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In this lab, students will use chemical and physical properties to identify an unknown powder. There’s an aspect of “who done it” to make the activity a little more interesting to students.
Middle School, High School
This activity will help prepare your students to meet the following scientific and engineering practices:
- Scientific and Engineering Practices:
- Analyzing and Interpreting Data
- Planning and Carrying Out Investigations
- Engaging in Argument from Evidence
By the end of this lesson, students should be able to
- Identify an unknown from testing physical and chemical properties.
This lesson supports students’ understanding of
- Chemical changes
- Identifying an unknown
- Experimental design
Teacher Preparation: 20 minutes
Lesson: 45 minutes
- Unknown powder sample
- Sodium chloride, NaCl
- Baking soda, NaHCO3
- Corn starch, C6H11O5
- Powdered sugar, C12H22O11 (*see note below)
- Hot plate
- 5 small beakers
- Spot plate
- Conductivity tester
- pH paper
- Always wear safety goggles when working with chemicals.
- Acetic acid (vinegar) vapors can be irritating. Work in a well-ventilated area. In the event of eye contact, flush with water. The concentration of acetic acid in this experiment does not present any significant hazards.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- Make sure the powder found in grandma’s kitchen is the same as the one found at the house of the guilty suspect.
- Provide the white powders in labeled containers.
- Make sure that you use powdered sugar that has not been cut with cornstarch. It's often used as an anti-clumping agent, so if you're looking for a negative starch test for the powdered sugar, check to make sure you have the right kind of powdered sugar. Alternatively, use regular sugar.
- Label bottles with the following and put the specified powders in each:
Grandma - Kitchen floor (corn starch—or a different powder, teacher’s choice)
Suspect #1 - Betty Crump (corn starch)
Suspect #2 - Gladys Smith (baking soda)
Suspect #3 - George Jones (sodium chloride)
Suspect # 4 - Millie Malone (powdered sugar)
For the Student
It's Thanksgiving morning and you answer a frantic phone call from your grandma. She tells you she spent all day yesterday baking her famous pies for Thanksgiving dinner and left them on the counter to cool overnight. When she woke up this morning, they were gone. She doesn’t have time to make more pie, and Thanksgiving dinner won’t be the same without them. She suspects the thief is someone she knows, so she has already collected some evidence. She is depending on you and your chemistry skills to help her track down the thief and confirm her suspicions.
When you arrive at her house, you notice some white powder that has been tracked across her kitchen floor. You ask her what it is, but she used so many white powders in the creation of her masterpieces, she’s not sure which one it is. You head to her pantry and discover the following:
- Salt, NaCl, an ionic compound used for flavoring
- Baking soda, NaHCO3, an ionic compound that is used as a leavening agent in baked goods
- Corn starch, C6H11O5, a covalent compound used as a thickening agent
- Powdered sugar, C12H22O11, a covalent compound used as a sweetener
You take some notes then ask her about her preliminary investigation. She hands you the following profiles:
Name: Betty Crump
Occupation: Best friend and nosy neighbor
Evidence: White powdery substance collected from the stairs of the back door to her house
Backstory: Betty has extended family coming to town for Thanksgiving dinner and is a terrible cook.
Name: Gladys Smith
Occupation: Retired teacher and vocal city councilwomen
Evidence: White powdery substance collected near her kitchen sink, right next to where she keeps her dentures
Backstory: Gladys is Grandma’s biggest Bingo rival, and she is jealous of Grandma’s recent winnings.
Name: George Jones
Occupation: Works for the city’s transportation department
Evidence: White powdery substance collected from his work boot soles
Backstory: George is a big fan of Grandma’s pies and has been begging her to make him one for him to take to his own family’s Thanksgiving.
Name: Millie Malone
Occupation: Retired lunch lady and county fair blue ribbon contender
Evidence: White powdery substance collected from the door handle leading into her kitchen
Backstory: Millie’s pies placed second in the last county fair behind Grandma’s, and she still hasn’t gotten over it.
You collect the white powder from the kitchen floor, the white powders from the pantry, the evidence collected at the homes of the suspects, vinegar from the pantry, iodine from the medicine cabinet, a hot plate, beakers, a spot plate, a conductivity tester, and pH paper from your chemistry lab, and get to work.
Design a series of tests to determine what the unknown substance on the kitchen floor is. Write out the steps you take in the space below – make sure your instructions are clear and could be easily repeated by another scientist.
Create a data table to clearly organize the information you gather from your tests.
Who stole Grandma’s pies?! Explain how your tests led you to this conclusion.