In the Heat of the Summer: Blood Test

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Objective

Students will perform a chemical test in order to determine which evidence taken from suspects is blood.

Big Idea

The Kastle-Meyer blood test relies on chemical change as an indicator of the presence of blood.

Introduction

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:

  • Kastle-Meyer solution (available from Frey Scientific)
  • 70% ethanol
  • distilled or deionized water
  • 3% hydrogen peroxide
  • cotton swabs
  • dropper or pipette
  • sample of dried blood on filter paper
  • samples of fake blood on filter paper

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/Activator

10 minutes

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

10 minutes

Mini-lesson: I review with students the procedure of the Kastle-Meyer Test To Detect Blood on the about education website:

  1. Students moisten a cotton swab with water and touch it to each of the four samples that might contain dried blood. I have the samples on filter paper for each suspect, and they are in containers with the name of the suspect written on the container. I note that only a small amount of sample is needed.
  1. Students then add a drop or two of 70% ethanol to each swab. I notes that the alcohol exposes the hemoglobin in blood so that it can react more fully, to increase the sensitivity of the test.
  1. Once students have done the first two steps, they come to me. Each student is holding two swabs. I add a drop or two of the Kastle-Meyer solution. My reagent is fresh so I am not worried about it immediately changing color; if it did, it would mean that we would need a new batch of reagent.
  1. I then almost immediately add two drops of hydrogen peroxide solution to each of the four swabs and instruct students to look for a color change. If there is reall blood then the swab will turn pink immediately. If the color does not change, the sample does not contain a detectable amount of blood. 

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.

Application

25 minutes

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.

Debrief

10 minutes

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.