Phenomena: Yellow writing will appear on the white paper.

Because lemon juice contains a component called cellulol, writing it on paper can reduce the ignition point of the paper. As long as it is roasted with fire, the part where the lemon juice is written will be burned first. The words that appear yellow will appear. This phenomenon is called carbonization reaction. Some people have used this principle a long time ago to transmit secret letters and periodicals.

Extended information

1. Process conditions of "carbonization":

Dehydration carbonization refers to the removal of other elements from organic matter to leave carbon. For example, concentrated sulfuric acid has a strong dehydration ability. When it comes into contact with organic matter, it removes the hydrogen and oxygen elements in the organic matter according to the composition ratio of water (2:1), leaving black carbon, thereby carbonizing the other party. What is actually removed is hydrogen and oxygen, which does not mean that the substance contains water.

2. Other effective substances:

Other acidic substances that can be used as invisible ink include orange juice, grape juice, onion juice, vinegar and wine. Although milk is not acidic, it can also be used to create invisible messages because milk heats slowly and therefore turns brown when heated.

Baidu Encyclopedia--Carbonization