Carbon dioxide in the air reacts with water to form carbonic acid:
CO2 + H2O -> H2CO3
The neutralization reaction is sodium hydroxide + carbonic acid -> sodium carbonate + water:
2 Na(OH) + H2CO3 -> Na2CO3 + 2 H2O
Let's make disappearing ink...
How to Make Disappearing Ink
By Anne Marie Helmenstine, Ph.D., About.com
When the ink is sprayed onto a porous material the water in the ink reacts with carbon dioxide in the air to form carbonic acid. The carbonic acid then reacts with the sodium hydroxide in a neutralization reaction to form sodium carbonate. Neutralizaton of the base causes a color change of the indicator and the stain disappears:
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