Pure water H₂O can turn into a poisonous gas through a chemical reaction with certain substances. For instance, when water comes into contact with a reactive metal like sodium, it can produce hydrogen gas and sodium hydroxide. If hydrogen gas is further exposed to chlorine, it forms hydrogen chloride, which can vaporize into a toxic gas. This transformation involves a series of chemical interactions where the initial harmless water molecule reacts with other elements to create a hazardous gas under specific situation.