Did you know you can write secret messages with just lemon juice? In this classic invisible ink experiment, kids learn how heat causes a chemical change that reveals hidden writing. Explore basic chemistry and oxidation while having fun with a simple kitchen science project!
Secret Writing with DIY Invisible Ink
Learn how to make invisible ink easily with this fantastic science experiment and have a little fun with chemistry and biology.
Science Concepts Explored:
Grade Level: Grades 1–6
How To Make Invisible Ink
Leftover lemons? Try this apple oxidation experiment, a lemon volcano, lemon battery or even make fizzy lemonade for kitchen science!
Supplies:
- Lemon juice
- Iron
- Paintbrush
- Paper
- Small bowl
- Towel
DIY Invisible Ink Instructions
Watch The Video:
STEP 1: Cut a lemon or pour juice into a small bowl.
STEP 2: Using your paintbrush, paint a secret message with the lemon juice onto your paper.
STEP 3: Let your message dry.
STEP 4: With adult help or supervision, press the hot iron onto the paper where your message is written.
Hold for 15 seconds and lift to reveal the secret message!
💡 Love solving codes? Also, be sure to check out our secret decoder ring activity.
How Does Invisible Ink Work?
Lemon juice makes a great invisible ink because it is made from organic material. Organic materials, like lemon juice, contain carbon compounds. At room temperature, the lemon juice on the paper looks clear. However, when you heat the paper with a warm iron or light bulb, something interesting happens!
The heat breaks down the carbon compounds in the lemon juice and releases carbon. Then the carbon reacts with oxygen in the air, a process called oxidation. This chemical change makes the writing turn brown, and your secret message appears!
A Little History of Invisible Ink
People have been using invisible ink for over 2,000 years! The Ancient Greeks and Romans were some of the first to write secret messages this way. Later, during World War I and World War II, spies used invisible ink to pass important information. They even hid the tools needed to reveal the messages!
In the past, some recipes for invisible ink were so important that governments kept them top secret.
Different Types of Invisible Ink
There are many different kinds of invisible ink! Some common types you can find at home include:
- Lemon juice 🍋
- Apple juice 🍎
- Onion juice 🧅
- Vinegar
- Milk 🥛
- Cola 🥤
These are all organic invisible inks. When heated, they burn at a lower temperature than the rest of the paper, causing the writing to turn brown while the paper stays mostly the same.
Other invisible inks are called sympathetic inks. These special inks are made with chemicals and need a certain “reagent” (another chemical) to reveal the secret message. For example, you can make cranberry secret messages that appear with a baking soda solution!
What Is the Best Invisible Ink?
💡 Lemon juice is one of the easiest and best invisible inks to use. Learn about the scientific method, writing a hypothesis, and choosing variables here.
Turn It Into a Science Project!
Question:
Which liquid makes the best invisible ink?
Hypothesis:
Example: “If I use lemon juice, then it will create a darker, clearer secret message compared to other liquids like milk or vinegar.”
Materials:
- Lemon juice, milk, vinegar, apple juice
- Cotton swabs or small paintbrush
- White paper
- Iron or light bulb for heat source
Procedure:
- Write the same message with each type of liquid on different pieces of paper.
- Let the papers dry completely.
- Carefully heat each paper one at a time using an iron (with adult help) or a light bulb.
- Observe which message appears the darkest and clearest.
Observation and Data:
Record how long it takes for each message to appear and how dark each one gets.
Conclusion:
Decide which liquid made the best invisible ink based on your results!
More Science Experiments with Lemons
Printable Science Projects For Kids
If you’re looking to grab all of our printable science projects in one convenient place plus exclusive worksheets and bonuses like a STEAM Project pack, our Science Project Pack is what you need! Over 300+ Pages!
