Disappearing Ink Demonstration
Recommended Grade Level(s):
Appropriate for: Grades 9–12
Time Requirements:
Prep Time: 10 minutes
Activity Time: 30 minutes
Teaching Topics & Concepts:
• The purpose of this demonstration is to illustrate the effect of pH change on indicator color.
Materials:
• Thymolphthalein
• Ethanol
• Sodium hydroxide Solution, NaOH (0.1 M)
• 100mL Beaker or Bottle
Safety
• Wear safety glasses, gloves and lab coat or apron.
• Sodium hydroxide is caustic; handle with care.
!
Procedure:
1. Dissolve a small amount of Thymolphthalein in 10mL Ethanol.
2. Slowly add 40mL of water while stirring
3. Add aqueous Sodium Hydroxide solution, 0.1 M, dropwise until solution turns dark blue
4. The resulting solution can be used as disappearing ink
Expected Results:
Results may vary based on materials used, setup, procedure, and other factors, however, here are a few examples on what to expect:
• When used as ink to write messages, the ink will disappear over time.
Follow-up Teaching Notes:
• Thymolphthalein is blue above pH 10 and colorless below pH 9.
• The ink solution becomes colorless as it absorbs carbon dioxide from the air
Disposal/Clean-up:
• Paper can be disposed of in the garbage
• Extra ink can be placed in a sealed and properly labeled container for reuse
2 NaOH + CO2 —> Na2CO3 + H2O
Blue Colorless
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