Summary

In this demonstration, 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
    • 4.4.A: Explain the relationship between the macroscopic characteristics and bond interactions for: a. Chemical processes. b. Physical processes.
• Unit 8: Acids and Bases
  • Topic 8.7: pH and pKa
    • 8.7.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.

NGSS Alignment

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

• HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
• Science and Engineering Practices: Refine a solution to a complex real-world problem, based on scientific knowledge, student-generated sources of evidence, prioritized criteria, and trade off considerations.
• Crosscutting Concepts:
  • Stability and Change: Much of science deals with constructing explanations of how things change and how they remain stable.

Objectives

By the end of this demonstration, 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 demonstration supports students’ understanding of

• Indicators
• Acid/base reaction
• Neutralization

Time

Teacher Preparation: 5 minutes

Lesson: 10 minutes

Materials

• Roll of paper towels
• Phenolphthalein Indicator solution
• Cleaning supply with ammonia in a spray bottle
• Vinegar in a spray bottle
• Small art-type paint brush
  

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Students should wash their hands thoroughly before leaving the lab.

Teacher Notes

• Phenolphthalein Indicator solution is often used as an indicator in acid/base titrations.
• Phenolphthalein is a weak acid with the formula C₂₀H₁₄O₄. It is often written as “H-In”. In an acidic environment it is colorless. In a basic environment it will donate a proton (H⁺¹) and the resulting anion (In^-1) is pink.
• This demonstration is often used to welcome students and parents to your classroom at the beginning of the school year. It can also be used to introduce the concept of indicators and acid-base titrations.

Instructions

1. Use a small paint brush write a message using the phenolphthalein solution on a paper towel.
2. Allow the paper towel to dry completely.
3. Hang paper towel on the front of your desk or lab table.
4. Spray the paper with the ammonia cleaning solution to unveil what you’ve written.
5. Spray the paper with vinegar to make the message change colors.