What’s Really Happening?

When lemon juice dries on paper, it doesn’t completely disappear.

The water evaporates, but tiny amounts of substances from the lemon are left behind in the paper fibres. Once dry, there may not be enough of these substances to see.

But when the paper is warmed, something begins to change.

The hidden writing becomes darker and the secret message appears.

Why Does Heat Reveal the Message?

Heat acts like the secret key.

Lemon juice contains organic compounds — substances containing carbon that came from the living lemon.

When these compounds are heated, they begin to break down

 and react with oxygen. This causes them to brown faster than the surrounding paper.

The darker areas reveal where the lemon juice was painted.

This is a chemical change because heating produces new substances with different properties.

Could Other Liquids Work Too?

Lemon juice isn’t the only liquid that can leave a hidden message.

Apple juice, milk and some other liquids contain organic substances that can also change colour when heated.

But they don’t necessarily behave in exactly the same way.

That gives us something to investigate:

Which liquid produces the clearest invisible message?

Scientists can compare:

• how quickly each message appears

• how dark it becomes

• how easy it is to read

Keeping everything else the same makes it a fair test.

Real-World Connection

For hundreds of years, people have searched for ways to hide information.

Invisible inks and secret messages have been used by explorers, inventors, spies and code-breakers.

Different invisible inks work in different ways. Some are revealed by heat, while others need a particular chemical or type of light to make the message appear.

Think Like a Scientist

You discovered that some apparently invisible liquids can leave substances behind on paper that become visible when heated.

But one investigation usually creates another question.

• Which liquid makes the darkest message?

• Does the amount of liquid change how clearly it appears?

• Does the type of paper make a difference?

• Does heating time affect the result?

Scientists don’t stop after one result.

They use their evidence to 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]