Elmer’s Disappearing Purple Glue is a standard school supply in most K-12 classrooms. Many students are familiar with its bright purple color that disappears as it dries on their school projects.
How does it work?
The purple color comes from a dye called o-Cresolphthalein (Figure 1), an acid-base indicator closely related to the familiar phenolphthalein. o-Cresolphthalein is colorless below pH 8.2 and purple at a pH higher than 9.8. The indicator is purple in the glue stick due to the liquid water used in the glue formulation. When the water evaporates, the o-Cresolphathalein turns colorless (see Video 1 below).1
Video 1: How Does Elmer's Disappearing Glue Work? The Science Classroom YouTube Channel, August 23, 2022.
Figure 1: o-Cresolphthalein
The activity described provides an interactive introduction to acid-base indicators. Students use paint brushes dipped in vinegar to write a secret message on a notecard, then messages are decoded by applying Elmer’s Disappearing Purple Glue. Since vinegar has a pH of around 2-3, the o-Cresolphthalein will appear colorless only where the vinegar is spread while remaining purple on the rest of the paper surface see Video 2 below.
Video 2: Writing Secret Messages with Elmer's Glue, The Science Classroom, August 30, 2022.
This activity can be completed in about 15 minutes.
Elmer’s Disappearing Glue Stick
Small amount of vinegar poured into a 100-mL beaker
Blow dryer (optional)
- Dip the paintbrush in the vinegar and then write a message on the notecard.
- Allow the vinegar to dry by waving the note card or blowing on it. (A blow dryer gives the best result because the words appear a lot sharper when completely dried).
- Apply glue to cover the notecard with an Elmer’s Disappearing Glue Stick and reveal the secret message.
Gather materials and pour a small amount of vinegar in a 100-mL beaker.
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies