there are many versions of the experimental demonstration of "invisible text" based on the color change of chemical reaction, but generally more of them show one color at a time. Today, I would like to introduce two versions that can display two colors at the same time with a "chromogenic solution".

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the first is a blue + orange-red version:

(source: FlinnScientific)

the solution sprayed here is ferric trichloride, and the solution used to write is potassium thiocyanate and potassium ferricyanide. When thiocyanate encounters trivalent iron, it produces the familiar red complex (it looks orange when the solution layer is thinner), while the blue part produces Prussian blue (ferrous ferricyanide, potassium ferricyanide + trivalent iron or potassium ferricyanide + divalent iron). It is also the blue you see in the blue).

the other is my own blue + pink version:

this is the use of acid-base indicator, pink is phenolphthalein, blue is thymol phthalein, write to dry and immerse in dilute sodium hydroxide solution. There are many indicators that can change color through pH changes, but the range of options for designing invisible text is much narrower because it needs to be colorless or light before the change. Phenolphthalein and thymol phthalein are colorless under acidic and neutral conditions, and show color in case of alkali, so they are more suitable.

(here the color of thymol is lighter, because I wrote it directly with the "vanishing ink" left before, and I didn't buy the reagent _ (: disappear "∠) _)

as a demonstration experiment, invisible text still has a lot of room for change and creativity, if you find a better way to operate, you can also tell me ~