Invisible ink has been used by spies for centuries. At the time of the Revolutionary War, invisible ink made of a mixture of ferrous sulfate and water was commonly used. The secret messages were often written in between the lines of a normal letter. When heat or a special chemical (such as sodium carbonate) were applied, the message that was placed in between the lines would appear. In modern times, inks containing special properties are used and require viewing under ultraviolet (UV) light to see the message.
- Put some lemon juice in a bowl and mix with a few drops of water
- Wet a cotton ball and use it to write a message on a blank piece of white paper
- Wait for the “ink” to dry and become completely invisible
- After the ink has dried, heat the paper up using a hot light bulb
Lemon juice is an organic substance that oxidized and turns brown when exposed to heat. The heat produces a chemical reaction in the lemon juice. We dilute the lemon juice with water to make it harder to see on the paper. Other substances that produce a similar effect include vinegar, onion juice, honey, and orange juice.
Interesting note
In 1999, the CIA noted that World War I era invisible ink technology was to remain exempt from their normal (and mandatory) declassification process. The reason? They noted that the invisible ink used was still relevant to national security.
Parent/Teacher/Advanced Notes [click to expand]
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