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DEMONSTRATION in Indicators. Last updated August 17, 2019.


Summary

In this lesson, students will observe a hidden message while understanding simple acid/base chemistry and indicators.

Grade Level

High, middle, or elementary school

AP Chemistry Curriculum Framework

This demonstration supports the following units, topics, and learning objectives:

  • Unit 4: Chemical Reactions
    • Topic 4.4: Physical and Chemical Changes
      • TRA-1.D: Explain the relationship between the macroscopic characteristics and bond interactions for: a. Chemical processes. b. Physical processes.
  • Unit 8: Acids and Bases
    • Topic 8.3: Weak Acid and Base Equilibria
      • SAP-9.C: Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak acid or weak base.
    • Topic 8.7: pH and pKa
      • SAP-10.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

Objectives

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

  • Understand that a chemical change is taking place.
  • Recognize that an indicator causes the color change.
  • Realize that an acid base reaction is taking place.

Chemistry Topics

This lesson supports students’ understanding of

  • Indicators
  • Acid/base reaction
  • Neutralization

Time

Teacher Preparation: 5 minutes

Lesson: 10 minutes

Materials

  • Goldenrod paper (not all kinds work, so test out your paper before you do this demo)
  • Cleaning supply with ammonia listed as an ingredient in a spray bottle
  • Vinegar in a spray bottle
  • Yellow crayon

Safety

  • Even though household items are used, proper safety precautions should be observed. Wear goggles, wash hands when finished, dispose of materials properly.

Teacher Notes

  • https://www.youtube.com/watch?v=vplONCxeqtE
  • Not all goldenrod paper works, so test yours before you do this demo.
  • Write a message in yellow crayon on the goldenrod paper. Spray the paper with the ammonia cleaning solution to unveil what you’ve written. Spray the paper with vinegar to make the message disappear.
  • The goldenrod paper has a chemical in the dye that interacts with base and turns red. By spraying the cleaning solution, you are exposing the paper to a base, which causes the indicator color to change to red. The wax from the crayon prevents the solution from interacting with the paper, and so that part doesn’t change color. By spraying vinegar over the cleaning solution, the acid reacts with the base, and the solution is now neutral or acidic, and the indicator color loses its red color.