The observation of a penny that has seemingly turned gold is a real phenomenon often demonstrated in chemistry laboratories. This color transformation is not the result of a natural process, but rather a deliberate two-step chemical process that turns the copper surface of the coin into a different metal alloy. The change revolves around manipulating the coin’s surface chemistry to produce a material known as brass. The process is a classic educational tool used to illustrate concepts like metal plating and the formation of metal alloys.
The Composition of the Modern Penny
The penny serves as an excellent starting material for this transformation due to its outer metal layer. Since 1982, the United States penny has been composed of a core of zinc (Zn) that makes up 97.5% of its mass, covered by a thin plating of copper (Cu) that accounts for the remaining 2.5%. This modern construction differs significantly from pre-1982 pennies, which were typically 95% copper and 5% zinc. The outer copper layer is chemically reactive enough to participate in the necessary surface-level changes required for the experiment.
Step One: Coating the Penny with Zinc
The initial stage of creating a “gold” penny involves a process of metal deposition that first turns the coin silver. The penny is submerged in a hot, aqueous solution containing zinc ions, often prepared using chemicals like zinc chloride or a mixture of zinc metal and sodium hydroxide. The solution facilitates the reduction of zinc ions (\(\text{Zn}^{2+}\)) into elemental zinc metal (Zn) directly onto the penny’s copper surface. As the reaction proceeds, a thin, uniform layer of metallic zinc forms over the reddish-brown copper, giving the penny a distinct silvery-gray appearance.
This silver-colored intermediate is chemically a zinc-plated copper coin. The process needs a warm environment, often achieved using a hot plate, to speed up the rate of zinc deposition. Once the penny is fully coated with this zinc layer, it is carefully removed from the solution, rinsed, and dried, preparing it for the second, color-changing step.
Step Two: Heating the Metals to Create Brass
The final step in this transformation produces the golden color. The silver-coated penny is subjected to a controlled heat source, such as a Bunsen burner flame or a hot plate. This application of heat initiates a physical process known as solid-state diffusion between the two metals. During heating, the atoms of the newly deposited zinc layer and the copper atoms from the original penny surface gain enough energy to move and intermix.
This intermixing process creates an alloy, which is a homogenous mixture of two or more elements. The specific alloy formed by the combination of copper and zinc is known as brass, which has a characteristic yellow or golden color. The golden color is achieved when the zinc content in the resulting brass alloy is typically less than 35%, a form known as alpha-brass. By carefully controlling the heating duration, a uniform layer of this gold-colored brass forms on the surface of the coin.
Dispelling Misconceptions and Safety
The “gold” penny is not made of the element gold (Au); it is a layer of brass, an alloy of copper and zinc. Because the process modifies currency, it is technically illegal to alter coins with the intent to defraud, though this demonstration is generally permissible in an academic setting. The experiment requires careful handling due to the nature of the chemicals and heat involved.
The solutions used in the first step, particularly if sodium hydroxide is present, are caustic and can cause skin and eye irritation. The heating step requires tongs and protective eyewear, as the penny becomes extremely hot. In a laboratory setting, the reaction is often conducted under a fume hood to ensure proper ventilation and mitigate potential hazards. This experiment should only ever be performed under the direct supervision of a qualified adult or instructor.