Gunpowder is a simple mixture that supports rapid combustion. Its composition has slightly varied over the centuries, and can in fact be made using different proportions of the ingredients. The different blends affect the speed and how completely the mixture burns.
The traditional ingredients for gunpowder are:
Carbon (usually ground charcoal)
Potassium Nitrate (also called salt peter)
Sulfur
Each of the three play a useful role.
Carbon is the primary fuel – the part that actually burns, producing the carbon dioxide and water found in most combustion reactions.
Potassium nitrate is an oxidizer. Its chemical formula, KNO3 shows that there is quite a bit of oxygen stored up in each formula unit. This is important for gunpowder because to burn rapidly requires an immediate source of oxygen. Most fires have to draw their oxygen from the air around them, so only the outer surface burns well, and can only burn as quickly as fresh oxygen can diffuse through the air to the flame. Without an internal oxygen source, combustion goes slowly. By having an oxygen source mixed in with the fuel, combustion is rapid and complete (if the proportions are correct).
Sulfur works primarily as a stabilizing agent, though it too burns. It’s responsible for the harsh smell and teary eyes that go along with gunpowder smoke. The reason for this is because it gives off sulfur dioxide (SO2) gas which is toxic, mainly because it forms a strong acid (sulfurous acid) when it mixes with water – like the water that keeps your eyes, throat and lungs moist.
Sulfur is not actually necessary in the mixture. Leaving it out still provides a mixture that burns rapidly, but it tends to sputter a bit more. Charcoal is not the only possible fuel in gunpowder either. Sugar also works, which makes sense, as it is basically a carbon ring with hydrogen and oxygen (the building blocks of water) mixed in.
You may notice a purple hue to the gunpowder flame (especially with the sugar). This is because of the potassium ions from the KNO3. The electrons in potassium emit a purple light when excited, which contrasts with the typical orange flame associated with carbon.
One fun fact about gunpowder – most people believe that gunpowder always explodes. This isn’t true, though it does help keep people safe. Gunpowder burns rapidly, releasing large amounts of gases (carbon dioxide, steam, sulfur dioxide) and heat. If this occurs in a confined space, the heated gasses can exert enough pressure to cause the container to rupture, or “blow up”. Without confinement, a pile of gunpowder simply burns. A large enough pile, however, can also function as a container – the outer portions of the pile behave like a walls, containing the center. As the center burns, it can then blow the pile apart with the rapid release of gas. Because of this behavior, gunpowder gets classified only as a “low explosive” – quite wimpy when compared to the likes of nitroglycerine.
