Disappearing ink

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. 

Equipment

Apparatus

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

Chemicals

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

Health, safety and technical notes

• Read our standard health and safety guidance.
• Always wear eye protection.
• Ethanol is highly flammable, see CLEAPSS Hazcard HC040a.
• Sodium hydroxide is an irritant, see CLEAPSS Hazcard HC091a.
• Thymolphthalein soluiton is flammable, see CLEAPSS Hazcard HC032.

Procedure

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.

Notes

• 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.

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.

Answers

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