MYSTERY NAME  (Phenolphthalein)

INTRODUCTION

This activity is appropriate for younger elementary school children in grades K-3 along with topics about "Color Changes" or for intermediate elementary school children in grades 4-6 to introduce acid-base reactions and the use of indicators.   We often use it under the guise of getting to know the names of certain students.
 

PURPOSE

The purpose of this activity is to introduce children to the concept that colors can change through special chemical reactions, in this case an acid-base reaction, which can often occur within an indicator paper.
 

MATERIALS
 

1  piece of filter paper large enough for the class to see easily¹

1  5-dram screw-capped vial or 15-mL dropper bottle of phenolphthalein solution (0.1 g of phenolphthalein in 10 mL of either ethyl alcohol or rubbing alcohol)²

1  tiny artist's brush

1  spray bottle of Windex (with ammonia)

PROCEDURE

1. Ask the class who has a name or nickname with only three or four letters.

2. Pick a child, and tell that child that you will figure out what the name or nickname is.

3. Give the child the piece of filter paper, the phenolphthalein solution, and the artist's brush.

4. Tell the child to paint the name or nickname in large capital letters on the filter paper, wave the paper in the air and/or blow gently on it to let it dry, and then hand the paper back to you right side up.

5. Spray the paper with Windex, and the name or nickname will appear in pink/red whereas the rest of the sheet will remain white.

6. Immediately wash and rinse your hands thoroughly to remove any Windex solution.

EXPLANATION

Phenolphthalein solution is a dye which colorless in its acid form and therefore cannot be seen on the white paper after the solution dries.   However, phenolphthalein is bright pink in its base form.  Phenolphthalein is normally in its colorless acid form because carbon dioxide, which we exhale constantly and which is produced from the burning of fossil fuels, is all around us in our environment and is a weakly acidic substance.  The ammonia in the Windex is a weak base and converts phenolphthalein to its basic pink form.  However, the white filter paper remains unchanged. 
 

ADDITIONAL SUGGESTIONS

See Voice-Activated Reaction for another activity using phenolphthalein as an indicator.

REFERENCES

1. Coffee filter or commercial filter paper large enough for the class to see easily works best, but even white paper towels can be used.

2. 0.1 g of phenolphthalein in 10 mL of either ethyl alcohol or rubbing alcohol.  Obtain as the solution or as a solid from a local high school or college teacher friend.