This popular chemistry demonstration visually transforms a copper penny into a gold-colored coin, providing an accessible lesson in material science. The color change is achieved through a two-step process that involves coating the coin with a thin layer of another metal and then applying heat. It is important to understand that this procedure is one of metal plating and alloying, and it does not result in the elemental transmutation of copper into actual gold.
Preparation and Required Materials
Before beginning the experiment, all necessary items should be collected and prepared in a designated work area. You will need a clean copper penny, granular or powdered zinc metal, and a solution that can be either sodium hydroxide (caustic) or a less hazardous alternative like zinc sulfate or zinc chloride solution. Essential equipment includes a heat source, such as a hot plate or Bunsen burner, a beaker or evaporating dish to hold the solution, and a pair of tongs or tweezers for handling the coins. Additionally, a separate beaker of water and a paper towel are needed for rinsing and drying the treated pennies.
The Plating Process: From Copper to Silver
The first procedural step involves preparing the penny’s surface and the chemical bath to ensure a smooth, uniform coating. Begin by thoroughly cleaning the penny to remove any surface oils or grime that could interfere with the plating reaction. Once cleaned, the penny should not be touched with bare hands to maintain its purity.
Next, the zinc plating solution is prepared in a heat-resistant container, combining the zinc metal with the chosen chemical solution. Gently heat this mixture on a hot plate until it is near boiling or steaming, which activates the zinc for the plating process.
Using tongs, the cleaned copper penny is carefully lowered into the warm solution, ensuring it is in direct contact with the granular zinc metal at the bottom of the container. The coin should be left in the solution until its entire surface is uniformly covered with a bright, silver-colored layer of metallic zinc.
The Transformation: Achieving the “Gold” Finish
Once the penny is completely coated and appears silver, it must be removed from the hot solution using tongs and immediately rinsed under running water. This step is necessary to wash away any residual zinc solution or loose zinc particles and to cool the coin slightly. After rinsing, the silver-colored penny must be dried completely with a paper towel, taking care to avoid excessive rubbing that could scratch or remove the delicate zinc layer.
The final transformation requires the careful application of heat to the dried, silver-coated coin. The penny is placed onto a hot plate set to a medium temperature or briefly held in a flame using the tongs.
As the temperature rises, the silver color will visibly shift to a golden hue, typically within a few seconds if a direct flame is used. Once the desired gold color is achieved, the penny must be removed from the heat immediately and cooled quickly by dropping it into a beaker of water. Overheating the coin will cause the gold color to darken and eventually revert to a copper appearance.
Understanding the Chemistry
The two distinct color changes observed in this demonstration are the result of two different chemical and physical processes. The initial transformation from copper to a silver appearance is a type of galvanization, where a layer of metallic zinc is deposited onto the copper surface. In the hot solution, zinc atoms are dissolved to form zinc ions, and these ions are then reduced back to metallic zinc on the surface of the copper penny, creating the silver coating.
The second transformation, from silver to gold, is a physical process known as alloying, driven by the application of heat. When the zinc-coated penny is heated, the zinc atoms on the surface diffuse and intermix with the copper atoms of the penny underneath.
This intermixing of the two metals creates a new substance called brass, which is a copper-zinc alloy. The golden color is characteristic of alpha-brass, which forms when the zinc content is below 35% of the total alloy composition.
Safety Guidelines for the Experiment
Since this experiment involves chemicals and high heat, safety measures must be followed. Wear protective equipment, including chemical splash goggles, lab gloves, and a lab apron, to protect the skin and eyes.
If using highly caustic sodium hydroxide, perform the work under a fume hood or in a well-ventilated area to avoid inhaling fumes. Use tongs for handling all coins and glassware, as both will become extremely hot during the heating stages.
All chemical waste and solutions must be disposed of according to local safety protocols. Never pour solutions directly down a sink without proper neutralization.