Disappearing ink

Barzu George, Hirtan Constantin, Onofrei Octavian, Pamant Alexandru, Popa Ciprian, Sandu Andrei
“Vasile Alecsandri” High School, Galati, Romania

Introduction
A blue liquid is made. This liquid is tested on a white page, it leaves a blue ink spot. In a few seconds, the blue spot disappears.

What to do
1. Place 10 cm3 of ethanol (Highly flammable) in a small beaker.

2. Add a few drops of thymolphthalein (Highly flammable) indicator solution.
3. Add just enough NaOH solution (Irritant), dropwise, to produce a deep blue colour in the solution.
4. Using a small paint brush test the ‘disappearing ink’ on a white page.

Safety
Wear eye protection. Care with teat pipettes that contain sodium hydroxide.

Questions
The colour change occurs because sodium hydroxide reacts with a gas in the air.
1. Which gas in the air causes this colour change?
2. Write a word equation for the reaction.
3. Write a formula equation for the reaction.