While you're waiting, you can switch on your lamp to give the lightbulb time to heat up being careful not to touch the hot bulb itself.
What happened to your invisible ink? How long did it take for the change to occur? Which ones work best? Observations and results What happened to your invisible message? What other liquids work well to make invisible ink that develops under heat?
When you painted the lemon juice solution onto the paper, the carbon-based compounds were absorbed into the paper's fibers. When you heated the paper, the heat caused some of the chemical bonds to break down, freeing the carbon.
Once the carbon came into contact with the air, it went through a process called oxidation, one effect of which is to turn a darker color. Oxidation doesn't always need heat to occur. Some fruits themselves can turn brown from oxidation. Think of an apple or pear slice that is left out on the counter for too long. Add more water and a little sugar, and you can turn your invisible ink into lemonade!
Some 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. Some UV-visible inks may be detected on a photocopy, due to the relatively strong ultra-violet component in light from the photocopier scanning head.
This topic could start with a group discussion or a short story woven around the facts given below, during which the teacher introduces the ideas especially the words in bold. This is a fun activity but does have some real chemistry behind it. However, for younger children probably the best are the heat-stimulated inks. Invisible ink is any substance that you can use to write a message that is invisible until the ink is revealed. Anyone can write an invisible message, assuming you have paper, because body fluids can be used as invisible ink.
Invisible ink has a history that goes back in time. Touch them with a little powdered charcoal and you will read them. At the other extreme, Mary, Queen of Scots , used invisible ink and ciphers to communicate with Catholic supporters.
Her supporters wrote to her using two common substances: alum hydrated potassium aluminium 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.
During times of secret love, war or in the world of spies in the cold war, high security is important and there is often the need for invisible ink with which a message can be written, be invisible, and when it reaches the recipient be brought back so it can be seen.
This meant they had to operate without being detected. Hence, messages were often passed using invisible ink. SOE agents were trained not to risk their lives so they used invisible inks as a back-up method of communication when other, more secure communication techniques were unavailable.
The agency supplied special inks to its field agents. When agents were forced to improvise, they were advised to dilute their invisible ink as much as possible to reduce chances of detection. The properties identified by the SOE could be discussed by students as to why they are important criteria for an invisible ink.
The students will enjoy the practical activity and the explanation is best given after they have results they can talk about. If they work in groups using different inks to write questions they can pass those messages to another group who can develop the message and send the answer to the question back.
A good way of recording this work is to get the pupils to write a story involving the ink they have used for their experiment. All ages can take part in this activity but at different levels since the aim is to gain some understanding of the thinking of the scientist and artist with regard to the properties of materials. It links with:. Suggest not using. Potassium ferricyanide should not be present with Ammonia because an explosive mixture can be formed.
It should also not be present where concentrated acid could be added because it would produce TOXIC gases. Some of the metal compounds such as copper, cobalt, and lead can be toxic in varying quantities and can be an irritant if the liquid is spilt or put on the skin. Ultra-violet light can be harmful to eyes so pupils should be supervised when handling the lamp or they can observe as an adult uses the lamp. Portable counterfeit money detectors can be used for this and are easily sourced and reasonably priced.
Pass the message to another group who will develop your message and then write an answer to the question and pass it back to you to develop. These inks are best for younger children since most of the chemicals are household and generally less hazardous. They can be developed by heat generally ironing them with a hot iron or placing them in an oven or on a white tile on a hotplate.
These are best used with secondary students and most could be used comfortably with lower secondary students. Many of them work as pH indicators, so paint or spray a suspected message with a base eg sodium carbonate solution or an acid e. Some of these inks will also reveal their message when heated e. The substances in milk weaken the paper and may be more susceptible to heat than the paper, so although the message dries clear, the paper weakens and darkens where the milk was applied.
These can be used with younger children but the chemistry is more complex so the theory is best left to secondary age students. Most of the inks that become visible when you shine an ultra violet light on them also would become visible if you heat the paper. Because of the wide range of materials, teachers should select either a few of the materials or group students to allow a wider range of materials to be investigated. If using groups then sufficient time needs to be allowed for whole class reporting on the results and evaluation of the materials being used.
Provide solutions of solids if preferred. The way invisible ink works depends on the method used to make it. There are several ways to make invisible ink and below you will find a few different recipes for making your own invisible ink and how it works.
Why it works: Grape juice has an acid that reacts with the baking soda. A different color appears wherever the secret message is written. Why it works: Milk is an organic product which means it comes from a living thing. When it's heated, it burns at a slower rate than the paper. Your invisible message shows up brown.
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