Water-reactive

Water is usually thought of as the safe way to put out a fire, but for some chemicals it is the problem. Alkali metals such as sodium and potassium, some metal hydrides, calcium carbide and certain powdered metals can react with water and release hydrogen or other flammable gas. The heat from the reaction can be enough to ignite the gas.

Not every water-reactive material is a fire hazard. Some, such as acid chlorides, react with moisture to give off corrosive or toxic fumes. Others generate strong heat and can spatter. The exact behavior depends on the chemical, which is why the SDS is the place to check what actually happens.

Moisture is everywhere, so control is about storage and housekeeping. Keep containers tightly closed, store them in a dry place away from sinks, sprinklers where practicable, and wet areas, and avoid storing them in basements prone to flooding. A damp spill-cleanup material can set off a reaction, so dry absorbents suited to the product are used.

In an emergency, responders need to know in advance that water or foam may be the wrong choice. Dry powder or sand-type agents are common recommendations for metal fires, but the SDS and local emergency plan should say what applies.