As the title says, dynamite will explode, but other fuels simply catch on fire. Is there a hidden difference? Or is it simply that one catches on fire more violently?
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*Most* flammable and explosive substances are, at their core, a hydrocarbon with a chemical formula of X*(1 carbon + 2 hydrogen).
The limiting factor in a hydrocarbon’s ability to react is the availability of oxygen. The only reason sugar doesn’t explode when you light it on fire is because there just isn’t very much oxygen in the air – when sugar burns it is using up quite literally 100% of the oxygen around it and the amount of heat generated depends on how quickly more oxygen can get sucked in.
One of the simplest explosives you can make is to take sugar and mix it with Potassium Nitrate, which is about 50% oxygen by weight. With all of the oxygen from the Potassium Nitrate mixed into the sugar, the reaction is no longer limited by the amount of oxygen in the air and the sugar will explode. So the difference between something that will burn and something that will explode often comes down to whether the substance has oxygen mixed into it.
The explosive in dynamite is a molecule that is made up of what is essentially propane that has had some nitrogen and oxygen stuck to it. The fact that the nitrogen is stuck to both the oxygen and propane keeps them from reacting, but it doesn’t take much to knock the nitrogen loose. Once that happens, all of the oxygen is free to react with the propane, so 100% of the available fuel burns in a fraction of a second.
Nitrogen bonds very weakly to both oxygen and carbon. As a result, a common way of making explosives is to take a low weight hydrocarbon, like propane, and then to jam a bunch of nitrogen and oxygen into it. The fact that the oxygen-nitrogen-carbon bond is so weak means that you can break molecules made up of those materials with physical force or small amounts of heat. Once broken, the nitrogen will react with some of the available oxygen to form nitrous oxide, while the remaining oxygen will instantly burn all of the available carbon.
At the end of the day, the difference between a fire and an explosion is how fast you release the energy contained in the fuel. If you release that energy over a long period of time, the heat has time to slowly dissipate and you get a fire. If you release all of that energy in a fraction of a second, the heat “dissipates” in the form of an explosion.
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