Invisible ink from copper sulfate and ammonia

Copper sulfate solution is used as an invisible ink.

Scientific name: NH3 replaces water molecules from the Cu coordination shell with formation of a blue well defined complex.


Invisible copper - Reveal a copper sulfate message with ammonia

by MEL Science

Невидимые чернила - химические опыты



Wear eye protection goggles and gloves. Perform this experiment in a well ventilated room.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Perform this experiment

Reaction formula

CuSO4 +6H2O → Cu(H2O)62+ +SO42- (light blue)

(Cu(H2O)6)2+ + 4NH3 →(Cu(H2O)2(NH3)4)2+ + 4H2O (blue)

Step-by-step instruction

  1. Wet a cotton swab or paint brush in copper sulfate solution.
  2. Write a message on a paper sheet.
  3. Let the paper dry. The written text disappears.
  4. Pour ammonia solution into a glass.
  5. Cover the glass with the sheet of paper so that the side with the message is on top.
  6. The text becomes visible.

Scientific background

Cooper sulfate dissociates in the water solution and gives hydrated Cu+2 ions (Cu(H2O)2+) and SO42- ions:

CuSO4 +6H2O → Cu(H2O)62+ +SO42- (light blue)

When it comes into contact with ammonia vapor, NH3 replaces water molecules from the Cu coordination shell. The formation of a bright blue well defined complex is observed.

(Cu(H2O)6)2+ + 4NH3 → (Cu(H2O)2(NH3)4)2+ + 4H2O (blue)


Published on 23 March 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst