Experiment at Home

Invisible Ink

Invisible Ink

Perform this experiment only under adult supervision.

What do you need?


What do you do?

  1. With the help of an adult: cut the lemon in half and squeeze each lemon half into the bowl.
  2. Soak one end of the cotton swab into the lemon juice.
  3. Use that same end to write a secret message on the piece of paper.
  4. Let the lemon juice dry on the paper. Meanwhile, with the help of an adult, preheat the toaster.
  5. With the help of an adult: place the piece of paper on top of the toaster and wait for the message to reveal itself!

What’s the science behind this?

When sugar is heated, it changes color and turns brown (a process called caramelization). So, what’s the connection between sugar, a piece of paper and lemon juice?

The paper is made mostly of cellulose fibers – a material made of chains of small molecules linked together. That chain is called a polymer.

The cellulose polymer is composed of a long chain of glucose molecules. When we use the lemon juice to write on paper, the acidity in the lemon juice breaks down these long chains. Once the glucose is unchained, it is less resistant to heat. So, where the lemon juice is absorbed into the paper, the paper is more sensitive to heat and the sugar turns brown.

Please note: the glucose in the polymer will also change color eventually, but it will need a longer exposure to a heat source.