magic trick.
Modern pennies need extra care when interpreting results. Many newer United States pennies have a zinc center with a thin copper coating. Deep scratches or long exposure to acidic liquid can damage that coating and reach the zinc underneath.
If this happens, the coin may develop dull spots, rough areas, or unexpected color changes. That is not better cleaning. It is evidence that the reaction has begun to affect the coin itself.
Use brief test times and rinse each penny thoroughly, then dry it with a paper towel. Do not taste the solution or use it for food afterward. Dispose of small amounts according to classroom or home guidance, and wash hands after handling the coins and liquid.
Key Facts
- Materials: dull pennies, white vinegar, table salt, clear cup, spoon, timer, paper towels, and water for rinsing.
- Basic cleaning solution: mix about 60 mL vinegar with 1 teaspoon salt until the salt mostly dissolves.
- Copper reacts with oxygen over time to form copper oxide on the penny surface.
- Vinegar contains acetic acid, which helps break down copper oxide.
- Salt helps the cleaning work faster because chloride ions help remove the tarnish from the copper surface.
- Always rinse pennies with water after cleaning because leftover acid and salt can keep reacting with the metal.
Vocabulary
- Tarnish
- Tarnish is a dull coating that forms on a metal surface after it reacts with substances in the air.
- Copper oxide
- Copper oxide is a dark compound that can form when copper reacts with oxygen.
- Acetic acid
- Acetic acid is the weak acid found in vinegar that helps dissolve the tarnish on pennies.
- Solution
- A solution is a mixture in which one or more substances are evenly spread through a liquid.
- Variable
- A variable is something in an experiment that can be changed, measured, or kept the same.
Common Mistakes to Avoid
- Using different amounts of vinegar and salt for each trial is wrong because it changes more than one variable at a time and makes results hard to compare.
- Forgetting to rinse the penny after cleaning is wrong because leftover vinegar and salt can keep reacting and may leave new stains.
- Scrubbing one penny but not the others is wrong because rubbing adds a physical cleaning method that changes the fairness of the test.
- Touching eyes or tasting the solution is wrong because even common kitchen chemicals should be handled carefully during an experiment.
Practice Questions
- 1 You mix 60 mL of vinegar with 1 teaspoon of salt for one trial. How much vinegar and salt do you need for 4 identical trials?
- 2 A penny is soaked for 1 minute, another for 3 minutes, and another for 5 minutes. What is the total soaking time for all three pennies?
- 3 If one penny is cleaned in vinegar only and another is cleaned in vinegar plus salt, explain which test is more likely to clean faster and why.