Summary

In this lab, students will investigate how an acidic, vinegar based solution can help to get “dirty” pennies clean. Students will observe chemical change and investigate a real-world connection to this lab.

Grade Level

Elementary school

Objectives

By the end of this lab, students should be able to:

• Describe their observations throughout a procedure and connect them to chemistry vocabulary.
• Explain what factors contribute to the development of copper oxide “dirt” on pennies.
• Describe how the copper oxide “dirt” was removed from the pennies.

Chemistry Topics

• Reactions
• Chemical Change
• Observations

Time

Teacher Preparation: 15 minutes

Lesson: Day 1 - 45 minutes; Day 2 -30 minutes

Materials

(Per Group:)

• 5-10 dull, dirty pennies
• 1/4 cup or ~60ml white vinegar
• 1 teaspoon salt
• a clear, shallow bowl (not metal)
• paper towels

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• I have found that this lesson works best if students work in cooperative groups of 2-3.
• The second day is used to make a real world connection to the penny activity. This can completed individually or in groups.
• Please also note this warning: Since 1982, pennies have been made mostly of zinc with a thin copper coating, rather than mostly made of copper. Because vinegar will react with zinc, use caution with the amount of time you allow the newer pennies to sit in the vinegar solution.
  

Day 1: Investigating why the salt and vinegar will get the pennies clean.

1. Ask students: What do you know about pennies?
   1. Have student write their answers on their student paper first, and then share their answers with the class.
   2. From the student answers make a chart of things they know vs. what they want to know
   3. Refer back to this chart during each day of the lesson to update it
2. Watch Dr. Floyd’s "Dirty Pennies".
3. Have students attempt to define the following chemistry vocabulary words in the appropriate column of the provided table (located on their student handout). Then discuss/review the meaning of the vocabulary words and associated each with a relevant example from that will be used in this activity. Students should add information to the second column of the table as needed:
   1. Atom – The basic unit that makes up an element. In this lab Copper is the element that pennies are made of (or coated with).
   2. Molecule – A chemical compound that is made up of two or more atoms. Copper Oxide is the greenish molecule that forms on the surface of the penny and makes them look dirty.
   3. Mixture – A combination of two or more substances that are not chemically combined. When the pennies are mixed with the liquid solution this will be a mixture.
   4. Solution – This is a homogeneous liquid mixture, where one component (solute) is dissolved in the other (solvent). In this lab the salt is the solute that will form a solution when dissolved in the vinegar solvent.
   5. Acid – A substance with a pH below 7. Often associated with foods/drinks that are very tart in taste.
   6. Reaction – Atoms combine or rearrange themselves to form new substances. The acetic acid in the vinegar will react with the copper oxide “dirt” coating on the surface of the penny to form new substances: copper acetate and water.
   7. Chemical change – any change that results in new substances. For example, this is why the copper oxide appeared – the copper in the penny reacted with the oxygen in the air to create copper oxide on the surface of the penny.
4. Students should work in groups of 2-3. Each group will need the following materials:
   1. Clear bowl
   2. ¼ cup or ~60ml of vinegar
   3. 1 teaspoon of Salt
   4. Stirring device
5. Students should combine the salt and vinegar in the bowl and stir until the salt dissolves.
6. Each student should dip one penny halfway into the liquid. The penny should be held there for about 10 seconds and then removed. Students should record their observations and draw a sketch on the student data sheet.
7. Place all the pennies into the solution. Students will see a change for the first few seconds. After that the change will slow.
8. After 5 minutes, remove the pennies from the solution. Rinse them very well with water. Put them on a paper towel to dry.
9. Students should finish the “Analysis” section of the student handout.
10. Discuss observations and student answers to the analysis questions. Also be sure to review what actually happened so there is no misunderstanding.

• What happened? Copper oxide is the green/dull colored layer that is often seen on pennies and other copper objects. This forms due to a reaction between copper and the oxygen in the air, and is why over time the penny will appear dull/greenish. When the “dirty” penny is placed in the solution of vinegar and salt, the acetic acid in vinegar reacts with the copper oxide on the outside of the penny, creating new products and effectively removes the “dirt” from the penny.
• Extension opportunity: You could clean pennies with combinations of salt and lemon juice or orange juice, because those are acidic solutions also. Students could compare the effectiveness or the time difference when using different solutions.
  
• Anticipated results are shown below (from left to right: before reaction, during reaction, after reaction—comparison of dirty and clean pennies):

Day 2: Real World Connections:

• As a class discussion, ask the students the following questions:
  • Does anyone know what the Statue of Liberty is?
  • What does it look like?
  • What is it made of?
• Watch the New York Historical Society “When Did the Statue of Liberty Turn Green?”
• As an extension, students can create their own Statue of Liberty.
• They can write down a reflection about the chemistry related to copper and the statue of liberty.