Have you heard the one about Chinese alchemists who were searching for an elixir of eternal life but ended up discovering the worlds first chemical explosive instead? No?

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It turns out combining saltpeter, sulphur, and charcoal does not equal the recipe for immortality, it equals gunpowder. [explosion]
The alchemists must have been surprised the first time that powdery concoction came in contact with a spark, a Tang dynasty era text from around 850 A.D. describes it as such: "Some have heated together the saltpeter, sulfur, carbon of charcoal, and honey; smoke and flames result, so that hands and faces have been burnt, and even the whole house where they were working burned down." Hopefully that's not going to us here at SciShow today. Turns out the honey was not a key ingredient, but something does make this black powder so potent. None of its components are particularly combustible on their own, but when ground up and combined in the correct proportions, BOOM.

Charcoal, which makes up about 15% of gunpowder is the primary fuel. It provides a porous Carboniferous structure with lots of surface area and stored chemical energy. The Chinese liked to used willow wood for their gunpowder charcoal, though they also used grapevines, hazel wood, and pine cones.

Sulfur, which makes up 10% of gunpowder, is the catalyst and the secondary fuel. It has a relatively low melting point of about 115 degrees Celsius, helping to lower the overall temperature at which the gunpowder ignites.

Potassium nitrate, or saltpeter, makes up the other 75% of the gunpowder. Historically it was made from leeching animal manures and wood ash with rainwater. Today it's mostly synthetically made, using the Haber process.

But what's the point of the saltpeter? It's not fuel; it doesn't contain much chemical energy. What it does contain is oxygen.

Burning consumes oxygen and with a normal fire, atmospheric levels of oxygen are fine. But if you want something to burn fast, and I mean really, really fast, you need a concentrated source of oxygen. Potassium nitrate, by weight, is 86% oxygen, and it's a whole lot heavier than air. With the low ignition point of the sulfur, and the high surface area of the ground up charcoal, you got a recipe for a really instantaneous oxidation. And so begins our chain reaction. The sulfur starts the burn on the charcoal. The burning charcoal raises the temperature of the nitrate molecules in the saltpeter, tearing them apart, releasing all that sweet oxygen to hasten the combustion. All told, the reaction takes a few thousandths of a second. When it's complete, our three ingredients have reacted to form mainly nitrogen, carbon dioxide, carbon monoxide, water, and potassium sulfide, which is the white smoke you see when a gun is fired.

So that explains the rapid burning, but where's the explosion? Well as the Chinese discovered over time, the explosiveness occurs when you enclose the gunpowder. During the combustion process, solids, which don't take up very much space, are converted to gases, which do. In fact, at atmospheric temperatures, the products of this reaction need over a thousand times more space as the original solids. The release of that energy is what makes gunpowder so explosive. Enclose the gunpowder and leave a tiny hole and suddenly you've got a bottle rocket. Enclose it completely and you have yourself a small bomb. Put it in a tube with a hole at one end and a lump of lead between the gunpowder and the outside and you've got a cannon.

Now the ratios of the ingredients in gunpowder are extremely important, which meant that large barrels of gunpowder often had to be carefully, like, re-stirred and mixed together if they sat for any period of time. The process of making gunpowder today has been refined to grind the ingredients into a very fine grain in a wet solution to ensure even distribution, and then they re-dry it. All told, it took centuries for the Chinese to first discover, and then perfect, these correct ratios of gunpowder. And like the recipe for coca-cola, they kept it a closely guarded secret. It took nearly 400 year for the Chinese to finally lose their monopoly on this potent invention. Though some historians theorize this is because China's history was so stable during this time that they actually had little call to use gunpowder in warfare. Once the secrets made it to Europe however, the chain reaction could not be contained. Warfare, and the face of the earth, would change forever.

Thank you for watching this episode of SciShow. I'm so glad that we didn't blow ourselves up. If you want to ask any questions or suggest future topics, we're down in the comments or on Facebook or twitter. And if you want to keep getting smarter with us at SciShow, you can go to https://youtube.com/scishow and subscribe.