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A Secret Letter with Invisible Ink

A Secret Letter with Invisible Ink

Introduction: (Initial Observation)

Developing recipes for invisible ink is the task of scientists. Putting it to use is generally the work of spies and others in pursuit of nefarious deeds. It’s the work of the sleuth to figure it all out, at least that’s the way most good mystery novels work! But invisible ink has also had its place in history.

Artifacts from World War II turn up quite frequently, but a  certain postcard turned up a few years ago which caught everyone’s interest. The postcard had been sent out of Poland during the occupation. There wasn’t anything of interest to the Germans on the postcard, or so they thought.

Beneath a one sentence letter on the postcard was a message written in invisible ink! The message makes an urgent request for supplies, including invisible ink. Fragmented sentences read like a nightmare, describing a Nazi death camp with gas chambers. It told a story about a person we might not have even known about otherwise. Invisible ink had made this possible, but what is invisible ink? And how does it work?

This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Adult help and supervision is required.

Information Gathering:

Find out invisible inks. Read books, magazines or ask professionals who might know in order to learn about the methods of making invisible ink and revealing them. Keep track of where you got your information from. Following are some related links:


Spy letters of the American revolution

HOLOCAUST Tales of Myth

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

What are some ways to write secret messages? What material can be used as invisible ink?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable in this project is the type of liquid that we may use as invisible ink. The dependent variables are the visibility of ink before and after the revealing process.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

My hypothesis is that onion and potato can be used as invisible inks. This hypothesis is based on my observation of roasting and frying onion and potato that makes them to change color and burn as soon as they lose their water. While performing experiments I will also test other material such as vinegar, lemon juice and milk to see if they can be used as invisible ink.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1:

In this experiment we will see if lemon onion juice can be used as in invisible ink. We will try to write secret messages with different possible invisible inks, and try revealing the secrets using a heat source.


Use any juice making method that is easier for you to make some onion juice. If your juice is not clear, use a coffee filter to filter any onion pieces and make clear juice.

Find something to write with such as a small brush, an old but clean fountain pen, or even just a Q-Tip or tooth pick. Providing that the paper is white, and providing you use your “ink” sparingly, the message is pretty much invisible to the eye. Be sparing in your use, though. Too much “ink” will cause the paper to buckle up a bit and the writing will show. And be sure that your writing implement is very clean and doesn’t leave traces of some old ink or paint.

Let the paper dry for about 10 minutes.

Use a hot plate, a burner or stove and carefully heat it up. Obviously, don’t let the paper catch fire…. (Pretty soon the message will turn up as brown writing.).

Repeat this experiment with other possible liquids that may be used as invisible ink. vinegar, lemon juice and milk are among the liquids that will work. Potato juice did not work as good, try it yourself.

Note: You may also use an iron to reveal a secret message.

How it works is really quite simple. There are chemical compounds in the onion and lemon juice that have a low burning point. These are carbon compounds such as those that make caramel. When you hold the paper over heat, these compounds scorch and turn brown before the paper does, so they leave their mark and reveal the writing.

Experiment 2:

In this experiment we use starch solution as invisible ink. Then we use a chemical reaction between starch and iodine solution to reveal the message.

Materials Needed:

  1. Corn Starch
  2. Hot Plate
  3. Toothpicks or Cotton Swab
  4. Pan
  5. Iodine Solution
  6. diluted iodine solution
  7. Water
  8. Paper
  9. Small Sponge


  1. Mix a small amount of water with cornstarch in the pan and stir until smooth. (You may try one tea spoon starch with one cup of water)
  2. Heat the mixture for several minutes. Stir.
  3. Dip a toothpick into the cornstarch and water mixture and write with the mixture on paper. Let the paper dry.
  4. To observe the message, dip the small sponge into the iodine solution and carefully wipe the paper. Do not get the paper too wet. The message should appear purple.

Note: Many papers specially those used as copy paper or laser printer paper contain starch, so they can not be used for this experiment. In order to find out which paper does not have starch at all, use a drop of iodine solution. If the paper does not turn blue, it does not have starch.

Experiment 3:

In this experiment we use another chemical reaction to reveal a secret message


  • phenolphthalein (available from MiniScience.com and ChemicalStore.com)
  • Rubbing Alcohol
  • Paper Toweling – higher quality ones work better
  • Windex™ – with ammonia D


  1. Use the rubbing alcohol to dissolve the phenolphthalein. If you a solution of phenolphthalein you will not have to do this.
  2. Use a paintbrush to paint your message on the paper towel.
  3. After it dries you can see nothing until you spray the Windex on the area. Then the words you painted will appear as pink letters.

Why does it work?

Phenolphthalein was first synthesized in 1880 and has long been known to change color depending on whether it is in acidic or basic conditions. It is colorless under acidic (or neutral) conditions and pink/red in basic solutions. The Windex™ contains a chemical, ammonia, that turns the conditions basic. By spraying the ammonia on the paper, the phenolphthalein turns pink and you can see the letters that were painted in advance.

Do I need to use phenolphthalein?

Old ExLax™ tablets contained phenolphthalein. You may find some old instructions for this experiment suggesting the use of ExLax instead of phenolphthalein. Please note that new ExLax tablets do not contain phenolphthalein.

Experiment 4:

In this experiment we compare different invisible inks to see which one will reveal faster. We will then enter our results in a data table and use it to make a bar graph.

Adult supervision and support is needed for this experiment.

Materials Needed:

  1. Milk
  2. Lemon or lime juice
  3. Onion Juice
  4. Toothpicks or Cotton Swab for writing
  5. Heat source (Hair dryer or toaster oven)
  6. Timer or clock
Lemon Juice


  1. Get a sheet of copy paper and cut it in 4 identical pieces.
  2. In the middle of each paper draw a 3 cell table (shown above)
  3. Use a toothpick and milk to write the word MILK in the upper cell.
  4. Use a toothpick and onion juice to write the word ONION in the middle cell.
  5. Use a toothpick and Lemon juice to write the word Lemon Juice in the lower cell.
  6. Repeat the steps 3 to 5 with all four pieces of paper and allow at least one hour for the papers to dry.
  7. Heat up one of the papers with a hair dryer or toaster oven and record the time it takes for each of the invisible inks to reveal. Record that time in your data table.
  8. Repeat the heating process with the remaining 3 papers (one at a time) and record the results.


Time to reveal

Milk Onion Lemon Juice
Paper 1
Paper 2
Paper 3
Paper 4

To calculate the average time to reveal for each ink you must add the four values you recorded and then divide the total by 4.

Make a bar graph:

Draw 3 vertical bars and name them Milk, Onion and Lemon Juice. The height of each bar will represent the average time to reveal for that specific invisible ink. Write the average reveal time above each bar. Name your graph “Relative Reveal Time for household invisible inks”.

Example: If the average reveal time for onion is 25 seconds, then the height of the bar may be 25 mm or 25 centimeters or any other unit you like.

Materials and Equipment:

  • Onion juice, vinegar, lemon juice, milk
  • toothpicks or paintbrushes
  • pieces of paper
  • a heat source

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


No calculation is required.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

At least 5 different kinds of the invisible ink can be made, using white wine, vinegar, lemon juice, apple juice, milk, iced tea, and orange juice. And if you have time, see if other types of acidic fruit juices also work.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References