Invisible Ink Reveals Cool Chemistry

What's your secret message?: With just a few household items--and a little bit of science--you can make an invisible ink message that will reveal itself under just the right conditions. Image: iStockphoto/JoKMedia

Key concepts
Chemistry
Acids
Oxidation
Heat

Introduction
Have you ever wondered how spies and secret agents could leave secret messages? Invisible ink might sound high tech, but you can create?and read!?a top secret message with one simple kitchen ingredient: lemons. George Washington's army used this same concept to send secret messages during the American Revolutionary War. What message will you write?

Background
Lemon juice?and the juice of most fruits, for that matter?contains carbon compounds. These compounds are pretty much colorless at room temperature. But heat can break down these compounds, releasing the carbon. If the carbon comes in contact with the air, a process called oxidation occurs, and the substance turns light or dark brown.

Materials
???? One half of a lemon (use caution when cutting)
???? One half teaspoon of water
???? Small bowl
???? Spoon
???? White paper
???? Q-tips
???? A lamp with a lightbulb that puts off a lot of heat, such as a 100-watt incandescent bulb or another heat source, such as a radiator
???? Optional: Pencil (to write a decoy message on your paper)

Preparation
???? Squeeze the juice of your lemon half into the bowl.
???? Add the water and mix with a spoon.
???? Think of a secret message you would like to write?and to whom you're going to deliver it!
???? Extra: If you want to be super secret, you can write a boring old message or draw a picture on the paper with a pencil before you write your secret message to disguise it even further.

Procedure
???? Soak the Q-tip in the lemon juice-and-water solution.
???? Use the damp Q-tip to write your top-secret message on the piece of paper.
???? Wait a few minutes for the paper to dry. While you're waiting, you can switch on your lamp to give the lightbulb time to heat up (being careful not to touch the hot bulb itself).
???? When the paper is dry, hold it up to the hot lamp for a few minutes (but don't let the paper get so hot that it burns). What happened to your invisible ink? How long did it take for the change to occur?
???? Extra: Try this activity with other acidic liquids, such as apple juice or vinegar. Which ones work best?

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