Crimes big and little happen every day. But how do you solve them? Maybe you find some clues but what then?
Science to the rescue!!!
Forensic science is of course based on… you guessed it SCIENCE. So lets use a little of that chemical science to solve today’s mystery of Who Stole the Cake?
In this lesson we will
- Practice deductive reasoning
- Perform scientific tests on gathered evidence
- Record observations
- Practice taking fingerprints
- Identify the culprit!
Time to Complete
10 minutes prep, 30+ minutes lesson
Crime Scene Investigation & Chemistry
Forensic science, deductive reasoning, chromatography, polarity, acid, base
Take a look at the lesson below or download the lesson PDF.
- Plate with crumbs – 1
- Flour – 1 or 2 Tablespoons
- Baking soda – 1 or 2 Tablespoons
- Vinegar – 4 or 5 Tablespoons
- Ice cube tray – 1
- Bowl or food storage container – 1
- Paper clips – 3 or 4
- Paper towel – 1
- 91% Isopropyl alcohol or rubbing alcohol – 1/2+ cups
- Suspect cards (pg 16) – 1
- Thief fingerprint & note cards (pg 22) – 1
- Detective pages (pg 15) – 1 per student
- 3 different brands of black marker
- Timer (watch or phone would work)
- Measuring tape (optional)
- Fingerprinting page (pg 23)
- Ink pad (optional)
Scenarios #1 & #2 supplies
The scenarios take all the same supplies except:
- Scrap of green fabric or string (scenario 1)
- Scrap of black fabric or string (scenario 2)
- Print 1 Science Detective sheet (pg 15) for each student
- Print the suspect and fingerprint sheets (pg 16). The sheets are designed to be printable front and back so the suspect is on one side with their fingerprint on the other.
- Print the Mystery Print and Note sheet (pg 22). The note is the same for both scenarios. Mystery Print 1 goes with scenario #1. [Mystery Print 2 goes with scenario #2.]
The Crime Scene
- Choose a spot for the initial crime scene.
- Place some kind of crumbs on an empty plate to show evidence of the stolen cake.
Clue #1 – time
Clue #1 is an eyewitness statement about time. The location needs to be exactly 1 minute from the crime scene.
- Choose a spot away from the crime scene and practice walking to it and timing for 1 minute.
**Only the location of Clue #1 matters. Put all of the other clues wherever you choose. Just spread them out in your backyard or at the park so they kids get some walking in.
****This prep is the same for scenario #1 and #2. ****
Clue #2 – cloth
- Place a scrap of green fabric or string 4.5 feet (1.4 meters) off the ground. You can make it look like it was snagged on a fence or bush.
- [Or for scenario #2, place the scrap of black fabric or string 2 feet (0.6 meters) off the ground].
Clue #3 – note
For the note, you will need 3 different brands of black marker. They can be permanent, washable, or dry erase.
- Cut a strip of paper towel about 1 inch (2.5 cm) wide and several inches long. It needs to be long enough to reach from the bottom of your bowl or food storage container to the top and be paperclipped to the edge.
- Draw a wide band of color across the strip of paper towel about 1 inch (2.5 cm) from the bottom.
- Label the top of the strip so you know it is a “mystery sample” from the marker that wrote the crime scene note. I used the letter M.
- For young kids, cut out 4 or 5 extra strips for their experiment. This is a great step to let the kids do themselves if they can uses scissors.
- Now you need to label each marker as Marker #1, #2, or #3. This corresponds with the marker numbers on the suspect cards. The marker you used to make the “mystery sample” MUST be labeled as Marker #1 [it needs to be Marker #2 for scenario 2]
Clue #4 – powder
- Temporarily mark an ice cube tray with numbers 1-12.
- Add about 12 teaspoon of the indicated powder to the tray.
- #1 – flour
- #2 – flour
- #3 – backing soda
- #4 – flour
- #5 – backing soda
- #6 – backing soda
- #7 – backing soda
- #8 – flour
- #9 – flour
- #10 – baking soda
- #11 – flour
- #12 – baking soda
- Put about 1/2 teaspoon of baking soda in a separate container as the “mystery sample” from the crime scene.
This is the general lesson format. Please refer to “The Science” section for more information on what science to teach during the lesson. That section is provided separately so you can pick and choose what is most interesting and appropriate for your learners.
Introduce the Scene of the Crime
This intro will happen at the crime scene with the crumb filled plate as evidence.
After the meeting of the Play Outside Forever group, someone stole the special cake for the party tonight! All there is left is a few crumbs on this plate. There were 12 people at the group meeting and all of them are suspects. We have interviewed them and gathered some evidence but we need you the Science Detectives to help us find the thief! Gathering and testing evidence from a crime scene is called Forensic Science.
Give out the Science Detective sheets (on pg. 6) where the detectives can write down their observations for each clue.
Clue #1 – time
Someone spotted the thief at 10:04am [spotted at 10:06am for scenario #2]. You need to walk to that spot you picked during the prep with the timer and decide how long it takes to get there. No one was seen running so you have to walk. This should take a minute or less.
Use what you know of the suspects time last seen at the crime scene to eliminate suspects who did not have enough time to walk to that spot.
- What time did they leave the crime scene?
- How much time between when the suspect left to when someone was spotted at 10:04am [10:06am]?
- Who had time to walk that far before 10:04am [10:06am]?
Example: Suspect #1 – Alice was last seen at the crime scene at 9:58am. That leaves 6 minutes to get to the eyewitness location. Plenty of time. She is still a suspect.
The way we are eliminating suspects is called deductive reasoning. Deductive reasoning uses a group of general known facts to draw a conclusion.
- Alice left the crime scene at 9:58am (fact)
- An eyewitness saw the thief at 10:04am [10:06am] (fact)
- It take 1 minute to walk from the crime scene to Clue #1. (fact).
Using these facts, we can deduce that Alice had plenty of time to walk to Clue #1 and is still a suspect.
Clue #2 – cloth
Now head over to Clue #2. Point out the cloth as the next clue. If you have a tape measure, measure the exact height of the cloth from the ground.
- How far is it off the ground?
- Would this cloth come from a shirt or pants?
- What color is it?
- Which suspect could have left this color of fabric at this height?
Eliminate more suspects due to what you know about Clue #2.
Clue #3 – note
Now for Clue #3. Show the note found at the crime scene (found on pg. 22). We have a sample of the marker that was used to write the note. We also have markers that were found at the homes of each suspect. We are going to do an experiment to decide which of these markers was used to write the note. We will then eliminate any suspects that do not have the right kind of marker.
You already prepared the Mystery marker sample. Have the kids prepare their marker samples the same way. The kids will make 1 sample strip for each marker (3 total) that they will compare to the Mystery marker sample.
- Fill the bottom of your bowl or food storage container with about 1/8 inch (3 mm) of alcohol.
- Paper clip the prepared sample strips to the side of the bowl with just the end of the sample touching the alcohol. DO NOT LET THE MARKER DIRECTLY TOUCH THE ALCOHOL. It will ruin the sample. The alcohol needs to climb slowly up the paper towel.
This experiment is a method called chromatography. We are using the alcohol to pull apart the different components of the marker ink. The marker may look black, but it is actually a mixture of colors. This is why we use 3 different brands. Each company makes their ink a little differently so the samples will be unique. Read more about chromatography in “The Science” section.
** You can move onto Clue #4 while this experiment runs. It will take about 10-15 minutes. **
Once the experiment has set for about 10-15 minutes, compare the samples.
Are they the same or different?
Are the black markers really all just black?
Which marker sample matches the “mystery sample” from the crime scene note?
Eliminate more suspects based on your observations about the markers.
Clue #4 – powder
The next clue is a mysterious white powder found at the crime scene. White powders were also found at the homes of each of the 12 suspects. Now we need to figure out if the powder from the scene is the same as the powder from the suspects.
You have prepped the tray of samples from the suspects and also the extra dish of the mystery powder. Now you need to add some vinegar to see how each powder reacts. Just add a little bit with a spoon. Some will be bubbly with vinegar and some won’t. Have the students take note of what each powder is doing.
Eliminate suspects by comparing their powder’s reaction to the mystery powder.
The vinegar’s reaction with the white powder (in this case baking soda) is a simple acid base reaction. The other powder (flour) is not a base so it is simple to see the difference. Read more about acid/base reactions in “The Science” section
Clue #5 – fingerprints
We have just gotten a fingerprint! Now that we are down to the last few suspects it will be easy to compare fingerprints of each suspect. Every person has a unique fingerprint so the Mystery Print will only match 1 suspect.
The students will compare the fingerprints of the suspects to Mystery Print 1 [or Mystery Print 2 for scenario #2]
- What are the similarities and differences in the fingerprints?
- Which fingerprint matches exactly?
You’ve caught the suspect!!
The Science Detectives have uncovered the real cake thief! Which suspect matches your observations? (see the answer on page 25 for scenario 1 or page 26 for scenario 2).
Being detective is hard work! But science makes it a little easier.
Extending the Lesson
Put some color on
Do the chromatography experiment again but this time use different colors.
- If you use 2 reds, do they look the same or different?
- Are the inks just one color or are they made up of a mixture of colors?
Where’s the print?
Instead of taking fingerprints off a friends hand, dust for fingerprints around the house! Check out this page about dusting for prints. You can also take the fingerprints of everyone in the family and compare.
- How is DNA fingerprinting used to identify a criminal? – animated video by Oxford Education https://www.youtube.com/watch?v=AkBUriMK9u8
- CSI Expert!: Forensic Science for Kids by Karen Schulz
- Crime Scene Whodunits: Dr. Quicksolve Mini-Mysteries by Jim Sukrach
- Eleventh Hour by Graeme Base
There so much extra on the PDF!
Our lesson plan PDF is just chock full of good stuff that is too much to put here. Grab the full lesson plan to get “The Science” section, vocabulary, and all the crime scene printable clues and pages.