The familiar copper penny can be temporarily transformed into a coin with a golden appearance using simple chemical processes. This demonstration is a classic in science education, illustrating fundamental concepts of metal plating and the creation of metallic mixtures known as alloys. The experiment involves a deliberate, two-step chemical sequence: first coating the copper with a white metal, which is then thermally converted into a yellow material that mimics the look of gold.
Necessary Materials and Safety Precautions
The experiment requires a copper penny, a source of zinc, a chemical solution to facilitate the plating, and a heat source. A common setup uses granular zinc metal and a solution of zinc chloride (\(\text{ZnCl}_2\)). You will also need a glass beaker or dish, a hot plate for heating the solution, and long metal tongs for safely handling the penny.
Safety is paramount because the process involves heat and chemical solutions. Chemical splash goggles must be worn to protect the eyes from splashes, especially when heating the solution. The reaction should be performed in a well-ventilated area, such as under a fume hood, because heating the metal may release fumes. Tongs are essential for moving the penny, as it will be extremely hot after both the chemical bath and the final heating step.
The Initial Plating Process
Before the chemical transformation can begin, the penny must be thoroughly cleaned to remove any oils or oxidation that would prevent the zinc from bonding. A simple preparation involves soaking the coin in a solution of table salt and vinegar until the copper surface appears bright and shiny. Once cleaned, the penny should be rinsed with water and dried, taking care not to touch the surface, as skin oils can inhibit the plating.
The first chemical step is to immerse the penny into the heated zinc solution containing granular zinc metal. The mixture is heated gently on a hot plate, which speeds up the chemical reaction. As the process occurs, the copper penny slowly begins to take on a silvery-white color, indicating that a layer of zinc metal is depositing onto the surface. This step is complete when the penny is entirely and evenly coated with the metallic layer, typically taking a few minutes.
The now “silver” penny is carefully removed from the hot solution using tongs and immediately rinsed in distilled water to stop the chemical reaction and cool the surface. The resulting coat is a thin, pure layer of zinc metal bonded to the copper. This intermediate product must be handled carefully to avoid scratching the newly deposited zinc layer.
Completing the Golden Transformation
The final stage involves applying heat to the zinc-coated penny to achieve the golden color. The silver coin is placed onto a hot plate, typically set around \(300^\circ\text{C}\) (\(570^\circ\text{F}\)). This high temperature is necessary to initiate the diffusion of the metals. Tongs should be used to flip the coin periodically to ensure even heating across both sides.
Within a few moments of heating, the surface color visibly changes from silver-white to a rich, uniform yellow-gold color. The heating must be closely monitored; overheating or heating for too long can cause the brass alloy to decompose or the zinc to evaporate, reverting the coin to its original copper color. Once the desired color is achieved, the penny is removed with tongs and submerged in cool water to rapidly halt the alloying process and cool the coin for safe handling.
The Science of the Color Change
The initial change from copper to silver is a process of chemical reduction, where zinc ions in the solution are converted into neutral zinc atoms that deposit onto the copper surface. In the presence of the granular zinc metal, the copper acts as the site for the reduction of the zinc ions, forming a metallic coating. The silver color is due to this pure zinc layer that completely covers the copper.
The subsequent transformation from silver to gold is not a chemical reaction but a physical process called alloying, driven by thermal diffusion. When the zinc-coated penny is heated, the atoms of the zinc layer gain enough energy to begin migrating and mixing with the copper atoms in the underlying penny. This intermixing of the two metals forms a new substance called brass.
Brass is an alloy composed primarily of copper and zinc, and its color is highly dependent on the ratio of the two metals. The golden appearance is specifically characteristic of alpha brass, which is a solid solution containing less than approximately 35% zinc. The heat causes the surface zinc to diffuse into the copper bulk, creating this brass layer that gives the penny its final golden hue.