In January, I wrote about how you can write a secret message with the spice turmeric. This time we’ll use an Elmer’s Disappearing Purple Glue Stick.
Materials:
• Disappearing Purple Glue Stick
• Vinegar (Caution: Vinegar is a weak acid and if it gets in your eyes it will sting)
• Q-tip or Paint Brush
• Sheet of White Paper or Cardstock
Experiment:
Using the Q-tip, dip it in the vinegar and write your message on the paper. Allow it to dry. Wipe over the message with the glue stick. Once the glue dries, the message disappears so you may need to work quickly.
What’s happening?
o-Cresolphthalein is the active ingredient in the glue stick (the little o means “ortho”). This compound is an acid-base indicator. It is a colorless molecule below a pH of 8.2 (acidic) but is purple above a pH of 9.8 (basic). The gluestick itself is slightly basic and thus it is purple when mixed with the o-cresolphthalein.
When the glue touches the acidic vinegar molecules, the o-cresolphthalein acidifies and becomes colorless. The compound needs to be wet in order for the purple color to exist so spraying the message with water will make the image reappear.
Extension:
Can you find other substances around the house that will change the gluestick from purple to colorless (juices, sprite, etc.)?
References:
https://www.youtube.com/watch?v=FtpCUxvbRrk
To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts.
