Welcome to Five Minute Science, where we offer up cool science experiments you and your kids can do together using everyday household items, in about five minutes!

In this experiment, you get to write secret messages with invisible ink, and learn some science from the experts at Let's Talk Science.

Purpose:

To show what happens when you apply heat to an acidic substance.

Be sure to ask your child what he or she thinks might happen before you do the experiment—what will happen when heat is applied to the invisible message on paper?

The Experiment

Materials:
  1. One half of a lemon or bottled lemon juice.
  2. Small bowl
  3. Cotton swabs
  4. White paper
  5. A heat source like an iron, a 100-watt incandescent light bulb, or a radiator. **PARENTAL SUPERVISION IS ESSENTIAL
  6. A pen or pencil to write a decoy message (optional).
Process:
  1. First, let your child decide what the secret message should be. If your child wants it to be super-secret, he or she can write a decoy message or draw a picture with pencil that will disguise the secret message.
  2. Pour 60ml (1/4 cup) of bottled lemon juice into the bowl or squeeze ½ of a lemon into the bowl.
  3. Dip the cotton swab into the lemon juice. You or your child can use the cotton swab to write the secret message on the paper.
  4. Let the paper dry completely.
  5. Using an iron on an ironing board, or another heat source, heat the paper for your child (children should not handle a hot iron). Do not let the paper get so hot that it burns.
  6. Watch what happens to the "invisible" ink?
Explaining the Science:
Did you see the invisible ink change colour and become visible? Why do you think that happened? Here's the scientific explanation. Lemons are naturally high in acid. When the acidic lemon juice is applied to the paper, it is absorbed into the paper’s fibres. When the paper is heated, the heat breaks down the chemical bonds and frees carbon-based compounds. The carbon-based compounds go through a process called oxidization and turn brown, and that is how you are now able to see the lemon juice ink.
 
Curriculum Connection:

This experiment is simple, safe and appropriate for any age. In the Ontario Science and Technology curriculum, Properties of and Changes in Matter is taught in Grade 5 where "“students will explore changes of state and investigate the difference between physical changes (which are reversible) and chemical changes (which are not reversible). Concepts learned in this strand about the use of heat to effect changes in matter will be relevant to the study of energy conservation in the next strand."

Extra Credit!
  • Try the experiment again using milk, or apple juice instead of lemon juice. What happens?
  • You do not need a heat source to create invisible ink. You can “develop” your ink in other ways. Try mixing 60ml of baking soda with 60ml of water in a small bowl. Dip your cotton swab into the solution and write a message. Let it dry. Paint over it with grape juice. What happens?
  • Play TVOKids Dr. Ben Senburner's Lab (click on Dr. Ben Senburner's Lab and then on "secret message") to write more secret messages.