Crime Scene Science: Solving Mysteries by Investigating Blood Stains
Do you enjoy watching mysteries or crime investigation shows? Every criminal leaves behind evidence at the crime scene. The trick to catching the criminal is collecting all of the evidence and making sense of it. This is what a forensic expert does. In this bloody, Halloween-themed science activity you’ll be correlating the size of blood stains to the distance they fell, but don’t get too grossed out; you’ll be doing it with fake blood. If you like figuring out mysteries, this is the activity for you!
Forensic science is any science that is used in the courtroom or judicial system and it is extremely vital. The goal of a forensic scientist is to remain impartial and to appraise all of the available evidence in order to determine the truth. Forensic science includes many areas of study, such as criminalistics, engineering science and pathology and biology. For example, if a sports player were to die suddenly while playing a game, a forensic scientist with a specialty in pathology and biology would be called in to find out the cause.
In crime scenes where people are wounded, investigators apply principles of blood spatter analysis by taking measurements, such as of the diameter of the blood spatter (drops), to figure out what happened. How do you think the diameter of spatter changes as blood is dropped from different heights? Find out by using some fake blood in this activity and trying your hand at collecting evidence!
- Measuring spoons
- Corn syrup
- Red food coloring
- Fork or spoon for mixing
- Plastic wrap (optional)
- Sheet of cardboard. You will want a total surface of at least four square feet. You can use multiple smaller sheets of cardboard instead of one large sheet. The cardboard should be able to lay flat and not have many folds or creases.
- Pencil or pen
- Medicine dropper
- Measuring tape
- Ruler with millimeter marks
- A helper
- A location to do your activity where the ground can be easily cleaned in case of spilling the fake blood. For example, concrete or linoleum may work well.
- Butcher paper or newspaper (optional)
- Prepare the fake blood. Start by measuring out two tablespoons (Tbsp.) of cornstarch and placing it into the bowl.
- Thoroughly mix two Tbsp. of water into the cornstarch.
- Add four Tbsp. of corn syrup to the bowl and mix thoroughly. The mixture should be smooth.
- Mix in one teaspoon (tsp.) of red food coloring. Stir until the mixture is a consistent color.
- You should now have a small bowl of fake blood! If you will not be using it right away, cover the bowl (such as with plastic wrap) and set it aside for now.
- Be careful not to spill the fake blood on furniture or carpeting as it can stain! If it does spill, be sure to clean up the fake blood immediately. You may also want to cover the ground where you will be doing the activity with butcher paper or lots of newspaper to help prevent stains.
- Lay the sheet of cardboard flat on the ground where you will do your activity.
- Fill the medicine dropper with some fake blood and hold the dropper completely vertically above the cardboard sheet. Have a helper use the measuring tape to make sure the end of the dropper is one foot above the cardboard sheet.
- Drop a single drop of fake blood onto the cardboard sheet.
- Repeat this process at least four more times so that you have dropped a total of at least five drops onto the cardboard sheet. If any of the drops land on another drop, or land in a crease or fold of the cardboard, do not count these drops for your total of five. Use a pencil or pen to label these drops “one foot” next to them on the cardboard. Do all of the drops look similar in size?
- Now have the helper use the measuring tape to make sure the end of the dropper is three feet above the cardboard sheet. Drop a total of at least five drops from this new height, three feet above the cardboard sheet, only counting drops that don’t land on other drops or in creases. Label these drops “three feet” on the cardboard. Do these drops look larger or smaller than the drops made from one foot?
- Lastly have the helper use the measuring tape to make sure the end of the dropper is five feet above the cardboard sheet. Drop a total of at least five drops from this last height, five feet above the cardboard sheet, only counting drops that don’t land on other drops or in creases. Label these drops “five feet” on the cardboard. Do these drops look larger or smaller than the drops made from lower heights?
- Let all of the drops dry, being careful not to disturb the drops while they dry. Note that the drops may look glossy and wet even after they have dried. You may want to make an extra drop on the cardboard somewhere that you can test (by touching it) to see when the drops have completely dried.
- When all of the drops have dried, measure the diameter of each drop in millimeters. Do your measurements very carefully as there may be only small, but measureable, differences between the drops. Write the diameters on the cardboard next to the drops.
- How did the diameter of the drops change as the fake blood was dropped from different heights? How do you think your results could help a forensic scientist figure out what happened at a crime scene?
Extra: Instead of dropping the fake blood straight down, try dropping it at angles to the ground, or dropping it while moving the dropper in a certain direction. Be sure to do this in a location that can be accidentally stained with fake blood! How does this change the size and shape of the drop’s impact and where the spatter ends up?
Extra: Repeat this activity over other surfaces, such as newspaper, printer paper, parchment paper, paper towels, butcher paper, or wood. How do your results change when using different surfaces?
Extra: In this activity you tried dropping fake blood from one, three and five feet. Try repeating the activity but use different heights (such as two, four, six and seven feet) and then graph your results. What kind of graph does the data make? What kind of mathematical correlation is there between the height the drop is dropped from and its diameter on the surface it lands on?
Observations and Results
You should have seen that the drops made from dropping the fake blood at five feet were the largest in diameter, while the drops made from dropping the fake blood at one foot were the smallest in diameter. However, the differences in diameters were likely small, with the five-foot drops possibly being about 12 millimeters (mm) in diameter, the one-foot drops being about 10 mm in diameter, and the three-foot drops being in between at about 11 mm in diameter. By understanding how blood spatter increases in diameter as the dropping height is increased, forensic scientists can better figure out what happened at some crime scenes where people are wounded.
Ask an Expert
- Forensic Science Facts, from Science Kids
- Forensics: How Does It Matter? Measure the Spatter!, from Science Buddies
- Science Activities for All Ages!, from Science Buddies