Ready for a top-secret assignment? In this experiment you're going to learn how a popular form of secret message is created so you can make your own. What will you write using your homemade invisible ink?

QUESTION

What makes invisible ink visible?

WHAT YOU NEED

• Lemon juice

• Cup

• Paintbrush

• Paper

• Friend

• Hot lamp

WHAT TO DO

• Squeeze some lemon juice into a cup. Don't worry if there are seeds in the juice.

• Dip your paintbrush into the lemon juice. Paint a message on the paper.

• Let the lemon juice dry.

• Now give the paper to a friend.

• Have your friend hold the paper up to a lamp. What happens?

Tasty

A chemist has a pile of tasty and nice smelling leaves. She wants to use the flavor and aroma but keep the dry and woody parts of the leaves separate. Connect the dots to find out one way she can do this!

WHAT'S HAPPENING

When you applied the lemon juice to the paper, you could probably see your writing at first, but then the juice dried and your writing disappeared. Presto! Your message was hidden.

To make your message appear, you held the paper close to the lamp. The heat in the lamp caused a chemical change—the water molecules in the lemon juice evaporated from the heat, leaving behind sugar molecules. The sugar molecules reacted to the oxygen in the air and turned brown, revealing the secret note.

Secret messages can be made and revealed using all kinds of methods. Ultraviolet light, for instance, is used by the United States Postal Service to print routing information on packages. The information is not visible without an ultraviolet light.

Sometimes, even though a message is “invisible,” it is still possible to tell that a blank piece of paper isn't really blank. If you look carefully, you might just see the juice sitting on top of the paper.

YOUR NOTES

Try this experiment with other citric acids, such as orange juice and grapefruit juice. Do they work as well as the lemon juice did?