Invisible Ink

Sometimes messages can be kept secret by the type of ink used to write them! Learn how to create your own secret messages using lemon juice!

Materials:

  • Lemon juice (if squeezed from an actual lemon and not from a bottle, mix with a little water so the message is less noticeable when dry)
  • An applicator (we used a Q-Tip meant for use on fingernails because it was the only kind we could find, but any kind of cotton swab works– paint brushes also work well)
  • Paper
  • A source of heat (we used a lamp, but you can also hold the paper over a flame or iron it)

Instructions:

Pour a small amount of lemon juice into a container. You won’t need that much, so just add enough to sufficiently saturate your applicator when writing your message. Dip your applicator into the lemon juice and write out your message. Make sure to add enough lemon juice to ensure that the message shows up clearly at the end of the experiment. Let the paper sit out to dry. If using a light bulb as your heat source, as we did, hold the paper up to the bulb until the message starts to appear. If using a flame or iron, be careful– not only can you burn your fingers, but also the paper containing your secret message.

How does this happen??

It’s posited that several processes contribute to this effect.

The first process is called oxidation. Oxidation is a chemical reaction involving the loss of electrons. You might have observed that cut apples turn brown after they have been sitting out in the open for a while. This is oxidation. More specifically, an enzyme called polyphenol oxidase reacts with oxygen in the air, creating melanin and turning the fruit brown. An enzyme is a protein that helps speed up reactions in living organisms.

Interestingly, if lemon juice is applied to fruits like apples that are susceptible to enzymatic browning, it can prevent said process from occurring thanks to the acids it contains.

So… what does this have to do with our experiment?

Well, a certain kind of oxidation is involved in our experiment, too. In lemon juice, there are carbohydrates present that start to carbonize when exposed to heat. This means that carbon is released and begins to oxidize when introduced to the air. The product of this oxidation is the brown that allows you to view your secret message.

Additionally, if you’ve ever seen really old pieces of paper in museums, you know that eventually, really old paper turns brown by itself. The lemon juice and heat actually just speed up this process. Paper is made up of cellulose fibers. These cellulose fibers are in turn made up of glucose. When applied to the paper, the citric acid in the lemon juice breaks down cellulose fibers in the paper into glucose. This weakens the paper, in turn causing these parts of the paper to turn brown more quickly.

Sources:

https://extension.purdue.edu/4h/Documents/Volunteer%20Resources/Science%20Made%20Easy/Brown%20Apples.pdf

https://www.sciencebuddies.org/stem-activities/invisible-ink#instructions

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