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Invisible Ink

✨ A blank sheet of paper suddenly shows your secret message in a different colour when you brush juice across it.

Three cards show invisible baking soda writing, the same card washed with grape juice so the letters appear blue-green, and a lemon juice message appearing pink, above a colour scale from acid to alkali.
How it works

🧰 You need

  • one spoon of baking soda
  • one spoon of water
  • a cotton bud or thin brush
  • white paper
  • dark grape juice
  • lemon juice

⚠️ Grape juice stains, so cover the table and do not use your best clothes.

👀 What you will see

The dry baking soda writing is almost invisible on white paper. Under the grape juice the letters show up in a clearly different shade from the background. The lemon juice message appears in a different shade again.

🧪 Do it

  1. Stir the baking soda into the water until it dissolves.
  2. Write your message with a cotton bud dipped in the solution.
  3. Let the paper dry completely, until you cannot see the writing at all.
  4. Brush grape juice evenly over the whole sheet.
  5. Read the message as it appears.
  6. Write a second message in lemon juice, dry it, and reveal that one the same way.

💡 Why it happens

Grape juice contains a natural purple dye called anthocyanin that changes colour depending on what it touches. A substance that changes colour like this is called an indicator. Baking soda solution is an alkali, so the letters turn a bluish or greenish shade, while lemon juice is an acid and pushes the colour towards pink or red. Chemists use indicators every day to find out whether an unknown liquid is acidic or alkaline.

🔁 Now try this

Paint stripes of six kitchen liquids on a grape-juice sheet and sort them into acids and alkalis from the colours you get.

🤔 Think about it

Why does a purple cabbage change colour when it is cooked with lemon juice?

🧘 Ask the Acharya

“Why does a purple cabbage change colour when it is cooked with lemon juice?” — ask him and he will explain it from your own chapters.

Ask Guru →

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