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LAB in Chemical Change, Physical Change, Identifying an Unknown, Acid Base Reactions. Last updated December 22, 2022.


In this lab, students will identify an unknown substance as the dangerous chemical X or a common household product. Students will perform tests on the physical and chemical properties of different substances to determine the identity of the unknown.

Grade Level

Middle school

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
  • Scientific and Engineering Practices
    • Planning and Carrying Out Investigations
    • Constructing Explanations


By the end of this lab, students should be able to

  • Identify and differentiate between physical and chemical properties of matter.

Chemistry Topics

This lab supports students’ understanding of

  • Physical change
  • Chemical change


Teacher Preparation: 1 hour

Lesson: 50 minutes


For each group:

Unknown sample (see identities in teacher notes)

  • Cabbage juice for pH test
  • Biuret reagent (1% strong base followed by CuSO4 (aq))
  • Vinegar
  • Iodine


  • Always wear safety goggles when working in a chemistry lab.
  • 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.

Teacher Notes

  • This activity is engaging, quick, inexpensive, and requires students to use deductive reasoning to solve a problem. I use it after a similar lab that uses only physical properties to identify an unknown. It’s a nice parallel to draw because in this lab students identify substances using physical and chemical properties.
  • Students should understand that reactivity and flammability are independent of the size of sample.
  • Unknown identities:
    • A: Baking Soda
    • B: Corn starch (Chemical X)
    • C: Salt
    • D: Gelatin
    • E: Baking powder

Suggested lab implementation:

  • Engage: To engage students, ask them to list some examples of physical change. Sample answers: tearing paper, crumbling cookies, melting ice. Also ask students to list some examples of chemical change. Sample answers: sugar caramelizing, vinegar and baking soda reacting to produce a gas, iron rusting.
  • Explore: Students complete the lab activity as describe din the procedures of the student handout.
  • Explain: Enforce the understanding that the cabbage juice reacts with acids and bases differently but does not react with neutral substances.
    • The biuret solution will react with certain substances and turn purple. If it stays blue there was no reaction since color change is a sign of a chemical change.
    • The vinegar produces bubbles when it reacts. The bubbles are a sign of a chemical reaction.
    • The iodine turns black when it reacts with certain substances.
  • Extend: Dalton's Atomic Theory Research: In 1808, John Dalton proposed what is now known as the law of conservation of mass. Students can research Dalton and his career. Task them with making a timeline to highlight his achievements.
Test results
Test Baking Soda Corn starch Salt Gelatin Baking Powder
Color White White White Ivory White
Crystal or Powder Powder Crystal Crystal Powder
pH Base Neutral Neutral Can be slightly acidic Neutral
Biuret Blue Blue Blue Purple Blue
Vinegar Yes No No No Yes
Iodine No Black No No Black

For the Student



You are an FBI agent and a package was sent to an important political leader. The politician was exposed to a white powdery substance. For precautionary reasons, he was rushed to George Washington Hospital in Washington, D.C. The treating physician has requested that you identify the substance to determine his course of treatment. You must find out whether the package was a hoax and is actually a harmless household product, or if the substance is dangerous Chemical X that leads to death within two hours of inhalation. The clock is ticking…


  1. Make observations of your sample. Record the color and type of solid in your data table.
  2. Test your sample and record your observations in the data table.
    1. pH test (acid/base properties)
    2. biuret test (peptide bonds)
    3. vinegar reactivity (confirms acid/base properties)
    4. iodine test (detects starch)
  3. For the pH test, use the chart below to help you determine the property.
Red Cabbage pH Indicator Colors













Greenish Yellow

Test Color Crystal or powder pH: acid, base, or neutral Biuret: purple or blue Vinegar: reaction or no reaction Iodine: no change or turned black


  1. What is the identity of your unknown?
  2. Why was it important to use different tests to determine which powder/crystal the unknown sample is?
  3. This lab tested how your substance reacted with different liquids. Do you think the property of reactivity is dependent on the size of your sample, or would the tests come out the same even if you used more or less?
  4. You previously completed a lab that used physical properties to identify a substance. Which properties do you think are MOST reliable to identify unknown substance, chemical or physical? Explain


Write 2–3 sentences explaining what you learned.