Water is typically used to extinguish fires by removing heat from the burning material. However, water-reactive substances, classified as Class 4.3 or noted with a “W” in fire hazard systems, are a major exception. These materials react violently upon contact with moisture or water. The resulting highly energetic reaction often produces a flammable gas, generating enough heat to cause spontaneous ignition or a rapid explosion. Introducing water to these Class D fires intensifies the hazard instead of extinguishing it.
Alkali Metals: The Most Common Igniters
The most famous examples of chemicals that ignite on contact with water are the Group 1 elements, collectively known as the alkali metals. This group includes lithium, sodium, potassium, rubidium, and cesium. They possess a single, loosely held valence electron, making them highly reactive reducing agents eager to form stable bonds.
Sodium and potassium are the two alkali metals most commonly featured in demonstrations of this extreme reactivity. When sodium metal is dropped into water, it melts and skitters across the surface, typically igniting with a bright yellow flame. Potassium is more vigorous, often igniting instantly with a lilac flame. The intensity of the reaction increases down the group, with cesium reacting explosively even with water vapor in the air.
The Chemical Mechanism of Ignition
The ignition process begins with a rapid oxidation-reduction reaction between the metal and the water molecule (\(\text{H}_2\text{O}\)). The metal atom loses an electron to the water, displacing a hydrogen atom. This results in the formation of a metal hydroxide and the liberation of gaseous hydrogen (\(\text{H}_2\)).
This chemical process is intensely exothermic, releasing a substantial amount of heat energy almost instantaneously. This rapidly generated heat raises the temperature of the hydrogen gas past its autoignition point. The hydrogen gas then ignites in the presence of atmospheric oxygen, creating the visible flame or explosion. For more reactive metals like potassium, the heat released is great enough to cause the metal itself to melt and sometimes ignite simultaneously with the hydrogen gas.
Other Types of Water-Reactive Materials
While alkali metals are the classic examples, many other chemical classes react violently with water, sometimes producing different hazardous byproducts.
Metal Hydrides
Metal hydrides, such as lithium aluminum hydride (\(\text{LiAlH}_4\)) or calcium hydride (\(\text{CaH}_2\)), are powerful reducing agents. They react with water to release large volumes of flammable hydrogen gas. These compounds require strictly dry handling conditions for chemical synthesis.
Organometallic Compounds
Organometallic compounds, including aluminum alkyls and alkyllithiums, involve a carbon-metal bond. These substances are often pyrophoric, spontaneously igniting upon exposure to air, and their reaction with water is extremely violent.
Other Reactive Materials
Certain metal phosphides, like aluminum phosphide, react with water to release highly toxic phosphine gas (\(\text{PH}_3\)), which presents a severe respiratory hazard. Non-metal halides, such as phosphorus trichloride (\(\text{PCl}_3\)), react with water to produce corrosive acids and heat.
Safe Handling and Emergency Response
Managing water-reactive materials requires stringent safety protocols focused on eliminating contact with moisture. These substances must be stored in airtight containers under an inert atmosphere, such as argon or nitrogen gas, or immersed in a protective solvent like mineral oil or kerosene. Storage locations must be kept strictly dry, away from sinks, water baths, and overhead sprinkler systems.
In the event of a fire, it is necessary to avoid using water or carbon dioxide extinguishers, as both will exacerbate the reaction. Specialized fire suppression materials must be used, such as a Class D fire extinguisher, which contains a dry powder like sodium chloride or graphite. Small spills or fires can be smothered using inert materials like powdered lime, soda ash, or dry sand. All work with these substances should be done inside a glove box or a fume hood to maintain an inert environment.