DEMONSTRATION in Indicators. Last updated April 26, 2019.
In this lesson, students will observe a hidden message while understanding simple acid/base chemistry and indicators.
High, middle, or elementary school
AP Chemistry Curriculum Framework
- Big Idea 3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons.
- 3.10 The student is able to evaluate the classification of a process as a physical change, chemical change, or ambiguous change based on both macroscopic observations and the distinction between rearrangement of covalent interactions and non-covalent interactions.
- Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
- 6.3 The student can connect kinetics to equilibrium by using reasoning about equilibrium, such as Le Chatelier’s principle, to infer the relative rates of the forward and reverse reactions.
- 6.8 The student is able to use Le Chatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
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.
This lesson supports students’ understanding of
- Acid/base reaction
Teacher Preparation: 5 minutes
Lesson: 10 minutes
- 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
- Even though household items are used, proper safety precautions should be observed. Wear goggles, wash hands when finished, dispose of materials properly.
- 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.