Key concepts

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?

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.

•    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)

•    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.

•    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?

Observations and results
What happened to your invisible message? What other liquids work well to make invisible ink that develops under heat?

When you painted the lemon juice solution onto the paper, the carbon-based compounds were absorbed into the paper's fibers. When you heated the paper, the heat caused some of the chemical bonds to break down, freeing the carbon. Once the carbon came into contact with the air, it went through a process called oxidation, one effect of which is to turn a darker color. Oxidation doesn't always need heat to occur. Some fruits themselves can turn brown from oxidation. Think of an apple or pear slice that is left out on the counter for too long.

•    Use caution with the lamp, as the lightbulb can stay hot even after it is turned off.
•    Have extra lemon solution? Add more water and a little sugar, and you can turn your invisible ink into lemonade! And it's no secret that lemonade tastes good.

More to explore
"Carbonic Colors: Fizzy, Washable Sidewalk Paint," from Scientific American and
"Talk through a String Telephone," from Scientific American
"Invisible Ink and More: The Science of Spying in the Revolutionary War," from Scientific American
"Invisible Ink," from
George Washington, Spymaster: How the Americans Outspied the British and Won the Revolutionary War, by Thomas Allen
Amazing Kitchen Chemistry Projects You Can Build Yourself, by Cynthia Light Brown and Blair Shedd

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