Make your own invisible ink experiment
Spies have used invisible ink 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.
Mary, Queen of Scots (1542-1587), used invisible ink to communicate with Catholic supporters. Her supporters wrote to her using two common substances: alum (hydrated potassium aluminum sulfate) or gall ink (tannic acid from parasitic wasps in oak trees galls). The alum letters would be developed by soaking the letter in water, and the gall ink would be developed by soaking the letter in iron sulfate solution.
In modern times, more advanced inks containing special properties are used and require viewing under ultraviolet (UV) light to see the message.
Here’s how to make heat activated invisible ink out of lemon juice.
- Put some lemon juice in a bowl and mix with a few drops of water
- Wet a cotton ball, paintbrush, or toothpick with the solution 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 (or iron it with a hot iron)
How lemon juice oxidizes to create an invisible ink
Lemon juice is an organic substance that oxidizes 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.
Other heat-activated invisible ink materials
There are several other substances that can be used in place of lemon juice to create invisible ink. Any of the following can be used:
- Apple juice
- Orange juice
- Onion juice
- Baking soda (sodium bicarbonate) (or 1 M sodium hydrogen carbonate)
- Vinegar (or 1 M ethanoic acid)
- Dilute cola
- Diluted honey
- Soapy water (or 0.1 M sodium carbonate)
- Sucrose (table sugar) solution (1 tsp in 10 cm3 water)
Substances that can be used to create ultra-violet light activated invisible ink
Modern day invisible inks use ultra-violet lights (i.e., black lights) to activate the ink and make it visible. Any of the following can be used to create invisible inks that are developed by ultra-violet light.
- Dilute laundry detergent (the bluing agent glows) or 0.1 M sodium carbonate hydrated (Check packets for safety notes)
- Tonic water (quinine glows)
- Vitamin B-12 dissolved in vinegar
These inks glow faintly or fluoresce when under an ultra-violet lamp. This is a property of many substances, particularly organic substances, and body fluids. Other inks work in a near opposite way by absorbing ultra-violet light but without fluorescing. When these are used on fluorescent paper, the inked areas fluoresce less than the surrounding paper area when under an ultra-violet lamp. This is especially a property of inks with a yellow tint.
Substances that can be used to create invisible ink activated by a chemical reaction
Most invisible inks involving a chemical reaction work using pH indicators. You can paint or spray a suspected message with a base (such as sodium carbonate solution) or an acid (such as lemon juice).
Examples of such inks include:
- Phenolphthalein (pH indicator), developed by ammonia fumes or sodium carbonate (or another base)
- Thymolphthalein, developed by ammonia fumes or sodium carbonate (or another base)
- Vinegar or diluted acetic acid, developed by red cabbage water
- Ammonia, developed by red cabbage water
- Sodium bicarbonate (baking soda), developed by grape juice
- Sodium chloride (table salt), developed by silver nitrate
- Copper sulfate, developed by sodium iodide, sodium carbonate, potassium ferricyanide, or ammonium hydroxide
- Lead(II) nitrate, developed by sodium iodide
- Iron sulfate, developed by sodium carbonate, sodium sulfide, or potassium ferricyanide
- Cobalt chloride, developed by potassium ferricyanide
- Starch (e.g., corn starch or potato starch), developed by iodine solution
- Lemon juice, developed by iodine solution
Interesting weird science notes about invisible ink
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.
Make your own invisible ink experiment supplies
Supplies: Paper, Cotton ball, Lemon
In-Article Image CreditsInvisible ink revealed with UV light via Wikimedia Commons by Felix E. Guerrero with usage type - Creative Commons License. October 16, 2008
Featured Image CreditInvisible ink revealed with UV light via Wikimedia Commons by Felix E. Guerrero with usage type - Creative Commons License. October 16, 2008