Penny Alchemy

Chemistry:	Single Replacement Reactions, Metal Alloys, Basic Solutions
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on

Also known as the Golden Penny, this demonstration is fun for students and parents alike. They can watch the coins turn silver while in a simmering solution, and marvel over the change. They will be surprised when they see the coins turn from silver to gold in a matter of seconds, simply by being heated up!

Materials

• Pennies, Sorted Between Pre-1984 & Post-1984

Preparation

• Vinegar
• Salt
• Water
• 2 Large Bowls

Presentation

• Zinc Metal Flakes
• 3M NaOH Solution
• 1000mL Beaker
• Hot Plate
• Tongs
• Large Bowl
• Paper Towels

Safety Precautions

Please read the Liquid Chemical section of the Demonstration Safety page before performing this demonstration.

This demonstration requires: Safety glasses, hot gloves. A small blast shield is recommended, but not required. This demonstration uses a strong basic solution, so follow these steps when disposing of it:

Disposal

1. Clean the zinc: pour the solution into a separate beaker, being sure that the zinc stays in the first beaker. Once separated, rinse the zinc thoroughly with water, then soak it in vinegar for 10-20 minutes. Rinse it with water, then lay it out on several paper towels and pat dry. This zinc can be reused for the demonstration.
2. Neutralize the solution: put a pH meter into the remaining NaOH solution, and titrate vinegar into the solution while stirring until you reach a pH between 6 and 8. A magnetic stir plate is recommended for this process.
3. Once neutralized, the solution can be safely poured down a drain. Thoroughly wash the glassware and utensils to remove any residue.

Demonstration

Preparation;

• 3M NaOH Solution: In a 1000mL beaker, combine 60g fo NaOH with 500mL of water. It is suggested to use a magnetic stir plate to speed up the process. Once fully dissolved, bottle and label the solution, and store in a chemical cabinet.
• Cleaning the Pennies: For this demonstration to run smoothly, the pennies should be cleaned of any dirt and tarnish on them. To do so, make a 10:1 solution of vinegar and salt in one of the large bowls, being sure that all the salt dissolves. Fill a second bowl with water, and lay out several paper towels. Put the pennies into the vinegar and salt bowl, and you should be able to see them become nice and shiny in 10-20 seconds. Once they are, move them to the bowl of water to rinse them, and then dry them out quickly with the paper towel. Let them air dry as you continue with the rest of the pennies. When dry, bag the pennies and label them for the demonstration.
  • Note: Do not clean pre-1984 and post-1984 pennies together! The high zinc content in post-1984 pennies will act as an electrode for the high copper content of the pre-1984 pennies, and can cause a rapid oxidation reaction. You'll be able to tell if there are pennies of different metal contents in the same bowl, as the post-1984 pennies will start to bubble in solution. If not quickly removed, they will start decomposing in the solution, and will tarnish all the pennies near them, if not cause them to decompose as well!
  • Additional Note: There will be occasional pennies from 1982 to 1984 that will have zinc centers. This is because the U.S. mint did a transition from full copper to copper-zinc pennies during those years, so you may run into an occasional penny during those years which can cause the rapid oxidation. Keep an eye open for any rapidly bubbling pennies, and wash them in small batches to prevent any problems.

Presentation;

1. Set up: Pour 500mL of the 3M NaOH solution into the 1000mL beaker. Place the beaker on the hot plate and turn it to medium-high. Fill a large bowl with water and set it next to the hot plate. Once the solution starts to steam, put 30-40g of zinc into the beaker, being sure it is spread equally across the bottom of the beaker. Lay out several paper towels off to the side of the bowl of water, away from the hot plate.
2. Once the solution is steaming again, you can start placing pennies into it. At this point, it does not matter of you place pre-1984 and post-1984 pennies into the solution together, so long as the post-1984 pennies are not broken and exposing the zinc inside them. Use the tongs to place the pennies gently onto the bottom of the beaker, making sure they don't touch or overlap and are all on top of the zinc.
3. After the pennies have been in the solution for about 45 seconds, you can try taking them out. They should be fully silvered over! If not, place them back in the solution and wait an additional 30 seconds. Rinse them with the bowl of water, and dry them on the paper towel.
4. To turn the silver pennies gold, place them on the hot plate next to the beaker. Within 5-20 seconds, the penny will start to transition from silver to bronze, then to rose-gold or gold. Once it has finished changing colors, pick it up using the tongs and place it in the water, and it will then become bright yellow gold!
5. Allow students to keep either a silver or gold penny. This demonstration is intended to be continuously running during a science night or outreach event, and any extra silver and gold pennies can be saved for later events. Add more water to the solution as needed to keep it around the 500mL mark, and otherwise the solution can be reused several times at later events. See the Safety Precautions section for disposal of the solution.

Why This Works

Short Explanation

The pennies are being placed in a strong basic NaOH solution, which is being heated by the hot plate. Basic solutions can be thought of as the opposites of acids: they accept protons, add negative ions to a solution, and increase the pH of the solution. Acids accept electrons, add positive ions to a solution, and decrease the pH of the solution. Most soaps are weak bases, so one can think of our NaOH solution as being a really strong "soap". The copper on a penny can dissolve into a basic solution, but only if two things are met: The solution needs to be hot, and the copper needs to be in contact with the zinc in the beaker. The zinc in the bottom of the beaker can dissolve, but just barely. The zinc would rather take the place of the copper as it dissolves off the penny, so once the penny touches the zinc we see this happen! The penny starts bubbling rapidly, and in less than a minute the outside layer of copper has dissolved, with it's place taken by the zinc! This type of reaction is called a Single Displacement Reaction, since a single element (Copper) switched places with another (Zinc).

The "silver" pennies have only a very thin layer of zinc on top of the copper underneath. These zinc-plated pennies can have their silver color rubbed off after repeated use, since the zinc is only sitting on top of the copper. When the zinc-plated pennies are heated up on the hot plate, the zinc layer and copper layer start to heat up together. This causes them to meld together, with the zinc atoms sliding between the copper atoms, and become brass! Brass is a Metal Alloy, or a metal which comes from the combination of different metal elements. Brass is often used to make instruments such as tubas, trumpets and saxophones, and is recognized by its yellow-gold color.

Full Explanation

The NaOH solution is strongly basic. Bases are effectively the opposite of acids: They accept protons, add negative ions to a solution, and increase the pH of the solution. NaOH is a strong base, and fully dissociates into two ions, Na⁺ and OH^-. The OH^- ions can give electrons, which is what drives this reaction.

When the zinc is added to the solution, it will weakly dissolve while the solution is being heated, and react with the OH^- ions:

Zn(s) ⇒ Zn2+(aq) + 2 e-

Zn2+(aq) + 2 OH-(aq) + 2 H2O(l) ⇒ [Zn(OH4)]2-(aq) + H2(g)

This only happens weakly, meaning that very little of the zinc actually dissolves. This, however, changes dramatically when the pennies are put in. The pennies are copper, or at least have copper on the outside. Copper gladly accepts electrons, and is commonly used as a conductor. It is used to drive this Electrochemistry reaction, or chemical reaction which is driven by an electrical charge or produces an electric charge as the reaction continues. The solution of free electrons was an Electrolyte solution, meaning it was ionic and could provide electrons. The copper, upon landing on the zinc in solution, was the Electrode, meaning it could accept electrons. This results in the following reaction:

[Zn(OH4)]2-(aq) + 2 e- ⇒ Zn(s) + 4 OH-(aq)

This reaction results in the zinc coming out of solution and plating onto the copper coin. This results in the coin looking to turn "silver", the first half of this interesting reaction. After the coin is plated, it can be removed and rinsed off to showcase the shine. If a coin does not appear to be plated enough, it can be placed back in to let the remainder of the coin be plated. The second half, when the coin is heated, is a surprisingly simple equation:

Zn(s) + Cu(s) ⇒ ZnCu(s)

This reaction, although easy to write out, is not as easily explained. When the coin is heated, the atoms of the zinc and copper start to move. The movement isn't enough for either metal to melt, but it is enough for the zinc to start sliding into the copper surface underneath. As this happens, the metal atoms form a Alloy, or a metal which isn't a unique element on the periodic table, but rather formed by the combination of metals. Brass is a common alloy, used to make instruments and largely as a decorative metal. Unlike copper, brass is a very soft metal and can be easily shaped and molded. The brass layer on the coin looks a bright yellow gold color, but over time it will tarnish and dull. It cannot be easily cleaned, as any attempt at cleaning the brass layer would remove it. It can, however, be re-plated with zinc, and then reheated to create a new brass layer on top.

Additional Information

• This demonstration works with any penny! It is a common misconception that you can only use pre-1984 pennies, but any penny can be used as long as it is cleaned first!
• This demonstration pairs well with the Rip The Can demonstration.