The question of whether fire can burn underwater challenges our basic understanding of combustion, which is typically an open-air reaction. Under normal circumstances, fire and water are fundamentally incompatible due to the physical and chemical requirements of burning. However, specialized chemical mixtures can bypass the need for atmospheric oxygen entirely. These reactions carry their own oxidizing agents, allowing them to sustain a high-temperature reaction even when fully submerged.
The Essential Components of Combustion
Combustion, the process we commonly call fire, requires the simultaneous presence of three components: fuel, heat, and an oxidizing agent. Fuel is any material that can burn, such as wood, paper, or gas. Heat supplies the activation energy needed to raise the fuel to its ignition temperature, starting the chemical reaction. The reaction is exothermic, meaning it produces more heat than it consumes, allowing the process to become self-sustaining. For conventional fire, the oxidizing agent is overwhelmingly the oxygen gas found in the Earth’s atmosphere.
Why Water Stops Normal Fire
Water is an effective agent for fire suppression because it attacks two components of combustion. The primary mechanism is cooling, where water absorbs heat from the burning material. Water has a high specific heat capacity, meaning it absorbs a large amount of thermal energy before its own temperature rises significantly.
The majority of the cooling power occurs when liquid water turns into steam, a process requiring a massive input of energy called the heat of vaporization. This energy is rapidly drawn away from the burning fuel, quickly lowering the temperature below its ignition point. If the temperature drops too low, the combustion reaction cannot sustain itself and stops.
The secondary mechanism is smothering, which interferes with the supply of the oxidizing agent. As water vaporizes into steam, its volume expands by roughly 1,700 times, creating a dense, inert gas cloud. This expansive steam displaces the surrounding air, effectively pushing oxygen away from the fuel’s surface. By blocking the oxygen supply and drawing away heat, water ensures that a standard fire quickly collapses.
The Chemistry Required for Submerged Combustion
For fire to occur underwater, the reaction must bypass the need for atmospheric oxygen and overcome water’s cooling effect. This is accomplished through two distinct chemical pathways involving either an integrated oxygen source or an extremely reactive fuel.
Pathway 1: Internal Oxidizers
This pathway utilizes materials that contain their own oxidizer, chemically bound within the fuel mixture. Specialized compounds, such as potassium perchlorate or potassium nitrate, are solid chemicals that release oxygen when heated. By mixing a fuel, often a fine metal powder like magnesium or aluminum, directly with this solid oxidizer, the combustion system becomes self-contained. The reaction sustains itself by supplying its own oxygen internally, making the surrounding water irrelevant.
Pathway 2: Extreme Reactivity
The second pathway relies on the extreme reactivity of certain elements, primarily alkali and alkaline earth metals. Metals like sodium, potassium, and magnesium react violently with water itself, using the water molecule (H₂O) as the oxidizer. This reaction strips the oxygen from the water, creating a metal hydroxide and releasing highly flammable hydrogen gas as a byproduct. The reaction generates enormous heat, which ignites the newly produced hydrogen gas, often resulting in explosive combustion. In the case of magnesium, the reaction is so intensely exothermic that the metal continues to burn even when submerged, as the heat prevents the water from extinguishing it.
Real-World Applications of Underwater Fire
The principles of submerged combustion are applied in devices designed to function in marine and military environments. Marine flares, designed to remain lit in wet conditions, employ the internal oxidizer concept. They are composed of powdered metal fuels, such as magnesium or aluminum, mixed with an oxygen-rich salt like potassium perchlorate. Once ignited, this chemical paste generates enough heat to sustain the reaction and boil the surrounding water, creating a visible flame.
Another example is the thermite reaction, a mixture of a metal fuel (usually aluminum) and a metal oxide (like iron oxide). The iron oxide serves as the internal oxidizer, allowing the reaction to produce temperatures exceeding 2,500 degrees Celsius. Thermite is used for specialized applications like underwater welding, where the extreme heat is necessary to melt and fuse metals despite the water’s cooling effect.
The explosive reaction of highly reactive metals is also utilized in certain military and industrial contexts. This intense, water-fueled burning highlights how engineered chemistry can overcome the limitations of a water environment.