Taken with kind permission from the Biological Science Initiative, sponsored by the University of colourado.
NOTE: Rather than providing the students with stains to test, you may
have them test the actual stains they collected from the crime scene.
Part One is intended to teach students about the catalase test for the presence of blood. While there are more sensitive tests for the presence of blood that an investigator might use, this is by far the cheapest. Following the student handout should be fairly straightforward. Students predict whether or not the substances provided will be catalase positive or negative, then they test their predictions. They also test whether each substance tests positive for blood using the phenolphthalein test. After this step they open the evidence packets provided, and test whether each stain that was found is likely to be blood.
Part Two addresses blood typing. A good way to avoid using actual human blood for this exercise is to purchase a simulated blood typing kit from a biological supply company. Their price range is approximately $35-$50.
Investigators often find blood stains during their examination of a crime scene. They also find stains that could be either blood or some other similar substance, like reddish-brown paint. What other things can you think of that might look like blood? How would you test a stain to see if it is blood?
Have you ever used hydrogen peroxide to clean a cut or a scrape? What happened when the hydrogen peroxide came in contact with the blood from the wound?
Blood contains an enzyme called catalase, which breaks down hydrogen
peroxide into water and oxygen gas.
When this reaction occurs, the oxygen gas is released as bubbles. The catalase enzyme performs an important function to living organisms because hydrogen peroxide is very toxic to living cells. Other organisms, including plants and some bacteria, also make catalase.
If you place a few drops of hydrogen peroxide on a substance that contains catalase, it will bubble profusely. These substances that bubble with the addition of hydrogen peroxide are said to test positive for catalase.
Criminal investigators do not typically use the catalase test at crime scenes. Other simple tests are better at detecting very dilute concentrations of blood sometimes so dilute the human eye can longer see the stain. These tests (listed below), while more reliable, require more expensive chemicals.
Most of these tests rely on the activity of peroxidase enzymes in blood to react with a chemical stain causing it to change colour, or in the case of luminol, glow in the dark.
In this activity, you will be comparing the results of the catalase test using hydrogen peroxide with the phenolphthalein test, to see how each reacts with blood and other substances.
Which of the following substances do you think would test positive for
catalase? Make a prediction for each, and explain your reasoning. Make
sure you make a prediction for each substance before conducting your test.
Test each of these substances to see if it is catalase positive or negative by placing a few drops of hydrogen peroxide on a small amount of each. Record results in the table above.
SAFETY NOTE: Even though you will not be using any real human blood in this activity, you should wear appropriate protection such as gloves.
Analysis of evidence from the crime scene
Which of these stains is probably blood? Could it be anything else other
than blood? Check your answers against the key provided.
Pattern and shape: The shape and pattern of blood drops can reveal important information about the nature of the wound from which the blood came. Was the bleeding person standing still or walking? What distance did the blood drop fall? Did the blood spatter in all directions? A good investigator would carefully photograph all blood stains from different angles both so that a forensic scientist could examine the pattern and to be able to present the evidence to a jury.
DNA: Blood contains DNA, and depending on the size of the stain
and its condition (old, new, dry, etc.), a forensic scientist may be able
to get enough information to obtain a highly probable match of a suspect
with the evidence.
Type: Blood typing can be used as an initial test to exclude some
suspected sources of a bloodstain. For example, if a blood stain at the
crime scene contains Type A blood, but the key suspect has Type O blood,
the suspect could
Investigators have collected blood samples from each of the suspects in the case. The samples and the evidence are labeled A-D. It will be your job to type each sample. You will determine both the ABO blood type of each sample as well as the Rh factor type.
ABO blood group:
The Rh factor:
Follow the directions provided with your blood typing kit and determine the blood types of the samples labeled A, B, C, and D. Remember to wear gloves while handling the blood samples. Record the blood types of each individual below. Consult the key to the labels and write in the identity of each sample.
Answer the following questions regarding your results: