Using lemon juice as invisible ink is a simple and commonly known method. It works due to the properties of lemon juice and the chemical reactions involved. Here's a basic explanation of how it works:
Acidic Nature: Lemon juice is acidic, containing citric acid. The acidity of the lemon juice makes it a useful substance for invisible ink because it weakens the paper's fibers without damaging it visibly.
Oxidation: The primary process behind the invisibility of lemon juice ink is oxidation. When lemon juice is applied to paper, the carbon compounds in the juice undergo oxidation when exposed to air. Oxidation is a chemical reaction where a substance combines with oxygen, resulting in a color change.
Heat Source: After writing or drawing with lemon juice, the paper needs to be exposed to heat to reveal the hidden message. The heat source, such as an iron or a flame, causes the oxidized lemon juice to darken and become visible.
When the paper is heated, the heat energy breaks down the weakly bonded carbon compounds formed during the oxidation process, resulting in a chemical reaction that darkens the lemon juice. The areas with lemon juice will turn brown or light brown, making the writing or drawing visible against the paper's background.
It's important to note that lemon juice is just one example of an invisible ink. There are other substances, such as milk, onion juice, or certain chemicals, that can also be used as invisible inks, each with its own unique properties and methods of activation.