Invisible Ink


To demonstrate how the acidic properties of lemons can be used to make lemon juice act as an invisible ink.

Additional information

Lemon's are primarily used for culinary purposes and are cultivated from wild plants. Their high citric acid level, which is about 5% of it's dry weight, gives lemons a tart taste and also makes them prime candidates for use in various educational science experiments. Beyond it's use in fun and exciting science projects, citric acid has many other applied uses including biotechnology and cleaning solutions.

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Required materials

  • Fresh whole lemons or concentrated lemon juice (NOT lemonade)
  • Small bowl
  • White paper
  • Small detail paintbrush with fine bristles
  • Light bulb, sunshine, or other heat source

Estimated Experiment Time

About 10 to 20 minutes

Step-By-Step Procedure

  • 1. If you're using whole lemons, cut the lemons in half and squeeze their juice into your small bowl. If you're using concentrated lemon juice, pour a small portion into your bowl.
  • 2. Place your brush into the bowl so the bristles are covered with lemon juice (your "ink").
  • 3. Write or draw something on your white paper with the "ink" from the paintbrush. You may need to re-coat your paintbrush several times to complete your message or picture.
  • 4. Allow several minutes for the paper to dry. Once dry, you'll have what looks like a blank sheet of paper.
  • 5. When you're ready to reveal your hidden message or picture, hold your paper up to a heat source (for example, an exposed light bulb from a lamp that gives off heat).
  • 6. After a short time being exposed to the heat, your message or picture will beginning to take form as it darkens to a pale brown color, revealing what it is!


Be VERY careful when holding your paper up to a heat source or light bulb! The heat emanating from the heat source may be strong enough to ignite the paper. Make sure you have an adult around to help supervise the revealing of the invisible message / picture to ensure safety.


Can you think of another way you could get the invisible ink to reveal the message or picture without using a heat source? Here's a hint... try using salt and wax crayons! How about using another liquid as your invisible ink? Can you think of any others that would make worthy substitutes? What would happen if you used a different style brush? What if you didn't have a brush, can you think of an alternative to paint your picture or write your message?


Lemons are naturally high in acid. When the acidic lemon juice is applied to the paper, it weakens the paper material. When the paper is heated, the remaining acid on paper turns brown, revealing the hidden message or picture.

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