Rust, Tarnish, and Verdigris

OK, so you certainly know what rust looks like, and may have seen tarnished silver, but what is that other thing? And what does it have to do with rust and tarnish?

Rust, tarnish, and verdigris (and the oxide coating on old pennies) all result from the chemical reaction of a bare metal surface with something in the air. That something may be oxygen, water, carbon dioxide, another chemical, or some combination. Rust forms when iron is exposed to air and moisture (water). If a piece of iron is left outside, it will eventually rust away to nothing. If you don't want an iron object to rust, you need to protect its surface by painting it or coating it with another metal, such as zinc (called galvanizing). You might think that silver tarnish also forms from a reaction with oxygen, but tarnish actually needs both oxygen and sulfur to form. Tarnished silver may look ugly, but the tarnish actually protects the silver underneath from further reaction!

Verdigris (VUR-di-gree) is the green coating that forms on copper or other copper-containing metals, such as bronze or brass. It forms from the reaction of copper with oxygen, carbon dioxide, and chloride (near seawater). Perhaps the most recognizable American example of verdigris is the Statue of Liberty. She is made of copper, and was originally brown! But over time, a verdigris coating has formed, giving her the familiar green color. You can sometimes see other examples of verdigris on copper roofs on old buildings, or on outdoor statues. Keep your eyes open, and you may spot some verdigris in your own neighborhood!

Shiny Pennies

You may know that U.S. pennies are made with copper, which gives them a brown color. New pennies are a bright, shiny, orange-brown, but over time, they become darker and duller. What happens to them?

The pennies are oxidizing, which means that they react with oxygen in the air. This forms a coating of copper oxide. But never fear, you can make your pennies shiny again!

What You'll Need:
Several dull pennies
Coca-Cola (not diet)
Plastic cup

Pour a few inches of Coke into the cup, and add the pennies. Let the pennies sit overnight, and check them the next morning. They will be shiny again!

IMPORTANT: Do not drink the Coke after the pennies have sat in it.

What did the Coke do to the pennies? Coke contains phosphoric acid, which reacts with the copper oxide and converts it back into copper metal. Other acids will do the same thing. Try putting pennies in lemon juice (citric acid) or vinegar (acetic acid).

Have you ever seen a metallic sheep??

What happened to the temperature in the last experiment? And how did the steel wool look at the end? You should have seen that the temperature went up by several degrees, and that the steel wool looked rusty after the experiment was done. What happened?

First of all, what exactly is steel wool? Is it the fleece from metallic sheep? Of course not. Steel wool is just very thin wires made of iron that is sold in clumps that looks something like sheep's wool. It is often used instead of sandpaper for removing paint or smoothing surfaces.

So, where did the heat come from? When iron comes in contact with the oxygen (O₂) and water (H₂O) in air, a chemical reaction called oxidation occurs. Rust is actually a mixture of several compounds of iron, oxygen, and hydrogen, with names like iron oxide and iron hydroxide. Do you remember learning about thermal energy in "Where Does Heat Come From?"? We said that a wood fire feels hot because the thermal energy of the wood was higher than that of the ashes and other material left over from the fire. The same thing applies to the iron. The thermal energy of rust is lower than that of the original iron, and you feel the energy lost as heat.

But why didn't the steel wool rust before you put it into the jar? Normally, steel wool is coated with oil or some other material that protects it from air. The vinegar you rinsed the steel wool with took the coating away, and the bare iron was exposed. So, the vinegar wasn't actually involved in the reaction, but it did allow the reaction to happen.

If you go to the drugstore, you can actually find a product that uses the oxidation of iron to generate heat! ThermaCare heat wraps contain iron (and some other chemicals) that react with air and produce warmth that helps relieve the pain of arthritis and sore muscles.