This is the fifth lesson in a forensics unit called In the Heat of the Summer. In previous lessons students have analyzed glass densities and boiling points, they have conducted a test for the presence of aspirin, and they have analyzed ink pens using chromatography as they work to amass a body of evidence that points to a specific culprit in an attempted murder.
In this lesson students use a Kastle-Meyer blood test to test blood-like substances found on the suspects in order to determine which suspect has real blood on their clothing after a break-in at the victim's home. Going into this lesson students have already studied chemical change using this lesson. However, this lesson could stand alone by spending a little more time discussing the indicators of chemical change.
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 to analyze potential blood samples and then interpret their data in terms of which suspect in the forensic case had blood on them. While this lesson does directly support a Disciplinary Core Idea, it does use phenolphthalien which students will use later in acid-base reactions, a chemical reaction that students will be explore as part of HS-PS1 Matter and Its Interactions.
The materials needed to do this lab are:
Note: I obtained real blood from a local butcher. I used tomato juice to make fake blood.
Here is the Master Data Sheet, which is a document which summarizes the evidence for each suspect for all of the lessons in this unit.
Do Now: Students begin class by reading the July 13 entries from the Investigation Log and the Investigation Notes from the In the Heat of the Summer Forensics Lab. The goal of this work is to re-engage students by reminding them of the story line that they have been working with.
Activator: After students have had a chance to do this reading, I cold call some questions in order to get the story line out in the open air.
Questions I asked, with the answers I received, are as follows:
What is the latest attempt on Dr. Rodriguez's life? (someone broke into his house and had an ax)
How was this attempt thwarted? (he awoke and scared off the intruder)
What evidence did police gather at the crime scene? (blood left on a broken window)
Mini-lesson: I review with students the procedure of the Kastle-Meyer Test To Detect Blood on the about education website:
In light of the fact that I only have 1 bottle of the Kastle-Meyer reagent, and in light of the fact that this is a quick reaction, I work with students on the last two steps because it was the most efficient way to have all the groups in class conduct the experiment.
Student Activity: Students conduct the Kastle-Meyer test. Here is a video of the final two steps being performed. The color turns pink quickly for the real blood, which I have assigned to the samples found in Mariana Goldbloom's possession. Students take their work back to their seats.
When students are finished with this final test, they work on their lab report; the directions are in the last page of the In the Heat of the Summer packet.
After I have worked with all students, I ask them to report the results. All students have the same result--that Mariana Goldbloom was the suspect with real blood in her possession. I then ask a student what the evidence was, and she replies that it was the cotton swab turning pink. I hold up her four swabs, and they are all pink. Some students experience a momentary surprise, but pretty quickly a student reminds the class that there is a time limit for the test--after a short period of time all the swabs turn pink.
This serves as a reminder to students that they need to understand the tests they are performing in order to make sense out of their data and in order to build a strong case against a suspect.