Decode This! Outta-Sight Ink

See more activities in: Fourth Grade, Physical Science

Secret messages with little more than lemon juice? That's right! With this activity, make invisible ink appear, while you reveal to your child some chemistry so cool it should be confidential.

What You Need:

  • Half a lemon
  • Small bowl
  • Cotton Swab
  • Paper
  • Lamp with an incandescent (heat-producing) light bulb

What You Do:

  1. Squeeze the lemon juice into the bowl.
  2. Add enough water to the lemon juice so that it's nearly colorless.
  3. Dip the cotton swab into the solution and write on the paper.
  4. Allow the paper to dry, and the hold it a few inches from the lamp and turn on the light. (Don't hold it too close to the bulb.) In a few seconds your writing will appear on the paper!

What's Going On?

Lemon juice contains citric acid. Heat encourages the acid to oxidize, which is a chemical reaction between oxygen in the air and a substance. (Oxidation is what happens when you cut an apple and leave it sitting out: it turns brown because the citric acid in the apple oxidizes.) So when the lemon juice ink is heated by the lamp, the lemon juice turns brown. The writing appears!