Learn about acid/base reactions as sodium hydroxide reacts with gas in the air to produce a solution that appears to disappear

Much like a spy, the chemists work is never done, and with this disappearing ink practical young learners can be both.

This experiment should take 30 minutes. 



  • Eye protection
  • Beaker, 100 cm3
  • Measuring cylinder, 10 cm3
  • Small paint brush to test the ink


  • Ethanol
  • Sodium hydroxide 0.4 mol dm–3
  • Thymolphthalein solution (50 per cent water, 50 per cent ethanol)

Health, safety and technical notes


CCE18_Disappearing ink_image1

  1. Place 10 cm3 of ethanol in a small beaker.
  2. Add a few drops of thymolphthalein indicator solution.
  3. Add just enough NaOH solution, dropwise, to produce a deep blue colour in the solution.
  4. Using a small paint brush, test the ‘disappearing ink’ on a white page.


  • This ink is the same as those sold in trick and joke shops.
  • The amount of indicator can be adjusted to give a deep blue colour.
  • The compound produced, Na2CO3, is commonly called washing soda.
  • Sodium hydroxide reacts with carbon dioxide in the air to form sodium carbonate.
  • 2NaOH(aq) + CO2(g) → Na2CO3(aq) +H2O(l)
  • Sodium carbonate is less basic than sodium hydroxide and causes the indicator to change from blue to colourless.
  • The colourless range for thymolphthalein is below pH 9.3.
  • The blue range is above pH 10.5 and the colour change takes place between these two.
  • The alcohol evaporates and leaves a clear and colourless residue.


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.


  1. Carbon dioxide
  2. Sodium hydroxide + carbon dioxide → sodium carbonate + water
  3. 2NaOH + CO2 → Na2CO3 + H2O