Kids, how can you send an invisible message? Some science projects don't require any chemicals that you don't already have around the house, and a great example is invisible ink. 

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

You use the ink by writing your message with it using a cotton swab, dampened finger, or toothpick. Let the message dry. To be extra sneaky, you may want to write a normal message on the paper so that it doesn't appear to be blank and meaningless. If you do write a cover message, use a ballpoint pen, pencil, or crayon (fountain pen ink could run into your invisible ink). Don’t use lined paper for the same reason.

Most invisible inks are made visible by heating the paper. Some messages are developed by spraying or wiping the paper with a second chemical; others are revealed by ultraviolet light.

Examples of common invisible inks are: any acidic fruit juice (e.g., lemon, apple, or orange juice), onion juice, baking soda (sodium bicarbonate), vinegar (acetic acid), white wine, dilute cola, dilute honey, milk, soapy water, and sucrose (table sugar) solution. Here is just one example:

1. Mix equal parts water and baking soda.
2. Use a cotton swab, toothpick, or paintbrush to write a message onto white paper.
3. Allow the “ink” to dry.
4. One way to read the message is to have your adult partner hold the paper up to a heat source, such as a light bulb. The baking soda will cause the writing in the paper to turn brown.
5. A second method to read the message is to paint over the paper with purple grape juice. The message will appear in a different color.

Tips:

1. If you are using the heating method, avoid igniting the paper - don't use a halogen bulb.   2. A cotton swab makes an excellent disposable 'paintbrush'.  3. Baking soda and grape juice react with each other in an acid-base reaction, producing a color change in the paper.  4. The writing turns brown because the weakened paper burns before the rest of the paper. Be careful not to overdo your heating and ignite the paper! 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
June 2008

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Reference:  Anne Marie Helmenstine on her April 27, 2008 blog at http://chemistry.about.com/.
See these links for baking soda and lemon juice “inks”: http://chemistry.about.com/cs/howtos/ht/invisibleink2.htm and http://chemistry.about.com/cs/howtos/ht/invisibleink3.htm .