Summary
Introduction
Have you ever wanted to send your friend a secret message that no-one else can read? Then you might know of invisible ink—a special type of ink that you can use for writing and that does not show up on paper. Only after a special treatment will it appear again magically, and the message can be read. How does this work? Find out in this activity and write your own secret messages!
Materials
Tools:
- Cardstock
- Measuring spoons
- 3 cups or small bowls
- Spoon
- Q-tips or a brush
- Old newspaper or cardboard
- Paper Towels
- Water
- Lemon or lemon juice
- Iron
- Baking soda
- Water
- Turmeric and rubbing alcohol OR concentrated grape or blueberry juice. Note: Instead of rubbing alcohol another high percentage alcohol (40 percent or more) can be used. Water does work as well but the results won't be as intense.
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Prep Work
- Squeeze the juice of half a lemon into a cup or small bowl.
- Add half a teaspoon of water to the lemon juice and mix it with a spoon.
- Mix 1 tablespoon of baking soda with one tablespoon of water in a second cup or bowl.
- Mix 1/2 teaspoon of turmeric powder with 3 tablespoons rubbing alcohol in a third cup or bowl. Note: Instead of rubbing alcohol, you can also use another high percentage alcohol (40 percent or more). Water is an option as well but the results won't be as intense. Note that the turmeric will only fully dissolve in the rubbing alcohol. In other solutions you will still see some undissolved turmeric. However, this does not affect your results.
Instructions
- Dip a Q-tip into the lemon juice-water solution.
- Use the damp Q-tip to draw a letter, symbol, or write a secret message on the white cardstock paper. Re-wet your Q-tip if necessary. Be careful to not soak the paper too much.
How does the paper look after you have written on it? Can you see your message?
- Let the paper dry completely.
Can you still see the message when the paper has dried completely?
- With the help of an adult, plug in the iron and put in on high setting. Carefully run the hot iron over the paper several times.
What happens when you apply heat to the paper? Why does this happen?
- Dip a Q-tip into the baking soda solution from your second cup.
- Use the damp Q-tip to draw a letter, symbol, or write a secret message on the white cardstock paper. Re-wet your Q-tip if necessary. Be careful to not soak the paper too much.
How does the paper look after you have written on it? Can you see your message?
- Let the paper dry completely. If you see baking soda residues on the paper, remove it with a paper towel.
How does the paper look when it has completely dried?
- Dip a Q-tip or brush into the prepared turmeric solution. Paint the whole paper with the turmeric solution being sure to cover the secret letters you wrote with your baking soda Q-tip.
What do you notice? Can you explain your results?
Cleanup
- Discard of all your solutions in the sink. Let the paper dry and dispose it in the recycling bin. Wash your hands and clean your work surface.
What Happened?
Were you able to reveal your secret message with both invisible ink methods? You should have been! Applying heat to the paper with lemon juice should have made your secret message visible. Lemon juice is a relatively strong organic acid as it contains citric acid. When you write on the paper with lemon juice, the acid weakens the fibers within the paper, and it starts to decompose. At the same time, the carbohydrates from the lemon juice—like the citric acid—are absorbed into the paper. Carbohydrates don't like heat and therefore when you use the hot iron on the paper with your secret message, the carbohydrates start to carbonize. This process releases carbon which oxidizes when it comes in contact with air. This oxidation reaction yields brown substances which make your secret message visible.
The baking soda on the other hand is alkaline or basic. It gets absorbed by the paper similarly to the lemon juice and once dried isn't visible anymore (although you might have noticed some baking soda powder residues on the paper, which you can wipe off easily). This most likely changed when you painted the paper with the turmeric solution as turmeric changes color depending on if it is in an acidic or alkaline environment. When it comes in contact with an alkaline or basic substance such as the baking soda on the paper, it turns from yellow to a deep red. Your secret message gets revealed! Concentrated grape juice should have also made your message visible as the grape juice contains acids that reacted with the alkaline baking soda absorbed by the paper. A similar reaction is possible with other acidic solutions such as blueberry juice or hibiscus tea.
Digging Deeper
Invisible ink is an ancient invention and has already been used over 2,000 years ago. Its purposes were manifold, ranging from plotting conspiracies or espionage to writing secret love letters. Since then, many different recipes for secret ink have been developed. All of the recipes are based on chemical reactions that make one of the components inside the ink visible. The type of chemical reaction can vary, but they all result in a colored end product that makes the ink visible.
There are several types of reactions that can be used to expose invisible ink including acid-base, oxidation-reduction, and heat reactions. Acid-base reactions occur when an acidic or alkaline component of the ink can be made visible by a special chemical (indicator) that changes color depending on if it is in an acidic or alkaline environment. Similarly, oxidation-reduction reactions can be used, in which a chemical compound changes color depending on its redox state. You can also make use of the fact that some chemical compounds are sensitive to heat or light and change color once they are exposed to these conditions.
Depending on the type of chemical reaction the invisible ink is based on, making the ink visible can take different forms such as adding heat, applying a certain chemical or using ultraviolet light. All these special treatments result in the secret message being revealed.
Ask an Expert
For Further Exploration
- Try to use different acids to write your secret message. Do lime juice, orange juice, milk or vinegar work as well?
- Can you find other ways to make your baking soda message visible? Try other acids such as blueberry juice, hibiscus tea, or simply vinegar with a couple drops of food coloring. Do you get the same results?
Related Resources
Activities
Links
- Art of Manliness: Man Knowledge: The History of Invisible Ink
- Wired: Researchers Create Invisible Ink From Engineered Bacteria
- Scientific American: Invisible Ink Reveals Cool Chemistry
- Compound Interest: The Chemistry of Turmeric - Fluorescence, Indicator, and Health Effects
