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.
- Joginder KukrejaVan R. FosterDana L. Conover. (2012) High-Strength Gluestick Formulation with Color Indicator (U.S. Patent No. 13/587,264). U.S. Patent and Trademark Office.
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