A water-reactive substance is a chemical that reacts dangerously with water or moisture, releasing flammable or toxic gas or enough heat to cause fire or explosion.
A water-reactive substance is a chemical that reacts with water, including moisture in the air, in a way that creates a hazard. The reaction may release flammable gases, such as hydrogen or acetylene, toxic or corrosive gases, such as phosphine or hydrogen chloride, or enough heat to ignite nearby materials or boil water violently.
Common examples include alkali metals such as sodium, potassium and lithium, metal hydrides such as sodium hydride and lithium aluminum hydride, calcium carbide, aluminum and zinc phosphide fumigants, some magnesium and aluminum powders, and reactive chemicals such as thionyl chloride, chlorosulfonic acid and acid chlorides. Concentrated acids such as sulfuric acid also release large amounts of heat when mixed with water.
Water-reactive substances are a particular concern in emergencies, because water is the usual first choice for firefighting and spill cleanup.
Under the Globally Harmonized System and OSHA's Hazard Communication Standard, chemicals that release flammable gases in contact with water are classified as substances and mixtures which, in contact with water, emit flammable gases. Category 1 chemicals carry hazard statement H260 and Category 2 and 3 chemicals carry H261. Information on other reactions, such as release of toxic gases, appears in section 10 (stability and reactivity) of the safety data sheet.
On the NFPA 704 diamond, a W with a line through it in the white special hazards section indicates a material with unusual reactivity with water.
The U.S. Department of Transportation classifies materials that become spontaneously flammable or release flammable or toxic gases when wet as Division 4.3, Dangerous When Wet, with a blue placard.
Fires involving combustible metals need Class D extinguishing agents designed for the specific metal. Water, carbon dioxide and many standard extinguishers can make metal fires worse. Spills should be covered with a dry, compatible material, such as dry sand or a specialized absorbent, and kept away from water. Responders need to know whether toxic gases may be released and use suitable respiratory protection. Pre-planning with the local fire department is important for sites that store significant quantities.
A university chemistry lab used small amounts of sodium metal for drying solvents. A graduate student rinsed a flask with water without realizing that small pieces of sodium remained inside. The sodium reacted, releasing hydrogen that ignited and caused a small fire in the sink, which was put out with a Class D extinguisher.
The lab's review found that the hazard was listed on the SDS but not explained in the lab's procedures. The lab wrote a step-by-step procedure for quenching sodium residues with alcohol before any contact with water, added warning labels to flasks used with reactive metals and included water-reactive hazards in annual lab safety training.
No. Water can make the fire worse or cause an explosion. Use the extinguishing agent recommended in the SDS, such as a Class D agent for combustible metals.
Check the label and SDS for hazard statement H260 or H261, sections 5 and 10 of the SDS, the NFPA 704 rating and any DOT Division 4.3 marking.
Some are. Pyrophoric materials ignite on contact with air, and many of them also react with water. The two hazards are classified separately.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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