A treasure map with invisible ink

Invisible ink is very useful for drawing treasure maps or writing secret messages. Only those who know chemistry will find a way to reveal your message. Let’s use onion juice to prepare the invisible ink.

You will need:

From 6 years

Difficulty : medium

This experience requires the help of an adult

Let's experiment

Peel your onion and cut a quarter of it into very small pieces.

With the garlic press, mash the onion pieces and collect the juice drops in a bowl.

With a toothpick, draw or write the messages you like using onion juice like ink.

When finished, let your sheet of paper dry for 5 to 10 minutes.

With the help of an adult, turn on an electric hot plate and allow it to heat for 2 to 3 minutes.

Then ask an adult to position the sheet on the heater plate and observe what is happening.

Et voilà 😊 Nous, nous avons fait une carte au trésor.

Understand the experiment

It smells like onion!

Onion juice, but also lemon juice or vinegar, are organic compounds. They are rich in acid.

At room temperature, these juices are colorless. Under the action of heat, these juices oxidize and decompose. That causes a color change. From invisible, the ink becomes brown or black. It’s your turn to be the spy!

You can use other heat sources like a candle to make your drawings appear worthy of spy movies.

Did you know?

The first secret messages in invisible ink date from Ancient Greece (-500 BC)

Spies used invisible ink during the First World War or in magic tricks. But the very first to have used it were the ancient Greeks and Romans who used the milk of a plant, the tithymalus. There are several invisible inks based on acids or based on the use of chemicals.