The STEM Hub Canterbury Christ Church University
The STEM Hub



Make Invisible Ink

On This Day

in 1660, the worlds oldest scientific academy, The Royal Society was founded.


Make Invisible Ink

You will Need:

  • Baking soda or lemon juice
  • Paper
  • Water
  • Iron
  • Paintbrush or swab
  • Measuring cup
  • Purple grape juice (optional)

Health and Safety: Be careful is heating up the paper, you don’t want to set it alight! Ensure you have adult supervision and a way of extinguishing flames at hand.

What to Do:

  1. Mix equal parts water and baking soda.
  2. Use a cotton swab, toothpick, or paintbrush to write a message onto white paper, using the baking soda solution as “ink.”
  3. Allow the ink to dry.
  4. With adult help, heat the paper by ironing it. The baking soda will cause the writing in the paper to turn brown.
  5. Another method is to paint over the paper with purple grape juice. The message will appear in a different colour. The grape juice acts as a pH indicator that changes colour when it reacts with the sodium bicarbonate of baking soda, which is a base.

How does it work?

  • Writing a secret message in baking soda solution slightly disrupts the cellulose fibres in paper, damaging the surface.
  • When heat is applied, the shorter, exposed ends of the fibres darken and burn before the undamaged sections of paper.
  • If you apply too much heat, there’s a risk of igniting the paper. For this reason, it’s best to use either the grape juice chemical reaction or else apply a gentle, controllable heat source.

Pass us your hidden message! Send a photo to or tag us on Twitter @STEMHUB_SE

« Back