Combustion Reactions

To start a fire, you need three specific ingredients: Fuel, Oxygen, and Heat. This is known as the Fire Triangle. If you remove any one of these, the reaction stops immediately. In chemistry, combustion is an exothermic reaction, meaning it releases energy into the surroundings as heat and light. Most common fuels are hydrocarbons—molecules made entirely of hydrogen and carbon atoms, such as methane ($CH_4$), propane ($C_3H_8$), or butane ($C_4H_{10}$).

When a hydrocarbon burns in a plentiful supply of oxygen, we call it complete combustion. This is the most efficient way to release energy from fuel. In this reaction, the carbon atoms are fully oxidized to form carbon dioxide ($CO_2$), and the hydrogen atoms are oxidized to form water ($H_2O$). A hallmark of complete combustion is a clean, blue flame, like the one you see on a well-adjusted gas stove.

If the oxygen supply is limited, the fuel cannot burn completely. This is incomplete combustion. Because there isn't enough oxygen to turn all the carbon into $CO_2$, other products form. These include carbon monoxide ($CO$), a colorless and odorless toxic gas, and soot (pure carbon, $C$). Incomplete combustion is less efficient, releasing less heat energy, and is characterized by a flickering yellow or orange flame.