Key concepts
Forensics
Liquids
Physics
Models
 
Introduction
Do you enjoy watching crime mysteries or reading about investigations? Every criminal leaves behind evidence at the crime scene. The trick to catching the lawbreaker is collecting all the evidence and making sense of it. This is what a forensic expert does. In this Halloween-themed science activity you'll be correlating the size of bloodstains to a distance traveled—but don't get too grossed out; you'll be doing it all with fake blood. If you like figuring out mysteries and you don't mind a little gore, roll up your sleeves and get ready to crack the case!
 
Background
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 study all the available evidence in order to determine the truth—which can often determine what happened in  a crime case. Forensic science includes many areas of study such as engineering, pathology and biology. For example, if an athlete 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 liquid analysis by taking measurements, such as of the diameter of a blood spatter (drops), to figure out what happened. How do you think the diameter of spatter changes as liquid travels different distances? Find out by using some fake blood in this activity and trying your hand at collecting evidence!
 
Materials

  • Measuring spoons
  • Cornstarch
  • Corn syrup
  • Red food coloring (or another color of your choice)
  • Water
  • 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 one. 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
  • Butcher paper or newspaper (optional)
  • A helper
  • A location to do your activity (It should be where the ground can be easily cleaned in case of spilling the fake blood. For example, concrete or linoleum may work well.)
 
Preparation
  • Prepare the fake blood. Start by measuring out two tablespoons (tbsp.) of cornstarch and placing them 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 of red (or other color) food coloring. Stir until the mixture is a consistent color.
  • You should now have a small bowl of fake blood (or other liquid, if you prefer less gore!). 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—it can stain! If it does spill, be sure to clean up the liquid immediately. (Before you lay down the cardboard you might also want to cover the ground where you will be doing the activity with butcher paper or lots of newspaper to help prevent stains.)
 
Procedure
  • Lay the sheet(s) of cardboard flat on the ground where you will do your activity (placing the extra paper below and surrounding if desired).
  • Fill the medicine dropper with some fake blood or liquid and hold the dropper vertically one foot above the cardboard sheet. Have a helper use the measuring tape to make sure the end of the dropper is one foot above the sheet.
  • Drop a single drop of fake blood onto the cardboard.
  • Repeat this process at least four more times while holding the dropper over slightly different areas so that you have dropped a total of at least five drops onto the cardboard. (If any of the drops land on another drop or in a crease or fold on the cardboard, do not count them in your total of five.) Use a pencil or pen to label the five successful drops "one foot" next to each of 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 sheet, in different spots, 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. (Have a taller helper hold the dropper that high if you need more height—or use a sturdy stepladder with a spotter.) Drop a total of at least five drops from this height, 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 them. Note that the drops may look glossy and wet even after they have dried. You might want to add an extra drop on the cardboard somewhere that you can test (by touching it) to see when the drops have completely dried.
  • After all the drops have dried, measure the diameter of each one in millimeters. Take your measurements very carefully, there may be only small, but measureable, differences between the drops. Write the diameters on the cardboard next to the drops.
  • How did the drops’ diameters change as the fake blood (or liquid) 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, being careful to avoid surfaces that should not be stained. 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 impacts?

 
Observations and results
Did the diameter of the fake blood drops increase as the dropping height increased?
 
You should have seen that the drops made from dropping the fake blood at five feet were the largest in diameter whereas the drops made from dropping the fake blood at one foot had the smallest diameters. The differences in diameters, however, were likely small, with the five-foot drops possibly about 12 millimeters (mm) in diameter, the one-foot drops around 10 mm in diameter and the three-foot drops  in between—close to 11 mm. 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. What other type of science could studying this sort of liquid behavior help with?
 
More to explore
Forensic Science Facts, from Science Kids
Episode: Forensics by Kalia and Carolyn, from PBS Kids
Forensics: How Does It Matter? Measure the Spatter!, from Science Buddies
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies