Kid's Activity: Let's Make Invisible Ink!

by Melissa Jaramilllo and Julie Snyder

Sending secret messages has been happening as long as people could write or draw. Histiaeus, in 500 B.C., wrote a secret message on a slave's head, waited for his hair to grow back, and sent his slave cross enemy lines. When his head was shaved the message appeared! Most methods were a bit simpler though!

Invisible ink has been used to conceal secret messages for a long time. During the Revolutionary War, it wasn't uncommon for a letter to bear an invisible message as well as visible one. One of the techniques for protecting secret information was the use of disappearing inks. These inks are invisible under some conditions and visible under others. It almost seems like magic, but it is really chemistry! In our experiment today we'll be making one or more different invisible inks and learning how to activate them.

Would you like to send a secret message? Try one of these recipes. Two recipes are acid/base neutralization reactions (#1 and #3). The other two are redox or oxidation/reduction reactions.

Magic ink #1

• Baking soda
• Paper
• Water
• Paint brush or swab
• Measuring cup
• Purple grape juice concentrate

Mix equal parts of water and baking soda. Use a cotton swab or paintbrush to write a message onto white paper using the baking soda solution as 'ink'. Allow the paper to dry. Send to a friend.

What happens?
Paint over the paper with purple grape juice. What happens?

Why does it work?
Baking soda is a weak base. The baking soda in the message neutralized the acid in the grape juice over the message. Grape juice is purple when acidic, pink when basic. Its color is caused by several different pH-sensitive anthocyanins

Magic ink #2

• Lemon juice from 1 lemon
• Small glass
• Water -- about 1/3 cup
• Paint brush, feather, q-tip or toothpick
• Paper
• Iron or hairdryer

In a bowl, mix the lemon juice with the water. Dip paintbrush (the point of the feather, q-tips or toothpick) in the juice mixture and write a "secret message" on the paper. Let dry completely. When the juice has dried, you will not be able to see your message. You may also write a visible message on the paper if you wish. This was one technique to help hide a secret note. Deliver the message to your friend.

What happens?
If you heat the note with an iron or hairdryer, what happens?

How does it work?
The acids in the lemon juice burn through the paper when heat is applied. If you heat it too long, your message will burn away! Do you suppose other weak acids would work too? Try white grapefruit juice, milk, cranberry juice and orange juice. What can you conclude?

Magic ink #3

• Phenothalien indicator
• Paper towel
• Paint brush or cotton swab
• Windex solution, with ammonia
• Dilute white vinegar in a spray bottle

Using paint brush swab, write your message on the paper towel. Allow to dry thoroughly and send to your friend.

What happens?
If sprayed lightly with windex, what happens to the message? Spray with vinegar. Now what happened? Can you make this sequence repeat? [Extra idea - allow CO2 gas (dry ice evaporates to carbon dioxide) to flow past the unrevealed message. Can you see it now? Do you know why?]

Why does it work?
Phenothalien is an acid/base indicator. It is colorless in an acid solution and turns a bright pink in a basic substance. Ammonia (in the window cleaner) is basic and the phenothalien turns bright pink. If vinegar neutralizes the ammonia and the note becomes slightly acidic, then the message will disappear.