The Science of Secret Messages  

This article first appeared in Instructor magazine (February, 1994), published by Scholastic, Inc. and was reprinted in "Classroom Magazine," April 1994 with permission.

These chemistry experiments, by Lynne Kepler, will take science out of the laboratory and put it right in your classroom!

Announce to your class that you are going to teach them chemistry, and you might get a yawn, but tell them that you're going to show them the science of writing secret messages, and they'll be sitting on the edge of their seats!

Invisible Messages

Concept: Heat can produce a chemical reaction.

Skills: Observing, inferring, predicting.

What You Need

-Q-tips (one per student)
-citrus juice (lemon, orange or grapefruit juice all work well) sheets of plain white paper
-an electric iron or a light bulb
-piece of cloth

What To Do

    1. Using a Q-tip dipped in citrus juice, write a message on a sheet of paper. Show the message to the students. (It won't be visible.) Ask students what they might use to make the message appear. List their ideas, and tell them they're about to find out.

    2. Place a cloth over the message and run a hot iron over it for five or ten seconds (or hold it above a hot light bulb.)

    3. Ask students to explain what they observe. (The message appears because a chemical change takes place when the juice is heated.) Ask students why the rest of the paper didn't darken. (The juice burns at a lower temperature than the rest of the paper.)

    4. Allow each student to write an invisible message to a friend.

    5. Conclude the activity by asking students to think of all the things in their everyday lives that depend on chemical reactions. (Examples include baking bread or hard-boiling an egg.)

Alkaline Ink

Concept: Many substances are classified as acidic, alkaline (base), or neutral.

Skills: Observing, inferring, predicting, collecting and recording data, classifying.

What You Need

1 paper coffee filter for each student 1/3 cup rubbing alcohol 1/4 tablespoon turmeric powder newspaper mixing bowl 1 small plastic cup for every four students baking soda water Q-tips

Preparing The Coffee Filters

Prepare the coffee filters before the lesson. (Older students could prepare the filters themselves.)

    1. In a bowl, combine the turmeric with the rubbing alcohol.

    2. Dip the whole coffee filter in the bowl. It will turn a goldenrod hue. Do only one filter at a time.

    3. Place the filters on newspaper to dry.

What To Do

    1. Have groups of four students mix one tablespoon of water with one teaspoon of baking soda in a plastic cup. This is the "ink."

    2. Give each student a prepared filter and a Q-tip.

    3. Ask students to predict what the ink will look like when it comes in contact with the special paper.

    4. Have students write their messages on the paper. (The letters will immediately turn red.)

    5. Ask students why they think the messages turned red. (The turmeric paper serves as a test for alkaline materials. This means that the paper has a chemical in it that makes it change color when it comes in contact with alkaline substances.)

    6. Ask students if the alkaline substance is the water or the baking soda. (The baking soda.)

    7. Challenge your students to bring from home other common materials that they want to test for alkalinity. Which materials do they predict will turn the paper red? (The list is endless. Try dishwashing liquid, Epsom salts, talcum powder, and yeast.)

To make ink, students can mix one teaspoon of any of the materials with one tablespoon of water. They could draw up a chart to record their data individually, or as a class.


Make a secret writing recipe book.

Encourage your students to investigate other methods of making secret messages. The science section in the library is a good place to start. When students find methods that are successful, they can write detailed recipes to be compiled into a class book.

Display the book with a list of the supplies needed for other students to try the recipes!

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