Hidden Messages
From The Potion Laboratory
When we visited an old potion laboratory, we started wondering:
How did people hide secret recipes, coded messages and important discoveries before computers existed?
Some historians believe invisible inks were used by spies, inventors and explorers for centuries.
Today you’ll investigate one yourself.
5-12 yrs
Easy
20
min
Stage 2, Stage 3
Mission Briefing.
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Mackey
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Hidden Messages
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NESA Accredited Teacher
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High school chemistry & physics specialist 30+ years
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The Crazy Scientist in primary schools — 15 years
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International conference presenter on science education
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Creator of the LAB™ Learning System
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Curriculum aligned: NSW Science & Technology K–6 (2024)
A picture is worth a thousand words — check this out and see if you can spot the science hiding in plain sight.
Mission Equipment
• Lemon juice (or white vinegar, apple juice, or milk)
• Cotton buds or small paintbrush
• White paper
• Heat source: lamp with incandescent bulb, or iron set to low, or oven at 100°C
• Adult supervision for heat source
Let’s Investigate
1
Write your message
Dip a cotton bud in lemon juice. Write a word or draw a simple picture on white paper.
Use deliberate, clear strokes — the juice is hard to see as you write.
3
Make your prediction
Before you apply any heat — write down what you think will happen.
Will the writing appear slowly or all at once? Will it be light or dark?
Which part of the paper do you think will change first? Commit to your prediction before you find out.
5
Watch the magic
Within 1–2 minutes, the writing should begin to appear as brown marks.
Watch where the colour first develops and how it spreads.
7
Challenge
Write a message to swap with a partner.
Don't tell them where it's written on the page.
2
Let it dry
Set the paper aside for 5 minutes until the lemon juice is fully dry and invisible.
Test that you can't see it before proceeding.
4
Apply gentle heat
Hold the paper about 10–15 cm from an incandescent light bulb, or iron it on a low setting with no steam, or place in an oven at 100°C for 3–4 minutes.
6
Test other inks
Repeat with white vinegar, apple juice, and milk.
Does each develop at the same speed and colour intensity?
Did it work? Share the science! Tag @the_crazy_scientist on Instagram — we love seeing your experiments!
The Crazy Scientist LAB Learning System™
Every experiment follows The Crazy Scientist Lab Learning System™ — a simple way to help kids think like real scientists.
We
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LINK to what they already know,
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ACTIVATE curiosity through hands-on discovery
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BUILD understanding that actually sticks.
Think about what happens when you toast bread. The outside turns brown — the inside stays white. The brown isn’t the same material as the white bread — something has chemically changed.
Heat triggered a reaction that can’t be undone. Now imagine you could write a message with something that looks exactly like water — completely invisible at room temperature.
Ancient Romans knew it.
WWII intelligence agencies relied on it. What could possibly make writing appear from nothing?
Watch where the message develops first when you apply heat.
Is it evenly distributed, or does it appear in some places before others?
Compare your different 'ink' tests.
Which revealed most clearly? Which was most invisible before heating?
Try heating a piece of blank paper with no lemon juice. What happens? Now compare that to your message paper. What’s the difference?
Which ink worked best — lemon juice, vinegar, or milk? Which surprised you most? What do you think those liquids have in common?
The blank paper didn’t change — but the lemon juice did. What does that tell you about the difference between the two? What does the lemon juice have that plain paper doesn’t?
Your message appeared before the paper burned. What does that tell you about which material reacts to heat first? Why is that important for the trick to work?
If you left your secret message in a hot car in summer, could it self-reveal? What does your experiment tell you about the temperature needed?
Ancient spies also hid messages in other ways: steganography (hiding text inside normal text), microdots (photographs shrunk to the size of a full stop), and coded languages. What do these methods have in common with invisible ink?
"Want the full teacher guide? The Crazy Scientist Lab includes classroom delivery tips, how to manage the WOW moment, differentiation for Stage 2 & 3, — ready to teach tomorrow."
Think Like a Scientist
Scientists don't just do ONE experiment; they change one part of the experiment (independent variable) and then see how it affects another part of the experiment
(dependent variable)
Change ONE variable and test again.
Ink concentration — full-strength lemon juice vs diluted 1:3 vs diluted 1:10. How does concentration affect visibility before and after heating?
Paper type — plain white paper vs newspaper vs cardboard. Which substrate gives the clearest result?
🧪 Try it! Change ONE thing and test again. What did you discover?
Dr Puddledrip’s Science Tip
Want to go deeper? Tap a section below to explore. ▼
The Science Behind It
What’s Really Happening?
When lemon juice dries on paper, it doesn’t disappear.
The liquid becomes almost invisible, leaving behind tiny amounts of natural substances from the lemon.
At first, you can’t see them.
But when the paper is warmed, something begins to change.
The hidden writing suddenly becomes visible.
Why Does Heat Reveal The Message?
Heat acts like a secret key.
As the paper warms, the materials left behind by the lemon juice change colour faster than the paper around them.
That’s why the hidden message appears before the paper itself starts to brown.
Scientists call this a chemical change because new substances are formed.
Could Other Liquids Work Too?
What do lemon juice, apple juice, milk and vinegar have in common?
Some can also be used to create invisible messages.
Before testing them, make a prediction:
Which one do you think would reveal the clearest secret message?
Real-World Connection
For hundreds of years, people have searched for ways to hide information.
Secret messages have been used by explorers, inventors, spies and code-breakers.
Even today, scientists and engineers use special materials that only become visible when the right trigger is applied.
Think Like A Scientist
You discovered that heat can reveal a hidden message.
But what if you changed one variable?
Would orange juice work better than lemon juice?
Does stronger lemon juice create a darker message?
Does the message appear faster with more heat?
Which liquid creates the best invisible ink?
Scientists don’t stop after one result.
They ask the next question.
Try next
• Explore another irreversible chemical reaction that permanently changes a material → [The Mummy Maker]
• See how chemistry from the ancient world was put to practical use → [The Aqueduct Challenge]
Extension: G&T Years 5 & 6
What makes a chemical reaction irreversible?
When you heated your lemon juice message, the organic compounds in the juice broke down into new substances — dark, carbon-rich compounds. Those new substances are fundamentally different from the original ones. You cannot heat them back and get lemon juice again.
This is what makes it an irreversible chemical reaction: the original substances are gone and cannot be recovered
Vocabulary
Irreversible reaction
A chemical change that creates new substances that cannot be changed back. Once lemon juice has turned brown on paper, it cannot be made invisible again.
Citric acid
A natural acid found in lemons, oranges, and other citrus fruits. It is one of the ingredients in lemon juice that reacts when heated.
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READY TO TEACH THIS
TOMORROW?
Running the experiment is easy; however, teaching it well is another challenge.
Teachers often ask:
How do I adapt this for Stages 1,2 or 3?
What do I do with fast finishers?
What misconceptions will they have?
How do I structure this for a full class?
What syllabus outcomes does it cover?
What do I say when they ask WHY?
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