This classic chemistry demonstration allows an ordinary copper coin to take on the bright, metallic appearance of silver. The goal is to coat the copper surface of the penny with a thin, uniform layer of a different metal, changing its color without altering the coin’s core structure. This process, known as zinc plating, serves as an introduction to the principles of electrochemical reactions and surface chemistry. The resulting “silver” coin showcases a deliberate manipulation of materials through a controlled chemical environment, though it is not made of the precious metal.
Preparation: Necessary Materials and Critical Safety Measures
Successfully executing this chemical change requires specific components. Core materials include a clean copper penny, a source of zinc metal (often granular or powdered), and a concentrated solution of sodium hydroxide (\(\text{NaOH}\)), commonly referred to as lye. You will also need laboratory glassware, such as a beaker or evaporating dish, a heat source like a hot plate, and metal tongs or tweezers for handling the coin.
Safety is paramount due to the caustic nature of the chemicals involved. Sodium hydroxide is a highly corrosive base that can cause severe chemical burns and permanent eye damage, especially when heated. Work must be conducted in a well-ventilated area, preferably under a fume hood, because the reaction releases small amounts of flammable hydrogen gas (\(\text{H}_2\)). Always wear chemical splash goggles, heat-resistant gloves, and a lab apron to protect skin and clothing from accidental contact with the hot, alkaline solution.
The Zinc Plating Process: Step-by-Step Guide
Preparing the copper coin is the first step to ensure maximum surface contact for the plating process. Pennies should be thoroughly cleaned, often by soaking them in a mild acid solution like vinegar and salt, to remove surface oxides or oils. Once cleaned, the penny must be rinsed completely with water and should not be handled directly to prevent recontamination from skin oils.
Next, prepare the zinc plating solution by combining the granular zinc metal with the concentrated sodium hydroxide solution in a beaker. Gently heat this mixture on a hot plate until it is near its boiling point, avoiding vigorous boiling to prevent splashing. The heat accelerates the chemical reaction that prepares the zinc for deposition.
Carefully place the clean penny into the hot solution using tongs, ensuring it makes direct contact with the solid zinc metal resting on the bottom of the beaker. The color change is typically visible within one to ten minutes, as the copper coin becomes evenly coated with a dull gray-silver layer of metal. Once the transformation is complete, remove the plated penny using the tongs and immediately rinse it under running water. This step washes away the caustic sodium hydroxide solution and any loose zinc particles.
The Chemistry Behind the Change
The visual change is the result of a non-electrolytic reduction, also known as cementation. First, the zinc metal dissolves in the hot, concentrated sodium hydroxide solution to form a soluble complex ion: the tetrahydroxozincate ion, \([\text{Zn}(\text{OH})_4]^{2-}\). The hot, alkaline environment enables the solid zinc to react and enter the solution phase.
The copper penny acts as a surface where the dissolved zinc complex ion is reduced back into its metallic form. The zinc ions in the complex gain electrons, causing them to deposit as a solid, metallic layer on the copper surface. This is a reduction half-reaction where the zinc ion becomes neutral zinc atoms (\(\text{Zn}^{2+} + 2\text{e}^- \rightarrow \text{Zn}\)). The overall process is driven by the difference in chemical reactivity between the two metals, though the alkaline solution facilitates the necessary ion formation. The final product is a penny coated with a thin layer of elemental zinc, which provides the characteristic silver color.