This is the third lesson in a forensics unit called In the Heat of the Summer. In previous lessons students have analyzed glass densities and boiling points as they work to amass a body of evidence that points to a specific culprit in an attempted murder. In this lesson students will apply an iron chloride solution to various solutions to see which one(s) contain aspirin. Going into this lesson students have already studied chemical change using this lesson. Knowledge about chemical change can be introduced or reinforced by this lesson.
This lesson aligns to the NGSS Science Practices Analyzing and interpreting data, and Planning and carrying out investigations because it requires students to carry out an investigation of different white powders using chemical change. The lesson also requires that students interpret the data they generate from the tests in order to identify which one is aspirin.
The materials needed to do this lab are:
Here is the Teacher Guide: Master Data Sheet, which is a document which summarizes the evidence for each suspect for all of the lessons in this unit.
Do Now: I ask students to read the Investigation Log and Investigation Notes in the "In the Heat of the Summer" (see Resource in previous section) packet for July 4 in preparation for today's class, and to highlight the key information that will need for their report, which is the facts of the attempted crime and what the police are investigating from each of the suspects. This will help them get some momentum and context for today's class.
Activator: Once I have taken attendance and students have completed the Do Now, I lead the class by asking these questions and receiving these answers:
What was the murder attempt today? Someone tried to poison Dr. Rodriguez by sneaking into his house and putting aspirin into his ice tea. He is allergic to aspirin.
How did he escape this attempt on his life? He noticed the white powder on the table and called the police. They identified the powder as aspirin.
What do the police now have to investigate? Whether white powders found in the suspects' possession is aspirin.
I review with students a number of points. First, I point out where the materials are. I have a white powder and a balance at four different stations. I have the distilled water in small portions, stationed with a place where students can pick up test tubes, pipettes, and stirring rods. Students collect data with a partner; I instruct one person from each set of partners to get the white powder and one person to get the other materials to increase traffic flow. I review the procedure, and caution students that due to the solubility of each of the chemicals, they may not completely dissolve in water.
Students conduct the aspirin test. Here is a picture of the test tubes after the test was complete. Goldbloom was the suspect that had aspirin in her possession. As students wrapped up their test, I met with them and discussed their results, as shown in this aspirin test results analysis video. Student data for this lab consistently looked like this aspirin test data.
Students have enough time in this class period to work on writing their report. During this time I walk around and check in with how well students are keeping up with their reports. I also answer questions students have about their work.
This debrief is short and to the point. I pose the question "Which suspect had the white powder that tested positive for aspirin?" Everyone in the class names Goldbloom. The results were quite precise, and I point this out to students. (They are also quite accurate, but I do not point this out because that would dilute students' experience of solving a mystery).
For homework students are asked to write a paragraph for the aspirin test as outlined in the last page of the In the Heat of the Summer packet.
Here is an aspirin paragraph that one student wrote. Overall, I think this is a good representation of what I was looking for. One thing that I would like to improve for the next time I do these series of labs is to work more with the ideas of accuracy and precision. In this example, the students writes "there could only be one result which was one powder turning purple" as an explanation of why this test was accurate. I need to help students understand that while the results are consistent, this is a measure of precision, not accuracy.