Invisible copper

Reveal a copper sulfate message using ammonia!

10 minutes
Experiment's video preview



  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

My secret message is not revealing. What to do?

Put the marker again into copper sulfate solution and simply wait a little bit more: probably, it hasn’t yet absorbed the solution. Proceed to the next step.

Step-by-step instructions

We will use an empty marker to write a secret message with copper sulfate CuSO4.


Write your secret message.


Apply the solution of ammonia carbonate (NH4)2CO3 to the paper with the secret message. Copper ions interact with (NH4)2CO3, forming an intensely-colored substance.


Expected result

Ammonia gets inside paper and reacts with copper ions Cu2+. The formed substance has blue-lilac color and we can read the secret message!


Dispose of solid waste together with household garbage.

Scientific description

Why does the secret message appear?

The solution of ammonium carbonate (NH4)2CO3 decomposes into carbon dioxide CO2 and a gas with a rather unpleasant odor – ammonia NH3 gas. Ammonia molecules (as with other gases) are able to permeate the piece of paper with the secret message. While penetrating through, these molecules bind with the copper ions already in the paper. As a result of this chemical reaction, the light-blue writing turns blue-lilac.

How is ammonia produced?

Ammonium carbonate (NH4)2CO3 decomposes, producing ammonia NH3 and carbon dioxide CO2:

(NH4)2CO3 → 2NH3gas↑ + CO2gas↑ + H2O

In addition to being water-soluble, both ammonia and carbon dioxide are highly volatile and therefore vaporize easily. You most likely noticed the distinctly unpleasant ammonia odor. Carbon dioxide will not react with paper or ink, but ammonia will bind to ink’s copper ions Cu2+. The change in color indicates that a chemical reaction has taken place.

Follow up

What else can be done with copper sulfate? Do not skip the chance of processing it with tannin (see “Follow-up” on the “Iron gall ink” experiment).

That's interesting!

Beauty and benefits of copper sulfate

Because copper sulfate CuSO4 is so useful, this substance is produced on a large scale. In particular, anhydrous copper sulfate is an effective dehumidifying agent. Moreover, its aqueous solution is utilized as an antiseptic (poisonous to most living organisms), as a pesticide to fight garden pests, and as a source of copper to be deposited on various metallic surfaces.

For us beginner experimentalists, copper sulfate can serve as an unusual decoration. If we leave an aqueous solution of copper sulfate in an open vessel, the water will gradually evaporate, and the sulfate will precipitate to form beautiful blue crystals. Unfortunately, since these crystals contain water, they dry out and wear away with time. Coating them with a substance like nail polish can help keep them from aging.

Cobalt chloride: between pink and blue

Another substance that can serve as a base for invisible inks is cobalt chloride CoCl2. Its aqueous solution is slightly pink, and is almost unnoticeable on white paper (and is totally invisible on a pink background!). However, when heated, this light-pink coloring turns blue, and any secret message is revealed. The naturally light-pink cobalt chloride CoCl2 turns blue when its Co2+ cations are deprived of water molecules. Interestingly, if such an inscription is exposed to humid air, it will disappear again as water is returned to the cobalt cations Co2+. In general, cobalt compounds containing a Co2+ cation easily fluctuate between pink and blue, even with just a slight outside influence.