Home >> Science & Nature >> Science

Invisible Ink Using Lemon Juice for a Science Project

Lemon juice can be used as invisible ink, because it is clear at room temperature but browns when even a small amount of heat -- such as from a light bulb -- is applied. In order to use this scientific process as part of a science project, you might want to consider utilizing the scientific method in your presentation. To use the scientific method, frame a question as to whether the lemon juice will work as invisible ink, form a hypothesis why, use the procedure outlined here to test your hypothesis, and conclude whether your hypothesis was correct.

Making Lemon Juice Invisible Ink
- Juice a lemon into a bowl, making sure not to allow pulp and seeds to drop into the juice. Any pulp or seeds remain visible on the paper after you write with the "ink" and ruin the invisible effect. Add water to the lemon juice to dilute it -- even just a few drops will do. The water will lighten the juice enough to add to the invisible effect once the "ink" dries, but it will also lighten the message once it's revealed. Stir the water and juice together in the bowl using a spoon.
Writing With Lemon Juice Invisible Ink
- Your writing utensil should be able to soak up enough lemon juice but remain fine tipped enough that you'll be able to make recognizable letters with it. A cotton swab works well, because the small amount of cotton at the end is able to absorb the juice but the swab's shaft isn't absorbent and keeps its shape. A brush also works well. For the clearest, easiest-to-read message after the ink is revealed, write on unlined, white paper. Other light-colored, lightweight paper is also likely to reveal a legible message, but heavier paper such as construction paper, won't work. Write on the paper using lemon juice absorbed onto the cotton swab or brush, and then allow it to dry completely -- by the time it's dry, it should be completely invisible.
Reading Lemon Juice Invisible Ink
- To read the message, hold the paper up to a light bulb. The heat from the bulb will cause the areas where the lemon juice was applied to turn brown, even while the rest of the paper remains white. The message should appear in a short time -- the higher the wattage of the bulb, the faster the message will darken. If you want to speed up the process, you can also fold the paper between two pieces of construction paper or in a brown paper bag, and run an iron over it.
Why It Works
- Lemon juice is an acid. When it's applied to the paper, it breaks down some of the paper molecules into sugars. When heat is applied to those areas where the paper molecules have been broken down into sugar molecules, the sugars caramelize and turn a brown color, which reveals the message.

Science