Disappearing Ink

DESCRIPTION:
This demo uses acid base chemistry and an indicator to make an ink that will appear or disappear depending on the solution added to it. Phenolphthalein, which is clear, reacts with a base, and then turns pink.

 

 

 

 

 

 

TOPICS COVERED:
acids and bases
indicators
color change
chemical change

MATERIALS NEEDED:
– 0.5 g of phenolphthalein
– 10 mL of ethyl alcohol
– 30 mL of water
– NaOH (aq) concentration doesn’t matter
– T-shirt (paper towels can be used but they don’t work nearly as well)
– ammonia
– vinegar

PROCEDURE:
1. Dissolve the phenolphthalein in the alcohol
2. Add water
3. Add NaOH until the solution becomes dark red
4. Use the solution to write a message on the shirt, it will disappear
5. The message can be uncovered by squirting some ammonia on it, and it can be hidden by adding vinegar

ADDITIONAL COMMENTS:
This demo could be used in a CSI themed demo day.

SAFETY:
Safety goggles should be worn at all times.

REFERENCES:
Shakhashiri, B.Z. Chemical Demonstrations;  University of Wisconsin Press: Madison, 1989; Vol. 3, pp 47 – 49.  

Handout