Citric acid

Iron a secret citric acid message to reveal it

Scientific name: Citric acid destroys when heated. Brown-colored compounds are formed in this reaction. This makes the secret message appear


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Put protective gloves on. Conduct all the experiments on the tray. Take protective gloves off before using iron.

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


  1. Citric acid

Step-by-step instruction

  1. Fill 1/4 of an empty plastic vial with 1M citric acid C6H8O7 solution (about 2 mL).
  2. Dip the brush into the citric acid solution.
  3. Write a secret message on white paper.
  4. Let the ink dry. It will take 3–4 minutes.
  5. Place a paper towel on a heat resistant surface. Put on top the paper with the message facing up. Warning! Take off your gloves before using hot iron.
  6. Iron the paper to reveal the message. It may take several minutes.
  7. Wash the brush with water for use in future experiments.

Scientific background

How the message reveals?

High heat destroys citric acid. As a result of this chemical reaction, brown-colored compounds are produced, which makes the secret message visible. Similar darkening effect may be observed when heating sugar: it melts and darkens, turning into caramel. Recall the “Burning sugar” experiment from the starter package. In the very beginning, as we tried to ignite a regular sugar cube, and it was darkening and melting instead of burning. Which other substances could be used instead of citric acid?

Substances suitable for such an experiment should satisfy the following requirements:

Be water-soluble, so it would be easy to write the secret message with their solution;

Be transparent or slightly colored, so that no one would read the secret message before its development;

Be thermolabile (easily destroyable by heat), coloring the paper and making the message readable.

Hence, we could replace the citric acid solution with milk, sugar syrup or orange juice. Refer to the “Follow up” section to learn how to conduct these experiments.


Published on 17 January 2016

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