Fire and water often seem like fundamental opposites, a classic pairing of elements that cannot coexist. The common understanding is that water extinguishes fire, making the idea of a flame burning beneath the surface seem impossible. This perception holds true for most everyday fires, yet there are exceptions where combustion can indeed occur underwater.
What Fire Needs to Burn
The “fire triangle” states that three components must be present for fire to ignite and sustain itself: fuel, heat, and an oxidizer. Fuel is any combustible material. Heat provides the energy required to raise the fuel to its ignition temperature. The oxidizer, typically oxygen from the surrounding air, supports combustion by reacting with the fuel. If any one of these three elements is removed, the fire cannot start or will quickly be extinguished.
Why Water Stops Most Fires
Water is an effective extinguishing agent because it primarily targets two elements of the fire triangle: heat and oxygen. When water is applied to a fire, it absorbs heat from the burning materials. This cooling action reduces the fuel’s temperature below its ignition point, preventing further combustion. As water heats up and turns into steam, it expands, displacing the air around the flames. This steam creates a barrier that prevents oxygen from reaching the fuel, smothering the fire.
When Fire Burns Underwater
Fire can burn underwater when the need for atmospheric oxygen is bypassed. This usually involves a fuel that contains its own oxidizer or a process that supplies an oxidizer directly. One example is the thermite reaction, which involves a metal and a metal oxide. The metal oxide acts as the oxidizer, supplying oxygen for the reaction, allowing it to proceed vigorously even underwater. This reaction generates extreme heat.
Certain metals, such as magnesium, can also burn underwater. When ignited and submerged, magnesium reacts directly with water molecules. This reaction is highly exothermic, producing magnesium oxide and releasing hydrogen gas, which can then burn intensely with any available oxygen. Similarly, some flares and sparklers are designed with built-in oxidizers, such as potassium nitrate, which release oxygen when heated, allowing them to burn underwater.
Practical Underwater Fire
The principle of sustaining combustion underwater has practical applications, particularly in industrial settings. Underwater welding and cutting operations frequently employ techniques that involve intense heat. For example, oxy-fuel cutting underwater uses specialized torches that supply both fuel gas and pure oxygen to create a flame. The oxygen forms a bubble around the cutting tip, displacing water and allowing the flame to interact directly with the material.
Exothermic cutting, another method, uses rods filled with materials that burn at extremely high temperatures once ignited. These rods often contain a mixture that includes a built-in oxidizer, allowing them to continue burning and cutting through materials like steel, concrete, and even rock underwater. This technology is essential for construction, repair, and salvage work on submerged structures like pipelines, ships, and offshore rigs.