by STEM Scouts
Ever wanted to pass a message to a friend that only they know how to read? You aren’t alone! Invisible ink has been around for millennia and has been used by prominent historical figures from poets and scholars to spies and even the first president, George Washington.
Here’s how to make three different versions of invisible ink with common household items:
- Baking soda
- Grape juice
- Measuring cup or spoons
- Cotton swabs
- Sheet of paper
- Using the measuring cup or spoons, measure out equal parts baking soda and water.
- Add the baking soda and water to the cup; stir the mixture thoroughly with a spoon to ensure it is completely saturated.
- Dip your cotton swab in the mixture and then, pressing firmly, begin to write your message on the paper. You may need to stir the mixture again each time you dip your cotton swab in.
- Let the message dry completely. While it is drying, pour some grape juice into a separate cup.
- Once dry, grab a new cotton swab, dip it into the grape juice, and rub it across your message.
- Your hidden message should begin to reveal itself.
- What happens if you add more or less of each ingredient in the mixture? Does the temperature of the water make a difference? Can you find a perfect solution?
- Do you think you can change the applicator (cotton swab) to something with a finer point, such as a toothpick, and still read the message?
- How about if you change the grape juice for a soda? Or for normal water?
How Does It Work?
You’re likely familiar with the classic baking soda and vinegar volcano experiment. This works for a similar reason. Baking soda is a base, which reacts chemically with acids, such as vinegar (in this case, grape juice). Grape juice is a milder acid and when it’s exposed to a base, it creates a color changing acid-base reaction.
- Yellow highlighter
- Warm water
- Cotton swabs
- Disposable safety gloves (optional)
- Paper (optional)
- Pliers or hammer (optional)
- Put on your gloves and begin with your highlighter. There should be a plastic cap holding the ink cartridge inside of the casing. Grip the cap with a pair of pliers and pull it out. Note: If you don’t have pliers you can lightly tap the casing with a small hammer to crack it or use your hands to wiggle the cap off.
- Once the cap has been removed, the ink cartridge should easily slide out. Set it to the side.
- Pour roughly a cup of warm water into the cup. Then, with gloves still on, squeeze the ink from the cartridge into the water. Feel free to dip, dunk, and swish the cartridge in the water. Once the ink has been completely removed, the cartridge should turn white. Note: Without the gloves, the ink would get all over your hands and is somewhat difficult to remove, but highlighter ink is non-toxic to skin. So if you don’t have gloves, don’t worry, just prepare for a bit of a mess!
- Stir the solution thoroughly with the spoon.
- Dip a cotton swab into the solution and begin writing your message on a sheet of paper. Tip: you can also write on your skin!
- Use the black light to reveal your message to the world.
- Is the ink still visible when you write? Or is it too hard to see even under the blacklight? What happens when you adjust the amount of ink or water in the solution?
- How does visibility change on different surfaces and in different lights?
How Does It Work?
This experiment is an easy way to get your child interested in luminescence and the many ways it’s present in our world, such as fireflies (bioluminescence), glow sticks (chemiluminescence), glow in the dark toys (phosphorescence), and our highlighters in this experiment.
Illuminating the solution with a black light, or ultraviolet light, is an example of fluorescence because once the ultraviolet light is turned off, the solution no longer glows. Now, why exactly do this solution and a highlighter glow? This is because the highlighter contains phosphors, which is an artificial substance that emits light when irradiated.
- Blow dryer
- Cotton swab
- Cut your lemons in half and squeeze the halves into your cup.
- Add a few drops of water to the lemon juice and stir.
- Dip your cotton swab in the lemon juice and then, pressing firmly, begin to write your message on the paper. You may need to stir the mixture again each time you dip your cotton swab in.
- Let the message dry completely.
- Once dry, turn on the blow dryer to high heat and point it at the paper. Your hidden message should begin to reveal itself.
- Can you use other sources of heat? Would an iron, stovetop, or even light bulb produce the same effect?
- Lemon juice is the simplest mixture listed here. Can you put it in a refillable pen and write with it? Or could you paint with it using a brush?
- What happens if you don’t add water at all, or at a lot more water?
How Does It Work?
This is a pretty simple one. While you might think the paper would burn or crinkle before the part of the paper that was wet not too long ago would darken, lemon juice is an organic material and it browns when exposed to heat. The small amount of water added masks the visibility of the lemon juice on the paper.