Lemon juice is used to write a hidden message that becomes visible when heated with an iron.

1. Cut a lemon in half and squeeze the juice into a small bowl.
2. Dip a thin paintbrush into the juice and write a message on paper.
3. Leave the paper to dry for 15–20 minutes without touching or covering it.
4. Place the paper on an ironing board.
5. With the iron on a high heat, have an adult carefully iron the paper for 15–30 seconds until the message appears.
6. Allow the paper to cool for one minute before handling.

Write Secret Messages With Invisible Ink! | STEM Activity - Science Buddies:

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📄 Invisible ink recipe - The National Archives: https://www.nationalarchives.gov.uk/education/families/celebrating-british-science-week/how-to-make-invisible-ink/invisible-ink-recipe/

• Use orange juice instead of lemon juice, as in the Gunpowder Plot of 1605.
• Try other fruit juices (lime, grapefruit) to compare how well they work.
• Test different heating methods, such as holding the paper near a light bulb (with caution).

• An adult should cut the lemon.
• An adult must handle the iron to prevent burns.
• Allow paper to cool before touching.
• Do not place flammable objects near the iron.

• Why does the lemon juice message appear only after heating? (The carbon compounds break down and react with oxygen, turning the writing darker.)
• Would the message appear if you used water instead of lemon juice? (No, because water lacks the organic carbon compounds needed to change color when heated.)
• Why does citrus juice work particularly well for invisible ink? (It contains organic acids and carbon compounds that oxidize and darken when heated.)
• How might people in history have used invisible ink for secret communication?