Materials
- Lemon or lemon juice
- Cotton swab or paintbrush
- White paper
- Heat source (like a lamp or hair dryer)
- Small bowl or cup
Procedure
- Squeeze the lemon juice into a small bowl or cup.
- Dip the cotton swab or paintbrush into the juice.
- Use the swab to write a message or draw a picture on the white paper.
- Let the paper dry completely until the writing is invisible.
- To reveal the message, hold the paper near a warm lamp or gently use a hair dryer. Watch your secret message appear!
Choose Your Learning Level
Elementary
Lemon juice looks clear at first, but when it heats up, it turns brown. That helps your hidden writing show up!
Discussion Questions
- Why does the writing appear after heating?
- Can you use other juices or liquids to make invisible ink?
- What other ways can you reveal the hidden message?
Middle School
Lemon juice is slightly acidic and weakens the paper. When heated, the acid reacts and oxidizes, causing the writing to appear darker than the paper.
Discussion Questions
- Why does acid change color when heated?
- What are other examples of oxidation in real life?
- How can you design an invisible ink that doesn't need heat?
High School
This experiment illustrates acid-catalyzed oxidation. As the lemon juice dries, the sugars and acids remain. Heat accelerates their oxidation, causing browning similar to caramelization or Maillard reactions seen in cooking.
Discussion Questions
- Describe the chemical changes that occur when lemon juice is heated.
- How does this experiment model acid-catalyzed oxidation?
- What factors affect the visibility and speed of the reaction?