Content warning: This page contains content that is generally considered disturbing. Also, do not try to make Hydrogen cyanide at home. It is extremely dangerous and often fatal.

Hydrogen cyanide (trivially also known prussic acid, commonly called by its formula HCN) is an extremely toxic, highly flammable, weakly acidic and organic compound that consists of Hydrogen, Carbon and Nitrogen (H-C-=N). With a boiling point of 25 °C, it constantly switches between its liquid and gaseous form. In both of these forms, it is colorless, but supposedly has an almond-like smell. The salts of Hydrogen cyanide are called cyanides, the most well known of them are Sodium cyanide and Potassium cyanide.

Hydrogen cyanide is found in nature, most commonly in certain types of plants. Thankfully, it is not very abundant in the atmosphere, as concentrations of the gas in the air above 300 ppm (0.03%) will definitely result in death. However, clouds of Hydrogen cyanide are known to exist on Saturn's moon Titan. Because of its extreme toxicity, Hydrogen cyanide was used to execute death row inmates in certain countries and as a chemical weapon in World War I. It was also notoriously used in Nazi concentration camps to murder innocent people. The Nazis called the hydrogen cyanide "Zyklon B".

Hydrogen cyanide was first isolated in 1752 by the French chemist Pierre Macquer, who extracted it from the pigment Prussian blue. It is assumed that he was the first victim of cyanide poisoning, as the dangers of hydrogen cyanide were unknown back then. The second isolation took place in 1782, when Swedish-German scientist Walter Scheele also extracted it from Prussian blue. He gave it the name Blausäure (which means Blue acid in English), because it was extracted from a blue pigment. In 1815, French chemist Joseph Louis Gay-Lussac unveiled the formula of hydrogen cyanide: HCN.

Hydrogen cyanide is most commonly created by the reaction of sulfuric acid with a cyanide salt, but it can also be easily made in smaller quantities by wetting cyanide salts with water. Do not try to make Hydrogen cyanide at home. It is extremely dangerous and often fatal.