The Statue of Liberty is an iconic national monument on Liberty Island that was dedicated on October 28, 1886 (1). Back then, it was not as we see it today; then the exterior was copper colored, because, of course, it was made of copper! But today it is tiffany blue (2) to mint to seafoam green (3), depending upon the lighting (1, see image) (4). This was the result of a series of chemical reactions that took place over the first thirty years after the statue was assembled (5) and provide the reason why the Statue of Liberty is green.
The first reactions involve a concept called reduction in chemistry. Reduction occurs when an atom that is being oxidized donates electrons to the oxygen atoms. Chemists say that the oxygen atoms have been reduced. (6). They assign an oxidation number to the atom being oxidized that indicates the number of electrons gained or lost by that atom: a positive oxidation number means electrons have been donated and a negative oxidation number means electrons have been gained. Since oxygen is highly electronegative, it is eager to steal electrons. So oxygen tends to take electrons, which reduces its charge. That is the origin of the term “reduction.” Oxygen is more electronegative than most other elements, so in a reaction it is generally the atom that takes electrons and the it is generally the other atoms that give electrons, since they are more electropositive. (7). Since most elements tend to donate electrons to oxygen, the losing of electrons to another element is called “oxidation.” Reduction and oxidation are opposites, but they always go together. Thus, a reaction involving the giving and taking of electrons is called a redox reaction. (6).
In the first reaction, the copper is oxidized, by oxygen (which is reduced), to form CU2O. This compound is pink or red. Then the copper cation continues to react with oxygen to form copper oxide, 4CuO, which is black to brown. In the first years after Libertas’ figure was erected near New York City, much coal was burned in that city. The resulting air pollution wafted over the Statue of Liberty, bringing with it sulfur. This reacted with the copper to form the compound 4CuS, which is black. Three final compounds form from these initial compounds with the addition of carbon dioxide and hydroxyl ions: CuCO3(OH)2 (green), Cu3(CO3)2(OH)2 (blue), and Cu4SO4(OH)6(green). (8).
These three compounds form the iconic blue-green verdigris that encases the Statue of Liberty today. The Statue of Liberty provides a great lesson in chemistry about redox reactions and successive reactions.
- “Liberty Enlightening the World.” (n.d.).National Park Service. https://www.nps.gov/stli/index.htm. Date-accessed: 4/10/2018
- Knapton, Sarah. (2017). “First new shade of blue discovered for 200 years to be turned into Crayola crayon.” See image at end of article. The Telegraph. https://www.telegraph.co.uk/science/2017/05/12/first-new-shade-blue-discovered-200-years-turned-crayola-crayon/. Date-accessed: 4/10/2018.
- Morris, Brian. (2015). “50 Shades of Green…and One Shade of Blue.” PsPrint. https://blog.psprint.com/de(signing/50-shades-green-one-shade-blue/. Date-accessed: 4/10/2018.
- “Plan Your Visit.” (n.d.). National Park Service. https://www.nps.gov/stli/planyourvisit/index.htm. Date-accessed: 4/10/2018
- “Why is the Statue of Liberty Green?” (2018). Wonderopolis. https://wonderopolis.org/wonder/why-is-the-statue-of-liberty-green. Date-accessed: 4/10/2018.
- Clarck, Jim. (2016). “Definitions of Oxidation and Reduction.” LibreTexts. https://chem.libretexts.org/Core/Analytical_Chemistry/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction. Date-accessed: 4/10/2018.
- “Oxidation-Reduction (Redox) Reactions.” (2018). Khan Acandemy. https://www.khanacademy.org/science/chemistry/oxidation-reduction/modal/a/oxidation-reduction-redox-reactions. Date-accessed: 4/10/2018.
- Helmenstine, Anne Marie. (2018). “Why is the Statue of Liberty Green?” Thought co. https://www.thoughtco.com/why-statue-of-liberty-is-green-4114936. Date-accessed: 4/10/2018.