The Gold Rush:Turning Pennies into Silver and Gold

Introduction

Turn an ordinary copper penny into “silver” and then into “gold”! Get rich quick by demonstrating this eye-catching and simple procedure!

Concepts

• Oxidation–reduction
• Alloys

Materials

Safety Precautions

Zinc chloride solution and granular zinc are severe skin irritants. Zinc metal dust can be flammable; dust may be present at the bottom of the bottle of granular zinc. Do not use zinc dust in this procedure. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. The zinc chloride solution may be poured off of the granular zinc and be disposed of according to Flinn Suggested Disposal Method #26b. The granular zinc can either be reused or discarded in the solid waste according to Flinn Suggested Disposal Method #26a. The zinc chloride solution may be poured off of the granular zinc and be disposed of according to Flinn Suggested Disposal Method #26b. The granular zinc can either be reused or discarded in the solid waste according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

1. Weigh out and place 2.5–3 g of sodium chloride and 15 mL of vinegar in a clean, 100-mL beaker.
2. Clean two pennies by placing them in the sodium chloride/vinegar solution until they are shiny.
3. Remove the pennies using tongs and rinse thoroughly with water. Dry thoroughly with a towel. Note: Do not handle the clean pennies with your hands. The oils from your skin may interfere with the zinc-plating reaction.

Procedure

1. In a clean 100-mL beaker, mix together 1.0 g of granular zinc and 25 mL of 1 M zinc chloride solution. Note: Chemical splash goggles must be worn.
2. Place the beaker with the ZnCl₂ and zinc on a hot plate set to a medium heat setting.
3. Carefully and gently heat the mixture until the solution boils.
4. Using tongs, immerse two pennies in the mixture until they appear “silver.”
5. Use tongs to remove the pennies. Caution: The pennies will be very hot. Carefully dip the pennies into a beaker of distilled water. Shine the pennies with a towel. Set one treated penny aside to be used for later comparisons.
6. Using tongs, place the other treated penny on the hot plate until the penny turns to a golden color. Using a heat-resistant glove or tongs, flip the penny every 30 seconds to avoid burning.
7. Use tongs to remove the penny from the hot plate and immediately dip the penny into a fresh beaker of distilled water. The penny will be extremely hot and should be handled with tongs until it has cooled for several minutes. Students will enjoy showing their friends their “silver” and “gold” pennies.

Student Worksheet PDF

13009_Student1.pdf

Teacher Tips

• Enough materials are provided to perform this demonstration seven times: 10 grams of zinc, 200 mL of 1 M zinc chloride solution, 25 g of sodium chloride and 110 mL of vinegar. Pennies are needed but not provided in the kit.
• Clean pennies are necessary in this lab because a smooth surface is needed for the “silver” and “gold” to plate. Pennies can be cleaned with the salt/vinegar solution as described in the preparation section. Or they can be cleaned by soaking them in dilute hydrochloric acid solution. The pennies can then be scrubbed with steel wool before starting, if necessary.
• Students will likely not believe that “silver” and “gold” were made in this lab. Have them determine what was made.
• Pre-1982 pennies are 95% copper and 5% zinc; post-1982 pennies are 97.6% zinc, coated with a thin electroplating of copper. Either pre- or post-1982 pennies can be used for this lab; as long as they are very clean and shiny.
• This demonstration has been revised from its original procedure due to safety considerations. The original procedure, which used zinc metal and 3M sodium hydroxide solution presented a possible fire hazard for disposal. The method described herein is safer to perform and eliminates the fire hazard.

Sample Data

Answers to Questions

1. Research the composition of pennies. What was the composition of each of your pennies prior to treatment?

   Pennies that were made pre-1982 are 95% copper and 5% zinc. Post 1982 pennies are 97.6% zinc coated with a thin electroplating of copper.

2. Why is it necessary the pennies be thoroughly cleaned before beginning this activity? 

   A clean smooth surface is needed for the “silver” and “gold” (or brass alloy) to properly plate. When the penny is placed in solution it is necessary that the zinc ions can interact with the copper of the penny so they can be reduced to metallic zinc.

3. After the pennies were dipped in the granular zinc/zinc chloride solution they had a silver colored coating. This was not actually due to the presence of silver, what was the silver coating?

   The coating is actually brass which is a copper–zinc alloy. The color of brass is dependent upon its percentage of copper and zinc. The silver coating on the penny is the gamma-form of the brass alloy with zinc content greater than 45%.

4. Unfortunately, the copper pennies were not turned to real gold. What is responsible for the gold coating?

   As mentioned previously, the coating is a brass alloy. Upon heating, the brass alloy develops a golden color due to the zinc migrating through the copper to convert to the alpha-form of brass alloy which has a zinc content of less than 35%.

Discussion

In this reaction, a penny is placed in a boiling solution of 1 M zinc chloride containing granular zinc. The penny develops a zinc “silver-colored” coating. When removed from the solution and placed on the surface of the hot plate, the brass alloy coating on the penny turns a golden color.

Placing copper, or a copper-coated penny, in a mixture of zinc metal and aqueous zinc chloride causes zinc metal to plate out on the copper surface. This reaction occurs due to electrochemical potential differences that result when different “forms” of zinc solid are contained in a solution of 1M ZnCl₂. The driving force in this reaction is formation of a brass alloy on the surface of the copper.

Brass is a copper-zinc alloy. An alloy is a mixture of two or more metals dissolved in each other when molten (or a metal and nonmetal fuse together). The percentages of copper and zinc in brass vary depending on the type of brass. These differences allow the penny in this activity to turn several different colors. When a copper penny is added to the zinc solution, the zinc ions migrate to the copper where they are reduced to metallic zinc and deposited. The silver coating on the penny is the gamma-form of the brass alloy with zinc content greater than 45%. This gives the penny its silver coloring. When the zinc-coated penny is heated, the penny becomes gold in color. The gold color is due to the zinc migrating through the copper to convert to the alpha-form of brass alloy which has a zinc content of less than 35%. This form of the brass alloy is a golden color.

References

Special thanks to David A. Katz, retired, Wilmington, DE, for providing us with this activity.

Szczepankiewicz, Steven H. Journal of Chemical Education; 1995; 72, 386–388.