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. When you heated the paper, the heat caused some of the chemical bonds to break down, freeing the carbon. When you painted the lemon juice solution onto the paper, the carbon-based compounds were absorbed into the paper's fibers. What happened to your invisible message? What other liquids work well to make invisible ink that develops under heat? Extra: Try this activity with other acidic liquids, such as apple juice or vinegar.What happened to your invisible ink? How long did it take for the change to occur? 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).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). Wait a few minutes for the paper to dry.Use the damp Q-tip to write your top-secret message on the piece of paper.Soak the Q-tip in the lemon juice-and-water solution.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.Think of a secret message you would like to write-and to whom you're going to deliver it!.Squeeze the juice of your lemon half into the bowl.Optional: Pencil (to write a decoy message on your paper).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.One half of a lemon (use caution when cutting).If the carbon comes in contact with the air, a process called oxidation occurs, and the substance turns light or dark brown. But heat can break down these compounds, releasing the carbon. These compounds are pretty much colorless at room temperature. Lemon juice-and the juice of most fruits, for that matter-contains carbon compounds. George Washington's army used this same concept to send secret messages during the American Revolutionary War. 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.
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