Water-reactive substances

Water-reactive substances are those that spontaneously undergo a chemical reaction with water, often noted as generating flammable gas. Some are highly reducing in nature. Notable examples include alkali metals, lithium through caesium, and alkaline earth metals, magnesium through barium.

Some water-reactive substances are also pyrophoric, like organometallics and sulfuric acid. The use of acid-resistant gloves and face shield is recommended for safe handling; fume hoods are another effective control of such substances.

Water-reactive substances are classified as R2 under the UN classification system and as Hazard 4.3 by the United States Department of Transportation. In an NFPA 704 fire diamond's white square, and in similar contexts, they are denoted as "W". The classification of substances as water-reactive is largely a consideration for the safety of firefighting and transportation operations.

All chemicals that react vigorously with water or liberate toxic gas when in contact with water are recognized for their hazardous nature in the "Approved Supply List", or the list of substances covered by the international legislation on major hazards many of which are commonly used in manufacturing processes.