Everyone is familiar with the Statue of Liberty found in New York Harbor near Ellis Island, where more than 12 million immigrants came to the United States from 1892 - 1954. The statue was a gift to the U.S. from the people of France in 1886. However, the statue was not always the green color that we see today. Because the statue’s coating is made of copper, it originally had the familiar reddish-brown color of a penny. So why is the statue green today? The transformation took place over the course of about 30 years as the copper metal reacted with the surrounding atmosphere. Let’s see if we can do something similar with a penny by speeding up the reaction.

Materials

• Dish soap
• Measuring cup
• Paper towels  
• Penny
• Salt    
• Small container (glass)
• Small plate
• Tablespoon (plastic)
• Vinegar
• Water
   

Be Safe: Vinegar will irritate eyes and skin. Avoid handling or contact with skin and eyes.

Experiment

Clean the penny using soap and water, then rinse it and place it in the glass container. Add a tablespoon of salt to the container, followed by ¼ cup of vinegar. Swirl the mixture to dissolve the salt. If all the salt has dissolved, add a bit more until no more dissolves. Let the reaction sit for about an hour.

Does anything happen to the penny? Fold a paper towel so that it is about 5 cm (2 inches) by 10 cm. Remove the penny using the plastic spoon and place the penny on the paper towel sitting on a plate. Pour some of the vinegar/salt solution onto the penny and fold the towel onto itself so that it is about 5 cm square and covers the penny. Pour the remaining vinegar solution onto the paper towel so it is saturated. 

Let the experiment sit until the liquid has evaporated. (It’s OK to check the penny periodically to see if anything is happening.) After 24 hours you should see some greening of the penny. If little color is seen, try wetting the towel again with more vinegar. 

What’s happening?

If you started with an old brown penny, you may have seen it turn shiny red when it was first exposed to the vinegar solution. Vinegar reacts with the surface coating on a penny, exposing the elemental copper metal underneath. Chloride ions from the sodium chloride (salt) help this process along by pitting the metal. (This is similar to what happens to cars that are exposed to roadway salt in the winter.)

As the vinegar evaporated you should have observed a green color begin to develop. Removing electrons from elemental copper converts it to positively charged copper ions, which can bond to negatively charged ions to form bluish-green solids. In the case of our penny exposed to air and vinegar, oxygen from the air removes two electrons from copper metal and the resulting copper ions bind to acetate ions found in vinegar to give copper acetate, a green solid. Because copper acetate is soluble in water, if you wash the coin in water the green coating should dissolve in the rinse.

Extension

Other web sites suggest using ammonia for this reaction to give a deeper blue color, or Miracle-Gro^® to make the reaction go faster. Miracle-Gro^® has copper ions in it so the final color is not derived from the penny. What effect would old pennies have (pre-1984 pennies are 95% copper) versus newer pennies that are only 5% copper? You can also experiment using red wine vinegar, which contains sulfite ions that are also found in urban atmospheres.

References

https://www.frugalfamilytimes.com/2019/11/how-to-turn-copper-green-patina/

https://www.acs.org/pressroom/reactions/library/the-statue-of-libertys-true-colors.html

https://www.snopes.com/fact-check/statue-liberty-different-color/

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.

- PAUL BRANDT

^{Photo credit: https://www.pexels.com/photo/figure-lady-liberty-landmark-monument-47086/}