Hydrogen cyanide
Common salts such as sodium cyanide are involatile but water soluble, so are poisonous by ingestion. Hydrogen cyanide is a gas, making it more indiscriminately dangerous, however it is lighter than air and rapidly disperses up into the atmosphere, which makes it ineffective as a chemical weapon. Poisoning by hydrogen cyanide is more effective in an enclosed space, such as a gas chamber. Most significantly, hydrogen cyanide released from pellets of Zyklon-B was used extensively in the extermination camps of the Holocaust.