Invisible Ink Science: Secret Messages with Baking Soda and Grape Juice
By Ahmed, founder
Secret messages are irresistible. Here’s an invisible ink made from baking soda, and a reveal made from grape juice or turmeric, that actually teaches some chemistry.
What you need
- Baking soda
- Water
- Purple grape juice (Concord type) or ground turmeric mixed with a little water
- Cotton swabs or a small paintbrush for writing
- A wide paintbrush, pastry brush or cotton balls for revealing
- White paper (thicker paper works better)
The method
- Mix equal parts baking soda and water in a cup, stirring until dissolved.
- Dip a cotton swab in the mixture and write a message on the paper. Let it dry completely. It will be nearly invisible.
- Brush grape juice over the whole page (or turmeric water, if you’re using that).
- The message appears. With grape juice, writing turns blue-green against a purple background. With turmeric, it turns reddish-orange against yellow.
Control: water only
Write a second message using plain water, and reveal it the same way. Nothing appears, which proves it was the baking soda, not the paper or the wetness, that made the message show up. That’s a fair test with a control.
The science
Grape juice and turmeric are indicators: they change colour depending on whether a liquid is acidic or basic. Baking soda is a base. Grape juice is slightly acidic and appears purple. Where the baking soda is, the juice turns blue-green. Turmeric is yellow in neutral or acidic conditions and turns red-orange in a base.
It’s the same idea as the red cabbage indicator, which uses a different pigment from plants. See our lesson Can red cabbage tell acid from base?
Why does it need to dry first?
The baking soda stays on the paper as a thin film after the water evaporates, and it’s invisible against white paper. If it’s still wet, the writing smudges when you brush on the reveal.
Ideas to extend
- Try lemon juice as an invisible ink, which is a different mechanism: the acid weakens paper fibres, so it browns faster when gently heated (grown-up only, with a warm lamp bulb or iron kept away from children).
- Test other liquids as reveals: red cabbage juice, tea.
- Compare which paper holds the message best.
- Write a coded message, and swap with a friend.
Questions kids ask
Is baking soda an acid? No. It’s a base, so it’s the opposite.
Why does the colour change? The pigment molecules change shape in a base, and the new shape absorbs light differently.
Can we use markers instead? Marker inks have their own chemistry. That’s another experiment.
Safety
Keep turmeric off clothes, since it stains. Don’t heat paper near flames. Nobody should eat or drink the experiment materials.
More kitchen science
This experiment is How do spies write secret messages? in Kitchen Chemistry for Families, along with a volcano that explains where the fizz comes from, a lava lamp and oobleck. See also our parents page, and sign up for three lessons free.