Secret Message with Lemon Juice – SICK Science

Lemon juice is great in lemonade, can be a garnish for foods, and doubles as an ink for your secret messages.

Ah, a tall, cool glass of lemonade on a hot day just makes everything better. Lemon juice can spark up a salad, add zing to some grilled fish, and keep your avocados and bananas from turning brown. Strangely, however, it turns paper brown and reveals secret messages you’ve written using plain old lemon juice. You can be a gourmet cook and a spy at the same time.

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Experiment Materials

  • Lemon juice (fresh or bottled)
  • Cotton swab or a small paint brush
  • Small cup
  • Plain white paper
  • Heat source (See NOTE in Step 4.)
  • Adult supervision

Experiment Videos

Experiment

1

Add about 1 tablespoon (15 ml) of lemon juice to the cup. Fresh squeezed or bottled juice will work just fine.

2

Soak an end of the cotton swab or put the paint brush into the lemon juice. You’ll use this to write your message.

3

Write your message on the plain paper. You’ll be able to see it as long as the paper is wet so let the lemon-juice message dry completely.

4

There’s no rush to revealing the message but when it comes time to read it, there are several methods of heat sources. NOTE: If you choose the open flame method, make sure an adult helps out. Setting the message and/or yourself on fire are not what you want to do. Whichever method you choose, start at the bottom of the message and work your way up since heat rises.

Here are some options:

  • You can hold the paper over an incandescent light bulb with OK results. It may be hard to tell if the bulb is hot enough, so you may not know whether your paper is blank or whether you just can’t see the message yet.
  • You can “iron” your paper but don’t use the steam setting. This may be the best method. A clean rag between the iron and the paper helps keep the iron’s hot surface clean.
  • Hold the paper over a hot burner on a stove. There will be some distortion of the paper as it gets hot and the message appears. Watch your fingers!

How Does It Work

Most invisible ink messages can be revealed by heating either side of the paper on which they’re written. The message discolors before the rest of the paper gets hot enough to do so. When you wrote your message using the lemon juice, carbon-based compounds in the juice were absorbed into the paper’s fibers. Also, since lemon juice ink is a weak acid, it softened the fibers in the paper. The added heat caused some of the chemical bonds to break down in the dried juice and some carbon was cut loose. When the carbon came into contact with air, it burned or oxidized. One effect of oxidation is things turn a darker color. (Some oxidation doesn’t need heat to occur; it just needs a little air. Think of a piece of apple, banana, or pear left out on a plate for a while.)

Lemon juice – like most fruit juice – contains carbon compounds. These compounds are pretty much colorless at room temperature. Heat breaks down these compounds and releases the carbon. When carbon comes in contact with air (specifically oxygen), oxidation occurs and the substance turns light or dark brown. If it oxidizes long enough, it can go all the way to black.

Other Secret Messages

Some secret messages are written with one kind of chemical and made visible by spraying or wiping them with a second chemical. Other messages are revealed by shining ultraviolet (UV) light on them. Another way to read the lemon juice message is to put salt on the drying juice. After a minute, wipe off the salt and then use a wax crayon to reveal the message.