Content area
Full Text
I often measure the success of a laboratory activity on how well my students are able to think like scientists: that is, to think analytically and solve problems. One laboratory activity that shows promise in promoting this kind of thinking begins after my class has studied redox reactions and some acid-base theory. I initiate the activity by simply asking my introductory chemistry students to determine if copper metal reacts with acetic acid. While the answer to this question is known (1), solving this problem does give students experience with important scientific processes that are generalizable to new laboratory experiences. For example, in the course of this investigation students hypothesize answers, control variables by designing an experiment, and make logical deductions based on what they know and observe. The following discussion will elaborate on the methods my students have used to solve this problem.
The Discrepant Event
When faced with the question, "Does copper metal react with acetic acid?" the initial response of most students is either to refer to their textbooks or to conduct an experiment. The general chemistry textbooks that I have surveyed do not answer this question explicitly. While most state that copper reacts with concentrated nitric and sulfuric acids, copper is usually described as unable to displace hydrogen from acidic solutions or unreactive with nonoxidizing acids (2-4). While it might be inferred from this that copper does not react with acetic acid, other publications such as The Merck Index states that "copper is attacked by acetic acid" (5). The inability to find a conclusive and well described answer from these sources stimulates students to conduct an empirical investigation.
Designing an Experiment
Most students initially design a very simple experiment such as "add acetic acid to copper and see what happens." Although this initial exploration is fine, I ask them to consider all chemical and physical variables, even the gases in the air, and how they might control for each variable. The following generic procedure and design give very good results.
Into four screw-top test tubes, add 3 mL of a 50/50 solution of glacial acetic acid and distilled water.1 Next, cut four pieces of copper wire each 4 cm in length, scrape them clean with fine sand paper, remove any particulate matter...