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.
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.