Reworking the Qualitative Analysis and Bonding Lab

Test for the melting point above 500 C.

I have redesigned the Qualitative Analysis and Chemical Bonding (QA) lab to increase student success in identifying an unknown substance.  This lab comes from the Flinn AP Inquiry Lab for Big Idea 2, Investigation #6.  I ran the introductory activity as written in the Flinn lab on the first day; students make observations of representative substances to differentiate between ionic, non-polar, polar, and metallic substances. The changes came in the second day of the lab when I introduced the unknowns. In the AP lab, the goal is to use their QA flow chart to determine the bonding in their unknown substance and then work as a class to match up the substances to a possible list. In past years, the results for the second phase of the lab were not very satisfying because my students did not have enough experience  to design tests for the possible substances. This year I created a list of suggestions/notes that would help direct the students toward confirmation tests that would help them either confirm the identity of their unknown or rule out some possibilities on the list. I modeled the changes after Steve Sogo's lab style in which students can do a lab at almost any level of chemistry experience. The addition of the suggestions was a great improvement to the outcome, especially with the lab occurring in the first month of school when my students have very little experience in the lab. Most of the students were successful in identifying the bonding and the identity of their unknown, and they learned some new lab techniques in the process.

Recording evidence about the unknown for the video CER.

I've been doing this lab for several years with my AP Chemistry class in the spring as part of our AP review. Last year I decided to move it to the fall to accompany the nomenclature unit. It's a nice fit with the introduction to ionic and molecular compounds, and let's face it, there aren't many labs about nomenclature rules. The problem with moving it to earlier in the year is the very limited lab experience my students have in September. At Woodstock Academy, I teach AP Chemistry as a one-year combined honors/AP course. My students come to me in the fall with no chemistry, and they sit for the AP test in May. The QA and Bonding lab was only their third lab of the year.

The lab starts with a study of the properties of a representative ionic, non-polar covalent, polar covalent, and metallic substance. They test solubility, conductivity, and melting point. These are simple tests that inexperienced students can handle, allowing them to learn a few new techniques and practice lighting the bunsen burners. The students use their observations to develop a qualitative analysis flow chart to asses the bonding in an unknown substance. I dedicated a full lab day to the introductory activity because these students have only been in the lab twice so far this year. Everything takes longer when they are doing it for the first time and they work so cautiously in September! We had two regular class days before the second day of this experiment. I dedicated some class time for the lab teams to create their flow charts together. This was time well spent. My kids went back into the lab on the second day with a plan. Up to this point the lab procedure follows the Flinn AP inquiry lab kit procedure.

The changes came in the unknown identification. I've tried this lab for several years, each time with frustrating results. I would watch kids flounder, wondering why they didn't think to test the pH of the solution to see if they had citric acid, or why they didn't just do a flame test to see if it contained sodium. I basically had to tell students how to test for each substance and then what they should expect from each unknown. Very few of the kids were able to determine the identity of the unknowns, even with a short list of possibilities. I knew that something had to change.

Testing for decomposition at high temperature.

My goal was to give the kids a way to successfully identify their unknown substance from a list of possibilities that would work with limited chemistry experience. First I took a closer look at the twelve unknowns that came in the kit. I expanded the list of unknowns to include a few more ionic compounds with distinctive flame test colors, more non-polar compounds with very different melting points, and sand for a network solid. Next I started to develop some "hints/suggestions" that would lead the students toward tests that to confirm the identity of their unknown. The suggestions were designed to compliment the flow chart, not replace it. I still wanted the students to use the QA scheme as the basis of their work. Some of the suggestions were general like "ionic compounds that contain Na, K, Ca, and Li have distinctive colors when burned, use the flame test station if you want to test for this property". Other suggestions were very specific like "when heated, glycine produces ammonia gas, test for ammonia by inserting a moist piece of pH paper into the mouth of the test tube after heating the sample". During the course of the lab we discovered another lab hint to include that would provide a confirmation test for iron(III) oxide in addition to the distinctive color. One group was testing the melting point with the burner and found that it turned black. Curious. Well, of course, iron(III) oxide decomposes to iron(II) oxide when heated. I added this to my suggestion list for next year.

Testing melting point in boiling water.

My inspiration for these improvements came this summer at ChemEd when I spent some time with Steve Sogo. I was so happy to reconnect with him and catch up on his new projects. Steve has a gift for creating labs that are teaching events that include lab techniques and the chemistry necessary to solve a problem. I went to his workshop at ChemEd and got several great ideas for what I call "challenge labs" for this year. I talked over my dilemma about this QA lab with Steve. He helped me brainstorm so possible directions to go with this lab to incorporate enough chemistry to turn this into a learning event. The results were good this year. Most of the kids were able to confirm the identity of their unknown substance with tests for physical and chemical properties using the suggestions. Many of the kids said they enjoyed the lab and felt confident in their final answer. Each student created a video CER to justify the identity of their unknown. More about that process in the next post.