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Science Activity
Invisible ink is a popular form of cryptography ' the science of preparing messages in a form that can only be read by those who know the secrets of the form. In this experiment, you'll learn how to create your own secret messages with simple household items.
Video length: 1:44 min View video description
Invisible ink is used to write messages that are undetectable to the naked eye and can only be revealed through specific developing methods, such as the application of heat or through a chemical reaction.
The most common use of invisible ink has been for sending confidential correspondence and secret messages during wartime. More than 200 years ago, during the American Revolution, invisible ink was used by both the British and American armies. Secret messages were often written between the lines of a letter and revealed by heating the message over a flame.
Various forms of invisible ink were also used during the First and Second World Wars. In April 2011, the CIA released six documents highlighting recipes for invisible ink that were used during the First World War and that included ingredients such as nitrate, soda, starch and potassium iodide.
Discover your inner “secret agent” by creating and revealing your own hidden messages in this fun and simple chemistry experiment.
Experiment #1: Ink developed by heat
Materials
- lemon
- knife
- water
- bowl
- spoon
- toothpick, cotton swab, paintbrush or other tool for writing
- white paper
- iron (use only with adult supervision)
Instructions
(1) Cut lemon in half and squeeze juice from both halves into the bowl.
(2) Add a few drops of water to lemon juice; mix together with spoon.
(3) Dip the toothpick (or other writing tool) into the “ink”; write your message on the paper.
(4) Allow message to dry completely until it becomes invisible.
(5) With the help of an adult, hold your paper over a hot iron to reveal the secret message.
What just happened?
Lemon juice is an organic substance that is mildly acidic. When you apply lemon juice to paper, the acid weakens the paper’s fibres. Once the lemon juice has dried and the paper is heated, the acidic parts of the paper will burn and darken faster than the rest of the “untouched” paper, revealing your secret message.
Other heat-developed ink
Try writing secret messages with these other common household materials. Compare your results. Which ink worked best?
• vinegar • apple juice • onion juice • orange juice • milk • sodium bicarbonate • white wine • diluted cola • diluted honey • soapy water • sugar
Experiment #2: Ink developed by chemical reaction
Materials
- baking soda
- water
- bowl
- spoon
- toothpick, cotton swab, paintbrush or other tool for writing
- white paper
- purple grape juice
Instructions
(1) Add equal parts baking soda and water to bowl; stir with spoon. Make sure the baking soda dissolves well so as not to be too powdery.
(2) Dip toothpick, cotton swab or other writing tool into the “ink”; write a message on the paper.
(3) Allow the ink to dry completely until it is invisible.
(4) Paint over the paper with purple grape juice to reveal your message.
What just happened?
This is a classic example of an acid-base chemical reaction. The acid in the grape juice is neutralized by the baking soda (the base), producing a colour change on the paper to reveal your message. 
ISSN 1927-0275 = Dimensions (Ottawa. Online)
