As the holiday season approaches, it seems like adults have lots of secrets. Why should they have all the fun? You can write secret messages to friends by using common household items to make invisible inks. And the neat part of it is, each one of these inks demonstrates that chemicals can interact with paper, with one another, and sometimes with heat.
Invisible Ink Number 1
Put a little milk in a small bowl. Use a cotton swab, pipe cleaner or small paint brush to write a message with the milk on a piece of paper. Do not use too much milk or the paper will wrinkle. Let the milk message dry completely. The person who gets your message must heat it so that the message reappears. This can be done with an iron or by holding the paper above a 100 watt light bulb. Safety tip: Avoid fire danger by having an adult help do this and never use a halogen light.
Why it works: Milk is an organic product, meaning it comes from a living thing. When it is heated, it burns at a lower temperature than the paper and turns brown. The invisible message reappears in brown.
Invisible Ink Number 2
You can write with lemon juice the same way as with milk and read it after it is dry by heating it. Get lemon juice either by squeezing a lemon or by using bottled lemon juice. To read the message, heat the paper as you did with the message in milk. There is another way to read an invisible message in lemon juice. You can read it by spraying the paper lightly with red cabbage water.
Click here to learn how to make red cabbage water. Use a plant misting bottle to spray the red cabbage water on.
Why it works: Red cabbage water is an indicator for acids and bases. Since lemon juice is an acid, the red cabbage water interacts with the dried lemon juice and turns a different color. The invisible message reappears.
Invisible Ink Number 3
Write your message in vinegar and use red cabbage juice to read the message.
Why it works: Vinegar is also an acid. The red cabbage water will turn a different color where the vinegar dried.
Invisible Ink Number 4
Make ink from 1 teaspoon (5 ml) of corn starch in 1/4 cup (60 ml) of water. Heat it a little, half a minute in a microwave is about right. Stir it and let it cool. Write your message on paper and let it dry. To read the invisible writing, wipe the surface of the paper with a sponge which has been wetted in iodine and water. The iodine mixture should be 10 drops in a 1/4 cup (60 ml) of water. The message will show up dark purple on a light purple background. Safety tip: Iodine can be used on cuts to kill germs because it is poisonous to living things. Drinking or eating iodine could make you sick.
Why it works: Iodine interacts with starch. Since the dry corn starch message has a lot of starch, it turns dark purple. The paper has some starch in it also, so it turns color too.
Invisible Ink Number 5
Mix about 1/4 cup (60 ml) of baking soda and 1/4 cup (60 ml) of water. Write with this mixture on paper. Let it dry completely. To read the secret message, paint grape juice concentrate across the paper from top to bottom with a paint brush or a sponge. Does it work with regular grape juice? Tip: Grape juice stains. Be careful not to spill.
Why it works: The acid grape juice interacts with the alkaline baking soda to produce a different color making the secret message appear.