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Secret Message
Indicators – Acids and Bases
Activity: This demo shows indicators react in the presence of acids or bases. In this case,
phenolphthalein is used to reveal “secret messages” that have been written using a basic
phenolphthalein solution
small paint brush
paper towels (still on a roll)
small petri dish or beaker
bleach (or ammonia)
spray bottle
Before Demo:
1. Pour a small amount of phenolphthalein solution into a petri dish or small beaker to access
easier with a paint brush.
2. Roll out paper towel roll and use a paint brush to write a message (or messages) on each sheet
using the phenolphthalein. NOTE: Mark the bottom of each paper towel, so you know which
direction to hang it once it dries.
3. Add a couple of drops of bleach to a spray bottle and add about 150 mL of water.
4. Explain you’ve made a solution that reveals secret messages.
5. Tell students you will need to spray your secret message decoder onto a paper towel so they
can see the secret message. Unroll the paper towel roll and tear off a sheet. Tape it up on the
wall or white board.
6. Spray with bleach solution to reveal your secret messages: “U R My Favorite Class” is always
a classic; “We have Chemistry!” is always one of my favorites; “You are made of atoms,
therefore U MATTER” and “I’ve got my ION you!” are always good standbys that get a laugh
(forced maybe, but hey, I’ll take it!)
7. Allow paper to dry so secret message can be used repeatedly for upcoming classes.
©2016 Nitty Gritty Science
What’s Happening:
Phenolphthalein is an indicator that is colorless in the presence of an acid but will change bright
pink in the presence of a base, such as the bleach solution used in the demo. When the indicator
dries, the message “disappears” until it is activated again by a basic solution.
©2016 Nitty Gritty Science
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©2016 Nitty Gritty Science
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