Experiment 78

Objective:

To demonstrate how the acidic properties of lemons can be used to make lemon juice act as invisible ink.

Introduction:

Lemons, known for their culinary uses, contain high levels of citric acid, making up about 5% of their dry weight. This acid gives lemons their tart flavor and makes them suitable for various science experiments, including this one, where lemon juice is used as invisible ink. Citric acid is also used in biotechnology, cleaning solutions, and other applications.

Required Materials:

1.    Fresh whole lemons or concentrated lemon juice (not lemonade)

2.    Small bowl

3.    White paper

4.    Small detail paintbrush with fine bristles

5.    Light bulb, sunshine, or other heat source

Procedure:

1.    If using whole lemons, cut them in half and squeeze the juice into a small bowl. If using concentrated lemon juice, pour a small portion into the bowl.

2.    Dip your paintbrush into the lemon juice, ensuring the bristles are covered with your "ink."

3.    Write or draw something on the white paper using the lemon juice. You may need to re-coat the brush several times to complete your message or picture.

4.    Allow the paper to dry for several minutes. Once dry, it will appear as a blank sheet of paper.

5.    To reveal your hidden message, hold the paper close to a heat source (such as a light bulb or other heat-emitting source).

6.    After exposure to the heat, your message or picture will begin to darken into a pale brown color, revealing the hidden content.

Observation:

Lemons are high in acid. When lemon juice is applied to the paper, it weakens the paper fibers. When the paper is heated, the acid reacts and turns brown, unveiling the hidden message or picture.

Questions:

1.    Can you think of another way to reveal the invisible ink without using a heat source? Try using salt and wax crayons to explore different methods of revealing the ink.

2.    Can you think of another liquid that could be used as invisible ink? Other household liquids, like vinegar or milk, could serve as substitutes.

3.    What would happen if you used a different style brush? Would a broader brush change the thickness or appearance of the message?

4.    What if you didn’t have a brush? Could you use something else, like a cotton swab or toothpick, to write or draw?