Why do some things explode instead of simply burn?
In: Chemistry
Its about how quickly they can react with oxygen
Wood slowly burns because only the outside of the wood is exposed to the oxygen in the air, there is no free oxygen to allow the middle of the wood to burn until the outer layer has burned away
Now if you powderize that wood and fluff that dust up in the air then it can explode, because now all of the wood is exposed to oxygen at the same time so one spark can light a piece of dust which lights its neighbors which light their neighbors and the whole thing goes up in a big [dust explosion](https://en.wikipedia.org/wiki/Dust_explosion)
Generally things we make that we want to explode include their own oxydizer (source of oxygen). They contain a compound that will break down and give off oxygen that can be reacted with when exposed to heat providing oxygen to the inside of the fuel so that all of it can combust very quickly causing a detonation rather than a deflagration.
it’s not only about energy release, which could just mean things get very hot, like thermite
What an explosion is, is a pressure wave. A rapid increase in pressure and that pushes away the things around it. Things that explode tend to go from a state where there is tightly packed atoms or molecules to a state of gas. In gas form, things want to take a lot more space. That forms the rapid increase in pressure.
Things that only burn don’t change from solid to gas, but rather they give parts of themselves to something that is already a gas – eg they give carbon molecules to oxygen in the air, forming carbon monoxide or carbon dioxide(CO2).
A fire and an explosion are two distinct phenomena. To simplify, a fire is the result of a chemical reaction while an explosion is a rapid expansion of gas. While in some cases a fire may cause explosions, they are not always related so things can burn without exploding and explosions can occur without fire depending on the situation.
To keep it really elementary, the reasons things explode is because the energy stored in them is released rapidly.
You could have a thick piece of hardwood and, say, a bottle cap of black powder that have the same amount of energy stored in them, but when you apply a match to both of them the outcome is very different. The energy stored in the chemical bonds of the matter that makes up the log is going to be released very slowly over a matter of minutes and hours as the wood burns. The energy in the black powder is going to be released almost instantly as the bonds between the matter that make up the black powder break within a fraction of a second.
Think of it like this: it’s the difference between a tire losing air over a few days beacuse of a tiny leak slowly letting the pressure out of the tire, versus the sidewall of the tire failing and *all* the air pressure escaping almost immediately. It’s the same amount of air pressure in both cases but the outcome of one over the other is pretty dramatic.
Some items burn much faster than others. Let’s take wood and gunpowder
You have the same amount of each
You set fire to the wood, it burns slowly and takes maybe an hour to fully combust
Meanwhile the same amount gun powder burns incredibly quickly, in a matter of milliseconds
This means roughly the same amount of energy is released in a much shorter time frame making it appear much more violent. And with such a fast release of energy, you get a pressure wave of air that is one of the more damaging parts of an explosion
(Disclaimer, it’s different quantities of energy stored in each, but that’s to do with chemical composition and reaction rates of certain chemicals. Wood has alot more junk in it, some of which doesnt burn)
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