Steganography is to borrow a special ink to achieve the purpose of invisible writing. This special ink is not high-tech, although most people don't know it, but the source is very convenient. In fact, it is alum water that is common in people's lives. Handwriting written in alum water can't be seen on paper after drying, but if it is immersed in water, it will suddenly appear. The principle of displaying characters is as follows:

The chemical composition of alum is potassium aluminum sulfate dodecahydrate KAL (SO4) 2. 12H2O. When it is dissolved in water, the ionization equation is:

Potassium sulfate = K﹢+Al 3+2SO4 2.

However, Al3- is easy to hydrolyze, and colloidal aluminum hydroxide Al(OH)3 will be generated.

Al3﹢ +3H2O？ Aluminum hydroxide+3H﹢

Writing Al(OH)3 colloid with aluminum hydroxide after drying is equivalent to covering paper with an impermeable film. When paper is immersed in water, it is not easy to absorb water where alum water is stuck, but the surrounding paper is easy to absorb water, which shows the handwriting.