Chemical indicators

Chemical indicators change color in the presence of acidic or a basic substances.

Two common indicators used to make disappearing ink are phenolphthalein and thymolphthalein.


Indicator pH change Color in acid  Color in base
Phenolphthalein 8.3- 9.8  Colorless  Pink - red
Thymolphthalein 9.3-10.5  Colorless  Blue



1% phenolphthalein indicator or 1% thymolphthalein indicator solutions.

To color each indicator solution add drop wise a solution of 0.1 M NaOH until the indicator changes from to red or blue.

Toothpicks and cotton buds to write messages


Spray mist bottle of 0.01M NaOH solution for developing. Made by dissolving 0.2 g of NaOH in 500mL of water

Caution: Sodium hydroxide is caustic. Avoid contact with the eyes and the skin. Wash with water on contact.



Use the cotton buds or toothpicks to write your message on paper using one of the indicator solutions.

Breath on the paper to help your message disappear.

Use the spray bottle to make your message reappear.



1. What is an indicator?

An indicator is a chemical that changes color in the presence of an acid or a base.

2. Why does the 'magic' message dissappear when exposed to the air or by breathing on it?

Air and your breath contain carbon dioxide which is weakly acidic. The carbon dioxide reacts with the sodium hydroxide in the indicator and lowers the pH. The  indicator changes to colorless when the pH is lowered.

3. Write a balanced chemical equation for the reaction of sodium hydroxide and carbon dioxide

Sodium hydroxide + carbon dioxide → sodium carbonate + water

2NaOH + CO2 → Na2CO3 + H2O