You are here
⚠️
Caution
— Perform this activity under the supervision of an adult.
- Objective — Discover a chemical change that "writes" with heat.
- What to do — Write a message on a piece of paper using lemon juice as ink and let it dry. The message will become invisible.
- Reveal step — Now use a warm iron over the paper (or hold the paper carefully over a candle flame, taking care that it does not catch fire).
- Observation — The invisible letters turn dark brown as the paper warms.
- Reflect — Lemon juice is an acid. When heated, it reacts with the paper (which is mainly carbon-rich) and "browns" before the rest of the paper. Can these changes be reversed?
- Type of change — The browning is a chemical change (new substances formed in the paper); it cannot be reversed.
