For our third day of class, a rare full-day Friday, we began with the question: What is Chemistry? Students shared what came to mind when they thought of chemistry, and we crafted those ideas into a simple definition: Chemistry is the study of matter.
Next, students observed two materials on their lab benches – a block of copper and a beaker with zinc shot. Without being told the identity of either, students wrote down observations about the materials as a team, and then shared out observations as a class in an effort to describe and then deduce the identity of the materials.
Finally, the class was led through a demonstration of the Penny Lab. Following the steps outlined on page 6 of Lesson 2 of the textbook (A Penny for Your Thoughts), we immersed a pre-1982 penny (95% copper, 5% zinc) in a warm beaker of sodium hydroxide (2 M, about 40 mL) and zinc shot (enough to cover the bottom of a 150 mL beaker). After watching the penny change color to silver, tongs were used to transfer the penny to cool water. The penny was then transferred back to the hot plate (still set to 4) where it changed to a gold-copper color. The penny was placed back in cool water where it completed the transition to having a gold appearance. Throughout the demonstration, students worked the Lesson 2 worksheet. During the longer steps of the demo, students read through Lessons 1 and 2 of the textbook and worked through the Lesson 1 Exercise questions focusing on lab safety.
We concluded the class period with a discussion of what students observed. The discussion generated an impressive number of hypotheses and testable follow-up questions, so we may revisit this as a student-led lab in the near future (once all those safety contracts are turned in!). Students clearly understood that the gold color of the penny was not an indication that the penny had literally turned to gold – the gold color is attributed to increasing the percentage of zinc as part of the penny, which manifests as a gold color after heating the penny to integrate the zinc atoms within the copper atoms of the penny and thus creating brass!
Students who would like to watch the Penny Lab in action should check out the video below: