Invisible Ink Demonstration - Experiment 23

This demonstration shows that phenolphthalein is a chemical that displays different colors depending on either the acidity and the basicity of the environment.

Materials Substitutions
phenolphthalein solution
cotton swab artist’s paint brush
white typing paper paper towel, roll
100 mL beaker glass or plastic cup
ammonia windex spray with ammonia
acetic acid vinegar


  1. Before performing your demonstration, roll out one sheet of paper towel. Dip the swab in the phenolphthalein solution and use it to write a message or draw a picture on the paper towel. Prepare two additional sheets in the same manner. Let them dry in the air. Roll up the paper towel.
  2. In front of the audience, roll out the paper towel and spray it with Windex (with ammonia). A message appears in pink ink.
  3. Spray the second paper towel with Windex (to which you have added acetic acid). Nothing will happen. Spray the third paper towel with Windex (with ammonia) and it works again. Ask the students to explain what happened!

Teacher’s Notes
Phenolphthalein is an indicator that is colorless in the presence of an acid. It will turn bright pink in the presence of a base, like ammonia.

The same “secret message” sheets may be used repeatedly, if multiple performances are required. I usually tape the tops and bottom of the sheets to a wall or surface that Windex will not harm.

You may wish to spray the sheets with the pink message with CO2 gas.
This gas may be obtained by capturing the gas as dry ice sublimes. Alternatively, you may make CO2 gas by pouring some acetic acid onto sodium hydrogen carbonate that is contained in a plastic soda bottle.

Paper towels should be disposed of in a solid waste container. Solutions of acetic acid and ammonia can be stored in suitable containers in the stockroom.