To find out how we have to travel in our minds back to the beginning of human civilization--specifically, the domestication of animals. One thing that large domesticated animals like horses and cows produce in bulk is urine (pee pee!). When these animals are kept inside some sort of shelter, their pee seeps into the dry plant-less ground beneath their feet, where microorganisms turn urea into ammonia. The little pee-digesting organisms don't stop there though, they combine the ammonia with oxygen, eventually giving rise to the nitrate ion, in a process known as nitrification.
Ammonia + Oxygen → Nitrates
NH3 + 2 O2 → NO3- + H+ + H2O
|Nitrification of Ammonia. Note: animals pee out urea, but microbes turn urea into ammonia, which then undergoes nitrification.|
Those nitrate ions combine with other minerals in the soil (metal carbonates, for example) to make a mixture of calcium, magnesium, and potassium nitrates (I will refer to these as "metal nitrates"). More and more of these metal nitrates can accumulate over time due to the absence of nitrate-ion-consuming life such as plants in the soil of the animals' stables. Eventually the soil can become so saturated with metal nitrates that crystals of these substances grow beneath stable floorboards or along the walls of adjacent cellars.
|Metal nitrates (a.k.a. saltpeter) being extracted from underneath stable floorboards by a "saltpeter man" or "peterman"|
|Metal nitrates (in this case 95% potassium nitrate) growing from the cellar wall of an old mill. source|
You might ask "What does all this have to do with explosives?". We're getting there, trust me. Before humans could use the metal nitrates for really big and reliable explosives, they had to figure out how to purify them. The problem with the mixture of metal nitrates produced by nitrification of urine is that it gives a mixture of metal nitrates, when what you really want is potassium nitrate. The calcium and magnesium nitrates soak up too much water from the air, which diminishes their explosive potential. Arab scientist Hassan Al-Rammah (inventor of the torpedo) was one of the first people in history to leave a clear record of how to isolate pure potassium nitrate in his epically-titled 13th century text "The Book of Military Horsemanship and Ingenious War Devices" (!!!!). He described dissolving the metal nitrate mixture in water, mixing the resulting solution with wood ash (which has lots of potassium carbonate, as we discovered in this fire post), then filtering and crystallizing. Using wood ash serves to swap out all the undesirable calcium and magnesium ions from the crude metal nitrate mixture for desirable potassium ions from the wood ash, resulting in pure potassium nitrate. 13th-century chemistry used pee-breakdown products and wood ash! Insane!
As the heap ripens, the nitre is brought to the surface by evaporation, and appears as a whitish efflorescence, detectible by the taste.What ingenuity humans have! Amazing! Also, tasting crystals that grew off a pee/straw mixture is pretty surprising!
Now that we have pure potassium nitrate, let's continue our trek towards understanding its use in explosions. The picture below is another way to think about nitrification, and it shows the HUGE volume of oxygen gas that it takes to make a tiny volume of nitrates. For this reason, you can think of potassium nitrate as if it is solidified oxygen. This is very important to why it is good for use in explosives.
|A comparison of the volumes of reactants and products in the nitrification process. Roughly 200:1, if you want numbers. Nitrates can be though of as an extremely dense form of oxygen.|
So why does "solidified oxygen" help with explosions? Most explosions are simply combustion reactions that happen really really fast. In normal combustion reactions, the rate at which something burns is limited by how fast the oxygen molecules can reach the flame. That is why blowing on kindling of a campfire helps it burn faster and brighter, because you are increasing the rate at which oxygen molecules reach the fire.
|Young dude, increasing the rate at which oxygen molecules reach his nascent fire.|
So, when you mix potassium nitrate ("solidified oxygen") with some sort of fuel (charcoal, for example) and set the mixture ablaze, the resulting reaction doesn't have to wait for the lazy lumbering oxygen gas molecules to arrive, because it has all the oxygen it needs right there in the form of the potassium nitrate. Therefore, the combustion occurs nearly instantaneously. Imagine taking all the energy that is released from a campfire throughout the course of an evening and releasing it all at once. That's right...KAPOW!!!!
Now this discovery didn't really revolutionize human civilization until we figured out how to use it to kill each other. Gunpowder is made by mixing potassium nitrate with two fuels (i.e. atoms that can combine with oxygen atoms and release energy), carbon (charcoal) and sulfur. The oxygen atoms leave the nitrates and move onto the carbon and sulfur atoms, releasing a buttload of energy. This process is so explosive because the "solidified oxygen" is turned into products that are mostly gases (carbon dioxide, etc). The gaseous products take up lots more room than the solid starting materials...which explains the KAPOW.
In order for this oxygen exchange reaction (aka explosion) to happen as quickly as possible (aka biggest explosion possible), you need to make sure each carbon and sulfur atom is very near a potassium nitrate molecule. This is why people who make gunpowder use these crazy ultra-mixing contraptions called ball mills:
MIND-MELT-RECAP: the dawn of agriculture gave us animal stables, which gave us LOTS of pee, which gave us metal nitrates, which we somehow learned how to purify, which eventually gave us the worlds' first explosives, which we eventually turned into gunpowder! Agriculture to cow pee to guns! Who knew?!!
"Some History of Nitrates" J. Chem. Ed. 2003, 80, 1393. link
"The Chemistry of Fireworks" RSC Publishing, 2000 link
"Chemical Technology in Arabic Military Treatises" Ann. N.Y. Acad. Sci. 2006, 500, 153. link
"The Nitrifying Bacteria: a review" J. Soil Sci. 2006, 4, 59. link
PeePeePost #1: "Why Is Pee Yellow?" or "Rainbow of Urine"
PeePeePost #2: "Why Does Pee Smell?" or "Aroma of Life"
PeePeePost #4: "PeePee Portal to Phosphorus" or "What the Alchemists Did Right"
PeePeePost #5: "How Pee United You With Rocks"
PeePeePost #6: "Pee, Our Connection with the Earth's Metabolic Cycle"