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20 Minute Labs: Invisible Ink

20 minute lab lemon logo | Yellow Scope blog



Ever wanted to make a super secret message to a friend, like you were a spy in the movies?

top secret | yellow scope blogIn this month's 20 Minute Lab, we'll be writing a message using the "ink" of a common fruit!

Planning to set up a lemonade stand this summer with your friends or family? In between customers, you can use the lemonade to write secret messages to each other!

If you don't have a lemon or lemonade, you can use bottled lemon juice, an orange, lime, other citrus fruit, or even vinegar!


egg supplies | Yellow Scope blog
  • half a lemon
  • small bowl
  • spoon and water
  • Q-tip
  • sheet of paper
  • a strong light bulb (100 watts), or an iron


  1. Squeeze about a tablespoon of lemon juice into the bowl.
  2. Add a teaspoon of water and mix with a spoon.
  3. Dip one end of the Q-tip into the lemon juice.
  4. lemon setup | Yellow Scope blogUsing the Q-tip like a pencil, write your message on the paper! (Note: Don't use too much lemon juice when writing each letter to avoid getting the paper too wet and warping it!)
  5. Depending on how long your message is, you may have to re-wet your Q-tip with lemon juice several times.
  6. Wait a few minutes for the paper to dry.
  7. light bulb | Yellow Scope blogNow hold your paper over the light bulb. (Note: incandescent bulbs work the best!) To use an iron, turn the steam setting to "off" and place a piece of fabric between the iron and paper. Have an adult help with this step!






If you could see your message after the paper dried - without applying any heat - you may have used too much lemon juice when writing. Try it again - this time letting the Q-tip rest on the edge of the bowl before writing to let some of the liquid drip off.

If your message does not appear after applying heat, the heat from the light bulb may not be hot enough. Try using a hotter light bulb or try the iron method.


What's going on?

Did your secret message magically appear? Did it turn dark brown? Why did this happen?

Lemon juice is made up of carbon-based compounds, which you applied to the paper with the Q-tip. As you saw, it doesn't have any color at room temperature.

However, heat breaks carbon bonds and sets some of those carbons free from the paper. When that free carbon contacts the oxygen in the air, it turns a brownish color. This process is called oxidation.

lemon ink message | Yellow scope blog

Sometimes substances will oxidize without any added heat. Have you ever had apple slices turn brown when they sit out for a while? That's because the carbon-based compounds in the apple oxidized without the need for the extra heat.

apples | Yellow Scope BlogFun trick: to prevent your apples from turning brown, add some lemon juice to them! Lemon is a natural antioxidant which prevents the oxidation process from happening. 



Let us know what you did. Share your photos and results with us on Facebook, Twitter, Instagram, or send us an email to We love getting your messages!

For more exciting experiments, check out our Yellow Scope Science Kits on the Shop tab of our website!


Chelsea Schuyler
Chelsea Schuyler