Fun Science Projects for Kids

Top Secret Invisible Ink

Lemon juice plus a light bulb equals invisible ink!
Lemon juice plus a light bulb equals invisible ink!
©2007 Publications International, Ltd.

It's easy to send secret messages to your friends when you write them in top secret invisible ink. The "secret" is in the combination of lemon juice and heat from a light bulb or an iron. The heat causes a chemical change in the lemon juice and makes it appear darker on paper.

It can be a little tricky to write with the "ink" -- it's invisible, after all -- but once you get the hang of it, it's a fun way to share secrets with your friends. Make sure an adult is nearby when you get to the part of the project that uses heat, but as for the secret messages -- those are all yours. Here's how to get started:

What You'll Need:

  • Toothpick
  • Lemon juice
  • Paper
  • Heat source, such as a light bulb or iron

Step 1: Dip the round side of a toothpick into lemon juice, and write a secret message with it on a piece of paper. Use lots of lemon juice for each letter you write.

Step 2: Allow the paper to dry until you can't see the writing any more.

Step 3: Now move the paper back and forth over a heat source. As the ink gets warm, your message is revealed.

What Happened?

The acid in the lemon juice breaks down the cellulose of the paper into sugars. The heat supplied tends to caramelize the sugars, making them brown and revealing the secret writing. Repeat this activity with vinegar or milk to find out which makes the best invisible ink.

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