The Case of the Stolen Cookie Recipe: A Forensic Investigation

Purpose

Participants identify an unknown substance through observation of physical and chemical changes. 

Learning Outcomes

Participants will:

  • Identify evidence of physical changes, including changing phase or shape and dissolving in other materials. (MN State Science Standard 6.2.1.2.1)
  • Identify evidence of chemical changes, including color change, gas evolution, solid formation, and temperature change. (MN State Science Standard 8.2.1.2.1)
  • Distinguish between chemical and physical changes in matter. (MN State Science Standard 8.2.1.2.2)

Timeline

75 minutes total

  • Welcome and introduction - 15 minutes 
  • Small group data collection - 30 minutes
  • Analysis and discussion - 15 minutes
  • Conclusion and reflection - 15 minutes

Instructor Resources

Participant Handouts

Other Materials

Note: Quantities listed will supply five groups of five participants unless otherwise noted.

  • Small containers with lids: 7
  • Baking soda 
  • Baking powder
  • Cornstarch
  • Cream of tartar
  • Plastic table cloths (optional): 5
  • Spoons: 5
  • Magnifying glasses, lenses, or microscopes: 5
  • Blue construction paper: 5 sheets
  • Water
  • White vinegar 
  • 1 mL disposable plastic pipettes: 2-10
  • Universal indicator bottle and 1-5 droppers 
  • Iodine bottle and 1-5 droppers 
  • Ice cube trays: 4-20
  • Stir sticks: 20-100 
  • Non-latex gloves: one pair per participant
  • Writing utensil: one for each participant
  • Cookies (optional

Procedure

Set-up

  1. Label four containers A, B, C, and D.  Measure 100g each of baking soda (A), baking substance (B), cornstarch (C), and cream of tartar (D) into the corresponding container.
  2. Choose one substance to be the mystery power (recommended: cornstarch). Label one container “Evidence” and measure 100g of the chosen mystery substance into the Evidence container.
  3. Set up five units. Cover work space with plastic table cloths, if needed. To begin, place one container of substance (labeled A-D and Evidence) and a spoon at each unit. Later on, participants will stay at their unit’s work space while the substances are rotated through the units.
    1. Physical analysis unit
      • Magnifying glasses
      • Blue construction paper
      • Unit handout (half-sheet per participant)
    2. Chemical analysis water unit
      • Container of water
      • Pipettes: 1-5
      • Ice cube trays: 1-5 (recommended: label wells for each substance)
      • Stir sticks: 5-25 (recommended: label stir sticks for each substance)
      • Unit handout (half-sheet per participant)
    3. Chemical analysis vinegar unit
      • Vinegar
      • Pipettes: 1-5
      • Ice cube trays: 1-5 (recommended: label wells for each substance)
      • Stir sticks: 5-25 (recommended: label stir sticks for each substance)
      • Unit handout (half-sheet per participant)
    4. Chemical analysis iodine unit
      • Iodine bottle and dropper
      • Ice cube trays: 1-5 (recommended: label wells for each substance)
      • Stir sticks: 5-25 (recommended: label stir sticks for each substance)
      • Unit handout (half-sheet per participant)
    5. Chemical analysis universal indicator unit
      • Container of water
      • Universal indicator bottle and dropper
      • Ice cube trays: 1-5 (recommended: label wells for each substance)
      • Stir sticks: 5 (recommended: label stir sticks for each substance)
      • Unit handout (half-sheet per participant)

Lesson

  1. Divide participants across five units.
  2. Present slides 1-20. Be as dramatic as possible with the crime scenario!
  3. Present slide 21. Hand out gloves to each participant. Discuss health and safety procedures:
    • Allergies to these baking ingredients are uncommon
    • Gloves should be worn at all times
    • Tasting/smelling substances is not permitted
    • Iodine and universal indicator will stain, so take care to avoid overfilling the ice cube tray wells and tightly cap the dropper bottles
  4. Hand out writing utensils to each participant. Everyone can write down their observations, but have one participant serve as recorder for the unit’s final observations. Use the instructor's choice of document camera to record observations, and share final observations with the large group using blank presentation slide 23 or the blank Substance Analysis Table document.
  5. Have units consult their unit handout for detailed instructions. Walk around the room to assist.
    • Everyone: Write detailed observations!
    • Physical Analysis unit: Hold the magnifying glasses close to the substance not close to their eyes.
    • Chemical Analysis units: Carefully label and use correct stir sticks and ice cube tray wells to avoid cross contamination. Don’t overfill wells. Try adding more/less substance or solution, stir, and be patient to observe accurate reactions. Make sure to let the Chemical Analysis units know that they may have to give the reactions some time and to keep mixing the solutions with the stirrers.
  6. Rotate the substances between units every five minutes so that each unit analyses all five substances.  Consider projecting a countdown (slide 22). After observations, each unit should analyze and discuss their own results and draw a preliminary analysis and conclusion about the unknown substance. Then, have each unit bring up their final observations and initial conclusion to be shared with the class through the instructor’s choice of document camera, typed into presentation (slide 23), or typed into a separate document. Participants can start cleaning up at this time.
    • Rinse out the ice cube trays with water. All materials are okay to go down the drain.
    • Collect durable materials, including rinsed ice cube trays.
    • Wipe up spilled substances and solutions and carefully throw away soiled items. Remember that iodine and the universal indicator will stain. 
    • Throw away other disposable items, including gloves.
  7. Have each unit present (slide 23) their observations and preliminary analysis and conclusion.
  8. Discuss each unit’s observations, analysis, and conclusion as a large group and come to a consensus about the final analysis and conclusion.
  9. Present slides 24-25, an exposé of the criminal. Be as dramatic as possible! Reward everyone with a cookie for their hard work (optional).
  10. Finish clean-up and lead a reflection on the purpose and outcomes of the activity (slide 26).