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LAB in Elements, Identifying an Unknown, Conservation of Matter, Culminating Project, Chemical Change. Last updated December 27, 2022.


In this lab, students will identify a mystery substance by thinking like an 18th Century Scientist.

Grade Level

Middle school

NGSS Alignment

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

  • MS-PS1-1: Develop models to describe atomic composition of simple molecules and extended structures.
  • 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:
    • Analyzing and Interpreting Data
    • Planning and Carrying Out Investigations


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

  • Identify an unknown gas based on experimental evidence.

Chemistry Topics

This lab supports students’ understanding of

  • Reactions
  • Chemical change


Teacher Preparation: 30 minutes

Lesson: 2 class periods (one for investigation, one for discussion)


For each group:


  • Always wear safety goggles when carrying out lab activities.
  • Be careful when lighting the match. Be cautious of your surroundings.
  • 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

  • I love this activity. It is a culmination of the atoms and elements unit. It is a true constructivist style activity allowing students to discover for themselves the properties of oxygen, apply the laws of conservation of matter and the understanding of elements and compounds, and compare the results of prior labs.
  • Before doing this lab with my students, we have done at least two previous labs where students practice identifying gases released, such as carbon dioxide gas and hydrogen gas. While we do not have the hydrogen lab on the AACT website, consider using the Preparation of Hydrogen Demo to show your students this phenomenon before completing this lab. It is important background knowledge.
  • An additional lesson that might be helpful to pair with this lab is from the Landmark Collection, John Priestley, Discoverer of Oxygen
  • Offering the Priestly Prize for Excellence in Elements (a small plant) for the first and most evidence based conclusion/claim gets students remarkably excited and engaged. Their discourse is high school level as they work to solve the mystery with evidence!

For the Student



Thank you for your valuable time to help us here at Priestly Labs, Inc. As you may have heard, we believe that we have made an important and valuable discovery! Last week two of our head researchers mixed hydrogen peroxide (_________) and manganese dioxide (_______) and something bubbled off as a result. We don’t know if we should be excited or worried! We are hoping that you can help us find out by carefully repeating this experiment to determine just what this mystery substance might be. As soon as you have made a determination regarding the outcome of your experiments, please fill out the CONCLUSION of this sheet and send it to us right away. We look forward to hearing about your results. Good luck!

For your convenience we have provided you with all of the necessary chemicals and equipment and assume that you have access to the periodic table, other chemistry reference materials, and participate in respectful scientific discourse. There are additional questions and notations we have included that may help to expedite your conclusions.


  • Wear safety goggles at all times.
  • Be cautious around an open flame. 
  • Wash your hands thoroughly before leaving the lab.


  1. Place a small scoop (green pea-sized) of manganese dioxide into a test tube.
  2. Carefully add 10 mL of hydrogen peroxide to the test tube and quickly cover the test with your thumb.
  3. Record any observations, keeping your thumb on the test tube.
  4. Light a toothpick with a match and let it burn for 4 seconds. Gently blow out the flame and immediately insert the glowing splint into the opening of the test tube.
  5. Record your observations.



  1. The compounds you used were (write the names in words) :
  2. The chemical formula for those compounds are:
  3. Can an element change into a new element during a chemical reaction?
  4. Why or why not?
  5. Can a compound change into a new compound during a chemical reaction?
  6. Why or why not?
  7. Did a chemical reaction occur during this experiment?
  8. What is the evidence for your answer to #7?
  9. How many elemental gases exist on the periodic table?
  10. How are noble gases different from other gasses?
  11. Which elemental gases could be the gas you generated?
  12. Can you eliminate any gases from the list in question #9 that you know you didn’t produce in this experiment?
  13. Why?
  14. What two gases are left? What do you know about those two gases?
  15. How could you test to see to see if it is one of these?
  16. Why would hydrogen peroxide be kept in a dark brown bottle?


Dear Priestly Labs,

I think the mystery substance is ____________because of the following: