Introducing Scientific Reasoning with the Penny Lab

Copper metal is slowly oxidized when it is exposed to the atmosphere for long periods of time, forming a blue-green coating called a patina. For example, the green color of the Statue of Liberty is a result of patina formation. A wide variety of Cu 2+ compounds are found in patinas, and they generally take long periods of time to develop. Here, an activity is presented in which patinas are rapidly generated on the surface of copper-containing coins. The process can be carried out using very simple materials, making this activity suitable for a wide range of audiences, and also for courses offered in an online format. Because the development of the patina occurs within tens of seconds, the experiment can be presented as an in-class demonstration. Given the vast number of conditions that can be varied in this activity, it is highly amenable to exploratory learning exercises.

Section: ■ Materials and Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Rapid Formation of Copper Patinas: A Simple Chemical Demonstration of Why the Statue of Liberty Is Green

Kuntzleman

Cullen²,

Milam³

et al. 2020

“…Thus, weighing a penny gives some information about minting date and penny composition. More details on penny experiments are available …”

Section: Exhibit Stationsmentioning

confidence: 99%

“…More details on penny experiments are available. 20 Station 9. Station 9 contains four commercially available density-related items intended to arouse curiosity: a Galileo thermometer, a hydrometer, a liquid motion bubbler, and a density bottle.…”

Section: Mass Density and Buoyancymentioning

confidence: 99%

Exploring Matter: An Interactive, Inexpensive Chemistry Exhibit for Museums

Murov

Chavez

2017

Despite its vital importance in our lives, chemistry is inadequately represented in most museums. Issues such as safety, replenishing and disposal of chemicals, supervision required, and cost are constraints that have limited the number and size of chemistry exhibits. Taking into account the constraints, a 21-station interactive and inexpensive chemistry exhibit has been designed, constructed, and installed in the Great Valley Museum on the Modesto Junior College West Campus. The exhibit is intended to stimulate interest in chemistry, demonstrate and emphasize the importance of chemistry in our lives, and provide a meaningful and enjoyable educational experience with chemistry. The centerpiece of the exhibit involves the identification of 11 elements from observations of relative density, electrical conductivity, magnetic properties, color, and luster. Descriptions of the 21 stations are included in this paper.

“…Although many laboratory investigations and demonstrations use United States pennies (1)(2)(3)(4)(5), relatively little chemistry has been done with the penny's big brother, the "nickel". A United States 5-cent coin is made of an alloy that is 25% nickel and 75% copper (6).…”

mentioning

confidence: 99%

Isolation of Copper from a 5-Cent Coin. An Example of Electrorefining

Sogo

2004

Although many laboratory investigations and demonstrations use United States pennies (1-5), relatively little chemistry has been done with the penny's big brother, the "nickel". A United States 5-cent coin is made of an alloy that is 25% nickel and 75% copper (6). In this experiment, students isolate copper from a 5-cent coin using electrolysis (an example of the process of electrorefining). The isolation of pure copper metal from a coin that, at first glance, appears to be devoid of copper is a transformation that is both entertaining and highly educational for students ( Figure 1). Electrolysis in a Chloride SolutionRefining of metals through electrolysis can be performed either through a process of selective oxidation or selective reduction (7). In this experiment, pure copper is obtained through a process of selective reduction. The 5-cent coin is used as the anode and a graphite rod is used as the cathode. 6 M HCl is used as the electrolyte. HCl is chosen as the electrolyte for two reasons: the acidity promotes the solubility of the transition metal ions produced during electrolysis, and the chloride ions form coordination complexes that also enhance the solubility of the transition metal ions. A schematic diagram of the experimental setup is shown in Figure 2.When electrolyzed in typical aqueous solutions, the metals that make up the 5-cent coin are oxidized according to the half-reactions:However, when a significant concentration of chloride ion is present in the electrolytic bath, the following oxidation halfreaction for copper is favored (8,9):CuCl is almost insoluble in water, but in a bath of 6 M HCl, the CuCl will react with additional Cl − ions to form soluble complexes such as CuCl 2 − and CuCl 3 2− (9). This solubility allows the copper ions to migrate through solution to eventually become reduced at the cathode. Experimental ObservationsDuring the experiment, the 5-cent coin is gradually dissolved, resulting in an intensely green-colored electrolyte bath. The green color of the bath results from aqueous Ni 2+ ions (which may react with excess chloride ions to form complex ions such as NiCl 4 2− ). The copper(I) chloride complexes are nearly colorless, but may slowly (over a period of hours) be air oxidized to form green copper(II) chloride complexes (10).In a bath containing aqueous H + ions, Cu + complex ions, and Ni 2+ ions, the important reduction potentials are:CuCl + e − → Cu(s) + Cl − E Њ = +0.14 V 2H + + 2e − → H 2 (g) E Њ = +0.00 V Ni 2+ + 2e − → Ni(s) E Њ = ᎑0.25 V During the first few minutes of this electrolysis experiment, only hydrogen gas is formed at the cathode, but soon, as the