chemical joke

Acid-base indicators are compounds that turn different colors depending on the pH of the medium. From the numerous substances of this type, we will choose a pair that will allow you to conduct a seemingly impossible experiment.

Some colors are created when we mix other colors together. But will we get blue by combining red with red? And vice versa: red from a combination of blue and blue? Everyone will definitely say no. Anyone, but not a chemist, for whom this task will not be a problem. All you need is an acid, a base, a Congo red indicator, and red and blue litmus papers.. Prepare acidic solutions in beakers (eg by adding a little hydrochloric acid HCl to water) and basic solutions (sodium hydroxide solution, NaOH).

After adding a few drops of Congo red solution (photo 1), the contents of the vessels change color: the acid becomes blue, the alkaline red (photo 2). Dip the blue litmus paper into the blue solution (Pic 3) and remove the red litmus paper (Pic 4). When immersed in a red solution, red litmus paper (photo 5) changes its color to blue (photo 6). Thus, we have proved that a chemist can do the "impossible" (photo 7)!

The key to understanding the experiment is the color changes of both indicators. Congo red turns blue in acidic solutions and red in alkaline solutions. Litmus works the other way around: it is blue in bases and red in acids.

Immersion of blue paper (a napkin soaked in an alkaline solution of litmus; used to determine an acidic environment) in a solution of hydrochloric acid changes the color of the paper to red. And since the contents of the glass were blue (the effect of adding Congo red first), we can conclude that blue + blue = red! Similarly: red paper (blotting paper impregnated with an acidic solution of litmus; it is used to detect an alkaline environment) in a solution of caustic soda turns blue. If you previously added a solution of Congo red to the glass, you can record the effect of the test: red + red = blue.