Cool Invisible Ink Experiment with Lemon Juice

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Learn how to write secret messages with invisible ink! Using commonly found household items such as lemon juice, your kids can send secret messages to friends and family.

Invisible Ink Experiment Lemon Juice Hearts

Lemon juice wears many hats. It delays the oxidation process in fruits and prevents them from turning brown. It decays our tooth enamel when we consume too much of it. And it also mixes with water to become one of the most refreshing drinks you can have on a hot, summer day.

In this invisible ink science experiment, we are going to use lemon juice to write secret messages on paper. Amazing, you don’t need some high tech equipment to create top-secret messages. All you need is a lemon!

How to Write Secret Messages with Lemon Juice

Materials:

  • Lemon juice (we happen to bottled juice, but you can easily just squeeze some juice from a lemon)
  • Small cup or bowl
  • Q-tips or cotton swabs
  • White paper
  • Candle or iron

Instructions:

1. Pour the lemon juice in a small bowl or cup.

Invisible Ink Experiment Lemon Juice

2. Soak the end of the Q-tip or cotton swab in the lemon juice.

Invisible Ink Experiment Lemon Juice Cotton Swab

3. Use the damp Q-tip to write your secret message on a piece of white paper. The writing will be difficult to see, but under good lighting, you should be able to still see it as long as the lemon juice has not dried yet.

Invisible Ink Experiment Lemon Juice Writing

4. Let the lemon juice dry completely.

Invisible Ink Experiment Lemon Juice Dry

5. Reveal the message by adding heat! Here are 4 options:

  • Use an open flame. I recommend using a candle that you can set on a stable surface so you only need to worry about holding the paper. If you are doing this with kids, you might want to take over this part. We don’t want to set the paper on fire or worse. 
  • Use an iron. You may want to place a clean rag between the paper and the iron so that the lemon juice doesn’t get onto the iron. Simply iron the paper and watch the messages appear! Make sure you don’t use the steam setting.
  • Use a heat lamp or incandescent light bulb. A heat lamp or light bulb can emit enough heat to reveal the secret message. 
  • Use the stovetop. You can turn on the stovetop and hold your paper over the hot burner. 

Whatever heat source you decide to use, just use good judgment and a lot of caution. 

We ended up trying both the candles and the iron. Don’t be alarmed, I know I am letting my kids near an open flame. But my kids are very careful and I was monitoring them carefully with a glass of water in my hand. 

Invisible Ink Experiment Candle Flame

We found the iron method to be the best way to reveal the secret messages. With the candle, you can see that the paper got too close to the flame sometimes and the paper turned black. Other times we couldn’t tell if we just didn’t use enough lemon juice, or if we didn’t hold the paper over the flame long enough or close enough.

Invisible Ink Experiment Secret Message

The iron was nice and easy. You just pass the iron slowly over the paper a few times, and the invisible ink showed up. If you feel like the writing was too light, you can try ironing the paper some more to see if it will get darker without worrying about burning the paper. Plus, the paper was nice and flat from the ironing!

Invisible Ink Experiment Iron

The Science Behind the Invisible Lemon Ink

When you painted lemon juice onto paper, the carbon-based compounds got absorbed into the paper’s fibers. The added heat caused some of the chemical bonds to break down, releasing the carbon.

When carbon came into contact with air, it turned brown as the result of oxidation. This is similar to what we have seen in the apple oxidation experiment. Other fruits such as banana, pear, and avocado also darken when you leave them out too long after you cut them.

Science Extensions

What other liquids or solutions can you use to write secret messages? We have tried writing secret messages with a baking solution on marshmallows and it worked like magic. You can also try this experiment with vinegar, milk, honey, orange juice… whatever you have at home!

Did People Actually Use Lemon Juice as Invisible Ink?

Yes! Lemon inks were used in history to deliver secret messages. Monasteries used them to protect trade secrets. German spies during World World I used it to report British troop movements to the Germans using lemon juice.

Since using lemon inks became a well-known way to write secret messages, everyone knew the trick of adding heat to make the writing appear. Since then, invisible inks had progressed far beyond fruit juices.

Final Thoughts on the Invisible Ink Science Experiment

My kids really enjoyed writing secret messages with invisible ink, and I hope yours will too!

This experiment is perfect for Valentine’s Day, Mother’s Day, Father’s Day, and any other holidays when you just want to say “I love you!” All you have to do is write the secret message and ask your loved ones to place the paper over a heat source. Then step back and watch the amazement on their faces when the secret messages show up!


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