« Return to AACT homepage

AACT Member-Only Content

You have to be an AACT member to access this content, but good news: anyone can join!

Need Help?

Breath Test Mark as Favorite (7 Favorites)

LAB in Observations, Chemical Change, Physical Change, Reversible Reactions. Last updated October 08, 2019.


In this lab, students investigate whether chemical reactions can happen only in one direction, or whether they can be "undone."

Grade Level

High school


By the end of this lesson, students will

  • Witness a reversible reaction to gain a better understanding of the concept.

Chemistry Topics

This lesson supports students’ understanding of

  • Reactions
  • Reversible reactions


Teacher Preparation: 20 minutes

Lesson: 30 minutes


  • Straw
  • Cup
  • Water
  • BTB indicator
  • Vinegar in a dropper
  • Baking soda


  • Safety goggles should always be worn when working with chemicals.
  • Each student should be given their own clean straw and if the straw touches the lab bench, students should ask for a new clean straw. Warn them ahead of time not to rest their straws on the lab bench.
  • 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 have done this activity a number of ways, but this approach was particularly helpful with students who had almost no background knowledge of chemistry.
  • This lab is effective in getting students to access their prior knowledge, formulate a hypothesis, record observations, revise their hypothesis, and support a claim using evidence. In addition, all students were surprised and engaged with the activity.

For the Student



There are lots of chemical reactions going on all around us every day. Do chemical reactions only happen in one direction? Or can we “undo” (reverse) a chemical reaction?

Prelab Questions

Is it possible to “reverse” any of these processes? Circle the process(es) that can be reversed, and cross out the process(es) that cannot be reversed.


1. Fill a clear plastic cup half full of tap water. Add a full dropper of BTB solution to the water.

● Initial color of the solution

2. Use a brand new clean straw (that has not touched a lab table) to gently blow air into the solution. Blow until you notice a change in color and then stop.

● Now what is the color of the solution?

3. Add a SMALL amount of baking soda to the solution and stir with your straw. Add just a small amount until you see a change.

● Now what is the color of the solution?

4. Blow into the solution until the color changes again.

● Now what is the color of the solution?

5. Repeat steps 3 and 4. Stop when your solution is BLUE, and go to step 6.

● What colors do you see? List all of them:

6. To your blue solution, add a few drops of vinegar until the color changes.

● What is the final color of your solution?


  1. This experiment showed that chemical reactions CAN or CANNOT (circle one) be reversible.
  2. What evidence have you gathered that supports this? What did you observe?
  3. Use this format to explain: _____ caused ______, while _____ caused _______.
    How does your evidence support your whether or not a reaction can be reversible?


  1. Do you think all chemical reactions are reversible? Why or why not?
  2. What kinds of biology processes do you think might be reversible? List them and describe each process briefly.