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).

Copperplating

Some metal items are not made from just one metals, but are composed of a thin layer of one metal on another. This is called plating, and very often, the base metal is cheaper than the coating. Silverplate is a common example of this. Plating is done by first dissolving the coating metal, then placing the base object in the solution. Sometimes electricity is needed to create the coating, but sometimes the reaction just happens spontaneously.

Here's an experiment you can perform to plate copper onto a nail, screw, or other iron or steel object.

What You'll Need:

20-30 pennies

1/4 cup vinegar

1 teaspoon salt

Water

Plastic or glass bowl

Spoon

1 or 2 nails or screws

Pour the vinegar and salt into the bowl, stir, and add water until the salt dissolves. Put the pennies into the liquid and let them sit for 10-15 minutes. What happens to the pennies? Remove the pennies from the solution and add the nails or screws. Let them sit for 30 minutes. Have they changed color? If not, check back in an hour. You will eventually notice a brown coating of copper metal forming on the nail or screw.

All About Alloys

There are some familiar metals that are actually mixtures of several different types of metals - they are called alloys (AL-oise). Here are a few you've probably heard of (and some that are likely new to you):

• Steel: mixture of iron and one or more other metals (there will be a separate post about steel later)
• Brass (copper and zinc): a shiny yellow metal that looks a little like gold
• Bronze (copper and tin): a dark gold to brown metal that gets darker in color as it ages
• Pewter (copper, tin, and lead): a dark gray metal used for bowls, flatware, jewelry and other decorative items
• Solder (lead and usually tin): an alloy that melts at a relatively low temperature; used to make electrical circuits
• Nitinol (nickel and titanium): this alloy remembers its original shape; used to make eyeglass frames
• Electrum (gold and silver): also known as white gold; used in jewelry
• Alnico (aluminum, nickel, and copper): used to make magnets
• Amalgam (silver, tin, copper, zinc, and mercury): a soft alloy that has been used to fill dental cavities
• Nichrome (nickel and chromium): an alloy that glows orange-red and gives off heat when an electrical current is passed through it; used in toasters and electric burners

Take a look around your house and see how many examples of alloys you can find!

A Cola Cleaner

Here's a quick and easy experiment you can do that will demonstrate how acids can be used to clean metal. Pour about 2 inches (5 cm) of a cola drink into a disposable cup (make sure that the cola lists phosphoric acid in the ingredients). Drop a dirty penny (made of copper) it into the cola and let it sit for a few days. When you take the penny out, you will see that it has become very shiny? Where did the dull brown coating from the penny go?

You've seen new pennies, and you know that they are bright and shiny. But with time, the copper metal in the coins combines with oxygen from the air and forms a dull coating of copper oxide). The acid reacts with the copper oxide and forms copper phosphate, which does not stick to the penny's surface. So when you remove the penny from the cola, you have clean, shiny metal again.

One final thing - do not drink the cola after you're done with it. It contains dissolved copper, which is not good for you.

Copper Coating

Many metal objects around you are plated - they have only a thin coating of the metal you see covering another metal (usually less expensive). In particular, a lot of things are gold-plated or silver-plated. Gold and silver cost too much to experiment with, but here's an easy way to copper-plate a nail.

What you'll need:
1/4 cup (50 mL) of lemon juice or vinegar
Plastic or paper cup
10 to 20 dull pennies
Salt
2 small iron nails (not the galanized kind)
Steel wool scouring pad

Pour the lemon juice into the cup and drop the pennies in. Add a pinch of salt. What do you see happening? Wait three minutes. While you're waiting, scrub one of the nails with the scouring pad to clean off the surface. Rinse the nail well. Just put the second nail off to the side. Drop the cleaned nail into the cup, and wait about an hour. After that time has passed, take the nail out and compare it to the one you set aside. What do you see? When you're done, make sure to pour the lemon juice or vinegar down the drain and to throw the cup away.

As you may know, pennies are coated with the element copper (Cu). When you dropped them into the lemon juice or vinegar, bubbles formed. Metals like copper often react with acids (like the citric acid in lemons or the acetic acid in vinegar) to form bubbles of hydrogen (another element!) gas. You may have noticed that this solution changed color as some of the copper dissolved. When you added the iron (yes, yet another element!) nail, the dissolved copper collected on its surface, giving the nail a brown coat of copper.

The instructions said that you should not use a galvanized nail (which is coated with zinc - these elements are everywhere!), but you might want to try one, just to see what happens!

Fun Facts 2

Here are even more tidbits about elements:

• The blood of the octopus is blue because it contains copper (Cu, #29), instead of iron like the blood of mammals.
• Many dandruff shampoos contain selenium (Se, #34), which helps relieve itching and flaking.
• Many expensive camera lenses are made with glass containing lanthanum (La, #57), which gives clearer photos than regular glass does.
• Erbium (Er, #68) lasers can be used to remove wrinkles the skin.
• Berkelium (Bk, #97) is named after the city of Berkeley, CA, where it was first created at the University of California.
• The gemstones emerald, aquamarine, and alexandrite all contain beryllium (Be, #4).
• Radioactive technetium (Tc, #43) is used to detect tumors in the body.
• In the future, computer chips may be made with the element hafnium (Hf, #72), instead of with silicon, as they are today.
• All fluorescent light bulbs, including the new compact fluorescent lights (CFL's), contain the poisonous element mercury (Hg, #80). Because of this, you cannot throw them away with the regular trash.
• Many new cars come with xenon (Xe, #54) headlights, which give off a blue-ish light. They are very bright and can bother other drivers.