Substances categorized as “dangerous when wet” represent a serious hazard when they encounter moisture, including water, humidity, or even damp air. These materials are chemically unique because their interaction with water is a rapid and highly energetic reaction, not a simple dissolution. The danger stems from the swift and often violent production of heat, fire, or flammable and toxic gases. Understanding this reactivity is important for public safety, especially for those involved in the transport, storage, or handling of these chemicals.
Defining Water-Reactive Substances
Regulatory bodies worldwide, including the U.S. Department of Transportation (DOT) and the United Nations (UN) Model Regulations, classify these substances under Division 4.3. This designation groups them with other flammable solids and is based on their behavior upon contact with water. A substance is officially classified as Division 4.3 if it is liable to become spontaneously flammable or if it releases flammable or toxic gas at a dangerous rate when exposed to water.
The specified rate for this classification is a gas evolution exceeding one liter per kilogram of the material per hour, as determined by standardized testing. This criterion separates materials that simply react with water from those that pose a risk of fire or explosion during transport or storage. The classification emphasizes the immediate threat posed by the rapid production of gaseous products.
The Chemical Reactions That Cause Danger
The mechanism for the “dangerous when wet” hazard is hydrolysis, where the substance reacts with water molecules. This reaction is intensely exothermic, meaning it releases a significant amount of heat energy very quickly. This rapid release of heat drives the danger associated with these materials.
The heat generated by the reaction is often sufficient to cause the spontaneous ignition of the gases being produced. For many of these substances, the primary gaseous product is highly flammable hydrogen gas, liberated through the displacement of hydrogen from the water molecule. When hydrogen gas mixes with air, it forms an explosive mixture that can be easily ignited by the reaction’s own heat or a nearby spark.
In other cases, the reaction may liberate a different flammable gas, such as acetylene, or a highly toxic gas like phosphine. The danger is amplified because the water itself acts as the fuel for the reaction, making traditional water-based fire suppression methods ineffective and dangerous. The violent nature of the reaction can also create a blast wave, spreading the material and increasing the risk of a widespread fire or explosion.
Commonly Encountered Dangerous When Wet Chemicals
Several groups of compounds exhibit water-reactivity, often appearing in industrial processes or specialized laboratory settings.
- Alkali metals, such as sodium and potassium, react vigorously with water. They displace hydrogen and form a metal hydroxide, with the resulting hydrogen gas frequently igniting due to the intense heat.
- Metal hydrides, compounds containing hydrogen bonded to a more electropositive element, include lithium aluminum hydride and sodium hydride. These react with water to release hydrogen gas, often violently or explosively.
- Metal carbides, specifically calcium carbide, generate acetylene gas upon contact with water. Acetylene is highly flammable and prone to forming explosive mixtures with air.
- Metal phosphides, like aluminum phosphide, react with moisture to release toxic phosphine gas. Phosphine is a severe inhalation hazard in addition to being flammable.
Essential Safety Protocols and Emergency Response
Storage and Handling
The primary safety protocol for water-reactive substances is to ensure they are stored and handled in an environment entirely free of moisture and humidity. Storage containers must be hermetically sealed and maintained in a cool, dry location, often within a secondary containment system to prevent accidental contact with water. Alkali metals, for instance, are commonly stored submerged under mineral oil or kerosene to shield them from atmospheric moisture and oxygen.
Handling these materials frequently requires an inert atmosphere, such as a glove box filled with nitrogen or argon gas, to eliminate any possibility of reaction with ambient humidity. Specialized personal protective equipment, including fire-resistant clothing, is necessary when working with these compounds. All glassware and equipment used must be meticulously oven-dried beforehand to remove any trace of water.
Emergency Response
In the event of a fire involving these substances, water must never be applied, as it will only accelerate the reaction and intensify the blaze or explosion. Specialized Class D dry powder fire extinguishers are required for metal fires, as they work by smothering the burning metal and absorbing the heat. For other water-reactive materials, a standard dry powder (ABC) extinguisher or dry sand may be appropriate for suppression. Immediate evacuation and calling emergency services are the first steps, allowing trained professionals to manage the unique hazards of the reactive material.