Most households keep baking soda, or sodium bicarbonate (\(\text{NaHCO}_3\)), for baking, cleaning, and deodorizing. This common white powder is a simple chemical salt with unique thermal properties. Given its presence near heat sources, understanding its reaction to fire reveals why this substance is safe and useful in emergency situations.
The Simple Answer: Flammability Status
Baking soda is classified as non-flammable, meaning it cannot ignite or sustain a flame. Its chemical makeup consists of sodium, hydrogen, carbon, and oxygen atoms, lacking the long carbon and hydrogen chains found in organic fuels like wood or gasoline. Flammability requires a substance to break down into gaseous, combustible products that react with oxygen, a process baking soda cannot perform. The National Fire Protection Association (NFPA) rates its flammability at zero, the lowest possible hazard level.
How Baking Soda Reacts to Extreme Heat
When sodium bicarbonate is subjected to extreme heat, it undergoes thermal decomposition rather than combustion. This decomposition begins around \(50^{\circ}\text{C}\) (\(122^{\circ}\text{F}\)) and occurs rapidly once temperatures exceed \(80^{\circ}\text{C}\) (\(176^{\circ}\text{F}\)). The reaction breaks down the solid sodium bicarbonate into three products: sodium carbonate (\(\text{Na}_2\text{CO}_3\)), water vapor (\(\text{H}_2\text{O}\)), and carbon dioxide gas (\(\text{CO}_2\)). This breakdown absorbs heat from the environment, making the reaction endothermic, which works against the energy required to sustain a fire.
The resulting products are all non-flammable, and the release of steam and carbon dioxide is significant. This process prevents the powder from burning, cools the heat source, and produces a smothering agent. This stability and heat-absorbing property ensure baking soda does not contribute to a fire’s fuel load.
Baking Soda as a Fire Suppressant
The products of thermal decomposition enable sodium bicarbonate to function as an effective fire suppressant for smaller, contained fires. The carbon dioxide gas released is denser than air, quickly sinking and blanketing the fire. This \(\text{CO}_2\) displaces the oxygen required for combustion, rapidly extinguishing the flames.
The water vapor released during decomposition cools the fire’s temperature. The remaining solid product, sodium carbonate, leaves a cooling crust on the burning material, which helps prevent reignition by creating a barrier. Baking soda is the active ingredient in many BC class dry chemical fire extinguishers, designed for liquid (Class B) and electrical (Class C) fires. It is useful for small grease fires on a stovetop but is not effective against deep-seated fires involving solid materials like wood or paper (Class A).